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Endocrinology Vol. 149, No. 10 5136-5145
Copyright © 2008 by The Endocrine Society

Inhibition of Cholesterol Biosynthesis Impairs Insulin Secretion and Voltage-Gated Calcium Channel Function in Pancreatic β-Cells

Fuzhen Xia, Li Xie, Anton Mihic, Xiaodong Gao, Yi Chen, Herbert Y. Gaisano and Robert G. Tsushima

Departments of Medicine and Physiology, University of Toronto, Toronto, Ontario, Canada M5S 1A8

Address all correspondence and requests for reprints to: Robert G. Tsushima, Department of Biology, York University, 4700 Keele Street, Farquharson 344, Toronto, Ontario, Canada M3J 1P3. E-mail: tsushima{at}yorku.ca.


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Insulin secretion from pancreatic β-cells is mediated by the opening of voltage-gated Ca2+ channels (CaV) and exocytosis of insulin dense core vesicles facilitated by the secretory soluble N-ethylmaleimide-sensitive factor attachment protein receptor protein machinery. We previously observed that β-cell exocytosis is sensitive to the acute removal of membrane cholesterol. However, less is known about the chronic changes in endogenous cholesterol and its biosynthesis in regulating β-cell stimulus-secretion coupling. We examined the effects of inhibiting endogenous β-cell cholesterol biosynthesis by using the squalene epoxidase inhibitor, NB598. The expression of squalene epoxidase in primary and clonal β-cells was confirmed by RT-PCR. Cholesterol reduction of 36–52% was observed in MIN6 cells, mouse and human pancreatic islets after a 48-h incubation with 10 µM NB598. A similar reduction in cholesterol was observed in the subcellular compartments of MIN6 cells. We found NB598 significantly inhibited both basal and glucose-stimulated insulin secretion from mouse pancreatic islets. CaV channels were markedly inhibited by NB598. Rapid photolytic release of intracellular caged Ca2+ and simultaneous measurements of the changes in membrane capacitance revealed that NB598 also inhibited exocytosis independently from CaV channels. These effects were reversed by cholesterol repletion. Our results indicate that endogenous cholesterol in pancreatic β-cells plays a critical role in regulating insulin secretion. Moreover, chronic inhibition of cholesterol biosynthesis regulates the functional activity of CaV channels and insulin secretory granule mobilization and membrane fusion. Dysregulation of cellular cholesterol may cause impairment of β-cell function, a possible pathogenesis leading to the development of type 2 diabetes.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
PANCREATIC β-CELLS secrete insulin in response to elevated glucose to maintain blood glucose homeostasis. Defects in β-cell insulin secretion lead to hyperglycemia and development of type 2 diabetes. The distal events underling stimulus-secretion coupling of insulin secretion have been well documented and are characterized by two major events (1). The first involves changes in electrical activity of β-cell ion channels, and the second, the function of secretory machinery regulated by the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins.

Uptake of glucose by β-cells enhances mitochondrial oxidation and ATP production. The elevation of ATP to ADP ratio closes ATP-sensitive K+ (KATP) channels, leading to membrane depolarization, the opening of voltage-gated Ca2+ (CaV) channels, and fusion of insulin-containing secretory granules with the plasma membrane. Voltage-gated K+ (KV) channels play an important role in repolarizing the membrane potential to suppress the entire process of glucose-stimulated insulin secretion (2). In this sequential glucose-stimulated insulin secretion, influx of Ca2+ through CaV channels and subsequent increase in intracellular Ca2+ concentration ([Ca2+]i) causes interaction of SNARE proteins to initiate exocytosis (3, 4, 5).

SNARE proteins play an essential role in the fusion of insulin granules with plasma membranes. vesicle-associated membrane protein (VAMP)-2 is a SNARE protein located on donor vesicles, whereas syntaxin 1A and synaptosomal-associated protein of 25 kDa (SNAP-25) are SNARE proteins located on target plasma membranes (t-SNARE). Based on the current view, SNARE proteins facilitate exocytosis by binding SNARE protein located on donor vesicles to their cognate t-SNARE proteins, giving rise to a tight complex that fuses secretory granules to plasma membranes (6, 7). SNARE protein conformational changes are believed to provide energy for membrane fusion. It is well established that glucose-stimulated insulin secretion is characterized by a biphasic pattern consisting of a transient first phase followed by a sustained second phase secretion (8). This is reflected by the sequential release of distinct pools of insulin granules; a limited readily releasable pool and a larger reserve pool, respectively (9, 10). Granules from the readily releasable pool can undergo exocytosis right after stimulation, whereas granules from the reserve pool undergo mobilization and/or priming to gain release competence, and both involve the formation of SNARE complexes (11, 12). Type 2 diabetes from human (13) and animal models (Zucker fa/fa and Goto-Kakizaki rats) (14, 15) manifest a reduced expression of SNARE proteins, which is partially accountable for the reduction of first-phase insulin secretion (16).

Constituting about 20% of the total membrane lipid, cholesterol is involved in several subcellular functions, such as influencing the thickness and fluidity of membranes and insulating membranes (17, 18). Cholesterol is tightly packed with sphingolipids to form specific microdomains termed membrane rafts (19, 20). Numerous membrane proteins are found to be associated with membrane rafts, in which the normal function of targeted proteins is regulated. Caveolins are constituent proteins of membrane rafts (21). We have demonstrated that ion channels (CaV1.2, KV2.1) and SNARE proteins (syntaxin 1A, SNAP-25, and VAMP-2) are targeted to these cholesterol-rich membrane raft microdomains in pancreatic β- and {alpha}-cells (22, 23).

Our observations demonstrated that acute depletion of membrane cholesterol with methyl-β-cyclodextrin (MβCD) implicated the association of ion channels and SNARE proteins with membrane rafts in β- and {alpha}-cells, which plays an important role in regulating insulin and glucagon secretion. However, less is known about the effects of chronic cholesterol depletion on β-cell function. Squalene epoxidase (a flavoprotein monooxidase located on the endoplasmic reticulum) is the second enzyme in the committed cholesterol biosynthesis pathway (24). Here we demonstrate that the squalene epoxidase inhibitor, NB598, significantly inhibits endogenous cholesterol biosynthesis, resulting in impaired β-cell insulin secretion. Furthermore, we demonstrate that the mediators of this effect involve the inhibition of CaV channels and the impairment of the exocytotic machinery.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Cell culture
Mouse MIN6 cells (kindly provided by S. Seino, Chiba University, Chiba, Japan) were grown in monolayer and maintained in DMEM (Sigma, Oakville, Ontario, Canada) containing 25 mM glucose and supplemented with 10% fetal bovine serum, 100 U/ml penicillin, 100 µg/ml streptomycin, 2 mM [;scap]l[r];-glutamine, and 0.05 mM 2-mercaptoethanol at 37 C in a humidified atmosphere (5% CO2). Cells were passaged every 4–5 d at 80% confluence.

Pancreatic islet isolation and dispersion
Mouse pancreatic islets from mouse insulin promoter (MIP)-green fluorescence protein (GFP)-transgenic mice (kindly provided by Dr. M. Hara, University of Chicago, Chicago, IL) were isolated by collagenase digestion as described previously (22). Human islets were isolated (25) and kindly supplied by Dr. Jonathan Lakey (JDRF Human Islet Distribution Program, University of Alberta, Canada). Upon arrival, islets were immediately hand picked. For electrophysiological studies, islets were dispersed into single cells with 0.25% trypsin in Ca2+- and Mg2+-free Hanks’ balanced salt solution (Invitrogen, Burlington, Ontario, Canada), placed onto coverslips, and cultured overnight before commencement of patch clamp experiments. Both intact islets and dispersed islet cells were cultured in RPMI 1640 (Sigma) media containing 11 mM glucose supplemented with 10% fetal bovine serum (FBS), 0.25% HEPES, 100 U/ml penicillin, and 100 µg/ml streptomycin. The cultured islet cells were used within 3 d. Mice were maintained in the pathogen-free animal facility at the University of Toronto, and all experiments were approved by the University of Toronto Animal Care Committee and conducted in accord with accepted standards of humane animal care.

RNA preparation and RT-PCR
Total RNA was isolated from cultured INS-1 and MIN6 cells and the pancreatic islets from rat, mouse, and human using Tri Reagent (Sigma) following the manufacturer’s protocol. Subsequent deoxyribonuclease I (Ambion, Austin, TX) treatment was performed to remove any residual DNA contamination. One microgram of isolated RNA was reverse transcribed using Omniscript RT kit (QIAGEN, Mississauga, Ontario, Canada) according to the manufacturer’s instructions. PCR was performed using Hot Start Taq DNA polymerase (Fermentas, Burlington, Ontario, Canada) with the primer pair targeting the squalene epoxidase gene (forward: 5'-AGCTATGGCAGAGCCCAAT-3'; reverse: 5'-TGGTAGATGAGAACTGGACT-3'). PCR protocol used was as follows: heat activation of polymerase at 94 C for 5 min, followed by 35 cycles of 94 C for 30 sec, 53 C for 30 sec, and 72 C for 60 sec. The amplified DNA from squalene epoxidase mRNA transcripts was visualized as a 280 bp band in a 2% agarose gel.

Subcellular fractionation of plasma membranes, endoplasmic reticulum, and insulin secretory granules
MIN6 cells (4 x 108) were cultured for 48 h at 37 C in the culture medium supplemented with 10% delipidated FBS (Cocalico Biological Inc., Reamstown, PA), in the absence or presence of 10 µM NB598. The cells were harvested and homogenized in fractionation buffers: 50 mM 2-(N-morpholino) ethane sulfonic acid (MES), 250 mM sucrose (pH 7.2) for plasma membrane (PM) and endoplasmic reticulum (ER); 10 mM 3[N-morholino]propanesulfonic acid-Tris, 270 mM sucrose (pH 6.8) for insulin secretory granules (SG). Fractionations for PM and ER were performed by sucrose density gradient ultracentrifugation established by Ramanadham et al. (26). Insulin secretory granules were fractionated with Histodenz (Sigma) gradient ultracentrifugation followed by Percoll (GE Healthcare, Baie d’Urfe, Quebec, Canada) purification, as established by Brunner et al. (27). The isolated subcellular fractions were stored at –20 C for protein concentration determination and cholesterol extraction.

Cholesterol content assay
MIN6 cells (5 x 105) or 20 pancreatic islets from mouse or human were cultured for 48 h at 37 C in the relative culture media supplemented with 10% delipidated FBS, in the absence or presence of 10 µM cholesterol biosynthesis inhibitor NB598 (Sigma). Cells and islets were collected and washed with PBS. Cholesterol was extracted by adding 50 µl of 2:1 chloroform-methanol mixture, followed by 100 µl of PBS. To extract cholesterol from subcellular fractions, 50 µl of 2:1 chloroform-methanol mixture was added to different compartments. The top water phase was removed after centrifugation for 3 min at 10,000 rpm. Cholesterol sample was dried and dissolved in 10–40 µl of immunoprecipitation buffer containing (in millimoles) 150 NaCl, 20 Tris-HCl, 5 MgSO4, 1 EDTA, 1 EGTA, and 1% Triton X-100. Cholesterol content was measured using a fluorescence assay kit (Cayman Chemical Co., Ann Arbor, MI), following the manufacturer’s instructions.

Insulin secretion assay
Krebs-Ringer bicarbonate (KRB) buffer containing (in millimoles) 129 NaCl, 5 NaHCO3, 4.8 KCl, 1.2 KH2PO4, 2.5 CaCl2, 2.4 MgSO4, 10 HEPES, and 0.1% BSA was used for insulin secretion assay for mouse pancreatic islets. The isolated islets were cultured for 48 h at 37 C in the islet culture medium supplemented with 10% delipidated FBS, in the absence or presence of different doses of NB598. Three hours before the secretion assay, glucose concentration in the culture medium was changed to 2.8 mM to recover the islets to a basal condition. Twenty islets were washed once with KRB and preincubated for 30 min in 1 ml KRB supplemented with 1 mM glucose. The islets were then incubated for 1 h with 1 ml of fresh KRB supplemented with 1 mM glucose, and the supernatants were collected for the assay of basal insulin secretion. One milliliter KRB supplemented with 16.7 mM glucose was changed to incubate the islets for 1 h at 37 C and the supernatants collected for the assay of glucose-stimulated insulin secretion. The islets were washed with ice-cold PBS and lysed with 1 ml of 75% ethanol/0.03 N HCl, and the tissue lysates were kept for the determination of total insulin concentration. All samples were kept at –20 C until assayed for insulin using a RIA kit (Millipore Corp., St. Charles, MO) and values for released insulin in the supernatants were normalized to total islet insulin.

Electron microscopy
Isolated islets from MIP-GFP mice were cultured for 48 h at 37 C in islet culture medium supplemented with 10% delipidated FBS, in the absence or presence of 10 µM NB598. They were then fixed with a Karnovsky style fixative [4% paraformaldehyde + 2.5% glutaraldehyde in a 0.1 M cacodylate buffer with 5 mM CaCl2 (pH 6.8)] for 1 h, postfixed with 1% osmium tetroxide for 30 min, and treated with 2.5% uranyl acetate for 30 min. The islets were then dehydrated using a graded series of ethanol and infiltrated with epoxy 812 resin in polyethylene capsules. A complete polymerization of the epoxy resin occurs for 48 h at 60 C. The solid epoxy resin blocks containing the islet samples were sectioned on a Reichert Ultracut E microtome to 70–90 nm thickness and collected on 200 mesh copper grids. The sections were counterstained for 15–20 min using saturated uranyl acetate, followed by Reynold’s lead citrate and then examined and photographed in a Hitachi H7000 transmission electron microscope (Hitachi Limited, Tokyo, Japan) at an accelerating voltage of 75 kV.

Electrophysiology
The dispersed islet cells were cultured for 48 h at 37 C in islet culture medium supplemented with 10% delipidated FBS, in the absence or presence of different doses of NB598. Pancreatic β-cells can be easily recognized as being green due to the expression of GFP in MIP-GFP mice, which we have been characterized as possessing normal physiological function (28). Single β-cells were voltage clamped in the whole-cell configuration to measure CaV, KV, and KATP currents as previously described (22, 23).

Photolysis of caged Ca2+ and cell capacitance measurement
Patch electrodes were pulled from 1.5-mm thin-walled borosilicate glass, coated close to the tip with orthodontic wax (Butler; Guelph, Ontario, Canada), and polished to a tip resistance of 2–4 M{Omega} when filled with intracellular solution. Standard bath solution for the experiments contained (in millimoles) 138 NaCl, 5.6 KCl, 1.2 MgCl2, 2.6 CaCl2, 5 D-glucose, and 5 HEPES (pH 7.4, adjusted with NaOH). Intracellular solution for flash experiments contained (in millimoles) 112 Cs-glutamate, 5 o-nitrophenyl EGTA (NP-EGTA), 3.7 CaCl2, 2 Mg-ATP, 0.3 Na2-GTP, and 0.2 fura-6 F (pH 7.2, adjusted with CsOH). NP-EGTA and fura-6 F were purchased from Molecular Probes (Invitrogen, Burlington, Ontario, Canada). Cell capacitance (Cm) was measured using an EPC-10 patch-clamp amplifier (HEKA, Lambrecht, Germany) controlled by the lock-in module of PULSE software. The capacitance traces were imported to IGOR Pro software (WaveMetrics, Lake Oswego, OR) for analysis. Flashes of UV light and fluorescence-excitation light were generated as described previously (29). In the flash experiments, exocytosis was elicited by photorelease of caged Ca2+ preloaded into the cell via the patch pipette. [Ca2+]i was measured with the Ca2+ indicator dyes fura-6 F. [Ca2+]i was determined from the ratio (R) of the fluorescence signals excited at the two wavelengths (340/380nm), following the equation (30): [Ca2+]I = Keff x (R – Rmin)/(Rmax – R), where Keff, Rmin, and Rmax are constants obtained from intracellular calibration as previously described (29).

Membrane raft isolation
MIN6 cells were cultured for 48 h at 37 C in the culture medium supplemented with 10% delipidated FBS, in the absence or presence of 10 µM NB598. The cells were then harvested and lysed by sonication with cold 1% Triton X-100 in MES-buffered saline [MBS; 25 mM MES, 150 mM NaCl (pH 6.5), supplemented with protease inhibitors]. Lysed cells were centrifuged at 2000 rpm for 15 min at 4 C. The supernatant was diluted with equal volume of an 80% sucrose solution in MBS (with 1% Triton X-100) and placed into the bottom of an ultracentrifuge tube. A 30% and 5% sucrose in MBS were loaded on top of the sample, which was centrifuged at 49,000 rpm in a MLS-50 rotor (Beckman, Fullerton, CA) for 22 h at 4 C. Ten gradient fractions (480 µl each) were collected from the top, and 20–30 µl of each fraction were loaded onto an SDS-PAGE gel for Western blot analysis.

Immunoblotting
Western blot was performed to detect the changes of protein expression and membrane raft association of ion channels and SNARE proteins. MIN6 cell lysate or the fractions from sucrose gradient ultracentrifugation were subjected to SDS-PAGE and transferred to polyvinylidene difluoride-plus membranes (Fisher Scientific Ltd., Nepean, Ontario, Canada). Membranes were probed with the indicated primary antibodies; anti-CaV1.2 and KV2.1 (Alomone Laboratories, Jerusalem, Israel), antisyntaxin 1A and SNAP-25 (Sigma), and anti-VAMP-2 generated as described previously (31). The bound primary antibodies were detected with the appropriate peroxidase-conjugated secondary antimouse or antirabbit antibodies (Jackson ImmunoResearch Laboratories, West Grove, PA) and then visualized by chemiluminescence (ECL-Plus; GE Healthcare, Mississauga, Ontario, Canada) and exposure to x-ray films (Eastman Kodak Co., Rochester, NY).

Statistical analysis
Data points represent mean ± SEM. An unpaired Student’s t test or a one-way ANOVA followed by a Student-Newman-Keuls test was used to compare control values from NB598-treated groups. P < 0.05 was used to denote statistical significance.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Inhibition of squalene epoxidase significantly decreases cholesterol levels in β-cells
Cholesterol biosynthesis is initiated from the reduction of 3-hydroxy-3-methylglutaryl coenzyme A, undergoing over 30 steps until the final cholesterol product. 3-Hydroxy-3-methylglutaryl coenzyme A reductase is an inhibition target for clinically used statins. However, inhibition of this enzyme has numerous side effects due to the blockade of secondary synthetic pathways upstream from cholesterol biosynthesis (24). Squalene epoxidase is the second enzyme in committed sterol biosynthesis, and inhibition of this enzyme affects only cholesterol synthesis. To confirm the cholesterol synthesis pathway in pancreatic β-cells, we first determined the expression of squalene epoxidase. RT-PCR detected the mRNA transcripts of this enzyme in pancreatic islets from rat, mouse, and human as well as the clonal INS-1 and MIN6 β-cells (Fig. 1AGo). The squalene epoxidase inhibitor, NB598, was used to reduce endogenous cholesterol biosynthesis in β-cells (32). The inhibition efficiency of this compound on cholesterol biosynthesis was then examined in β-cells. Delipidated FBS was used for this and subsequent protocols to prevent uptake of cholesterol (i.e. lipoproteins) normally found in FBS. Incubation with 10 µM NB598 for 48 h caused a 36 ± 7% reduction in total cholesterol level of MIN6 cells (n = 6, P < 0.01) (Fig. 1BGo). A similar reduction in total cholesterol content in mouse and human islets was observed: 40 ± 16% (n = 4, P < 0.05) and 52 ± 1% (n = 4, P < 0.01), respectively (Fig. 1BGo). To further examine the inhibitory effect of NB598 on cholesterol levels in different cellular compartments, we isolated PMs, ER, and insulin SGs from MIN6 cells. NB598 caused a significant decrease in cholesterol by 49 ± 2%, 46 ± 7%, and 48 ± 2% from PM, ER, and SG, respectively (n = 3, P < 0.05) (Fig. 1CGo). This demonstrates comparable reduction in cholesterol reduction throughout the cell.


Figure 1
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  		FIG. 1. Inhibition of squalene epoxidase significantly decreases endogenous cholesterol levels in β-cells. A, RT-PCR detected the mRNA transcripts of the committed cholesterol biosynthesis enzyme squalene epoxidase in INS-1 and Min6 β-cells as well as in pancreatic islets from rat, mouse, and human. Lower panel shows the glyceraldehyde-3-phosphate dehydrogenase (GAPDH) loading control. B, MIN6 cells, mouse and human islets were cultured with and without 10 µM NB598 for 48 h, and total cholesterol was extracted with chloroform-methanol and measured using a fluorescence assay kit. The squalene epoxidase inhibitor NB598 significantly decreased the cholesterol levels from MIN6 cells, mouse islets, and human islets (*, P < 0.05; **, P < 0.01, compared with control). C, Cholesterol levels of PMs, ER, and insulin SGs from MIN6 cells. Incubation of the cells with 10 µM NB598 for 48 h caused a significant decrease in cholesterol levels in the respective subcellular compartments (*, P < 0.05, compared with control).

 
Inhibition of cholesterol biosynthesis perturbs insulin secretion from mouse islets
The effect of inhibiting endogenous cholesterol on β-cell function was first examined by glucose-stimulated insulin secretion of mouse islets. Pancreatic islets isolated from MIP-GFP mice were incubated for 48 h with and without NB598. Before all experiments, pancreatic islets or dispersed islet cells were washed thoroughly to minimize any possible direct effects of the compound. NB598 was found to dose-dependently inhibit insulin secretion under both basal (1 mM glucose) and glucose-stimulated (16.7 mM glucose) conditions (Fig. 2Go). NB598 (2 and 10 µM) caused reductions in basal insulin secretion by 36% (n = 9, P < 0.01) and 51% (n = 9, P < 0.001), respectively, compared with control. The glucose-stimulated insulin secretion was reduced by 34% (n = 9, P < 0.01) and 75% (n = 9, P < 0.001) under the same concentrations of NB598, respectively (Fig. 2AGo). To preclude the possible direct effect of NB598 on insulin synthesis, we also measured the total insulin content of pancreatic islets. NB598 at all doses used for the above insulin secretion studies did no cause any change in the total insulin content of the islets (Fig. 2BGo). To examine the specificity of NB598 on insulin secretion, we overloaded cholesterol to the NB598-treated cells by incubation with 10 mM soluble cholesterol at 37 C for 1 h (33). Insulin secretion at low glucose condition was fully restored by cholesterol repletion, whereas glucose-stimulated insulin secretion was restored by 57% (n = 6, P < 0.01), compared with control (n = 3).


Figure 2
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  		FIG. 2. Inhibition of cholesterol synthesis perturbs insulin secretion from mouse islets. Pancreatic islets isolated from MIP-GFP mice were incubated for 48 h without and with increasing doses of squalene epoxidase inhibitor NB598. Basal insulin secretion (1 mM glucose) and glucose stimulated-insulin secretion (16.7 mM glucose) were measured. A, NB598 dose-dependently inhibits insulin secretion from mouse islets under both basal and high-glucose conditions (*, P < 0.01; **, P < 0.001, compared with controls). B, Total insulin of mouse islets was measured from the same samples corresponding to A. NB598 does not cause a significant change in total insulin level. C, Insulin secretion measured in mouse islets incubated without or with 10 µM NB 598 alone (NB) or after 1 h incubation with 10 mM soluble cholesterol (NB+Chol). Cholesterol overloading fully restored basal insulin secretion at 1 mM glucose and partially restored glucose-stimulated insulin secretion (*, P < 0.05; **, P < 0.01).

 
To study the ultrastructure of insulin secretory granules, electron microscopic analysis was performed on mouse islets cultured for 48 h without and with NB598. No gross change in the insulin granule size or density was observed (Fig. 3Go). These results suggest that the observed impaired insulin secretion by NB598 is the result of a deficiency in cellular cholesterol unrelated to changes in insulin content or granule morphology. Because ion channels and SNARE proteins play an essential role on β-cell stimulus-secretion coupling, we next explored the possible changes in channel activity and single-cell exocytosis after endogenous inhibition of cholesterol biosynthesis by NB598.


Figure 3
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  		FIG. 3. Electron microscopic analysis of insulin granules. Pancreatic islets isolated from MIP-GFP mice were treated as in Fig. 2Go. Insulin granules from islets incubated with 10 µM NB598 (right panel) displayed similar gross morphology and density as those from control islets (left panel). White scale bar, 500 nm.

 
Inhibition of cholesterol biosynthesis blocks CaV channels
Opening of CaV channels and the subsequent increase in [Ca2+]i are critical to trigger insulin secretion. We previously detected the association of CaV channels with cholesterol-rich membrane rafts in pancreatic β-cells (22). Therefore, impairment of insulin secretion caused by the inhibition of cholesterol production by NB598 could be mediated by the altered CaV channel activity. Dispersed islet cells from MIP-GFP mice were cultured with different concentrations of NB598 for 48 h, followed by repeated drug washout before electrophysiological measurements were made. The β-cells were identified by the expression of GFP. Consistent with the reduction in insulin secretion, NB598 caused a dose-dependent decrease in CaV currents (Fig. 4Go, A and B). The peak CaV current amplitude measured at +10 mV was –13.3 ± 1.0 pA/pF (n = 9) in control cells, –3.7 ± 1.2 pA/pF (n = 4, P < 0.001), and –2.1 ± 0.1 pA/pF (n = 10, P < 0.001) in 2 and 10 µM NB598-treated cells, respectively. To preclude any possible direct effects of the drug on CaV channels, we added the 10 µM NB598 acutely into the bath solution and observed no effect on CaV currents (data not shown). Furthermore, cholesterol repletion experiments restored CaV current amplitude to 78% of control (n = 6), which was not statistically different from control (Fig. 4CGo). We speculate the resultant decrease in CaV currents after cholesterol reduction is one of the primary mediators for the observed inhibition of insulin secretion by NB598.


Figure 4
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  		FIG. 4. NB598 inhibits mouse β-cell CaV channels. Pancreatic islet cells isolated from MIP-GFP mice were incubated for 48 h without and with increasing doses of NB598. β-Cells were recognized by GFP marker. A, Representative traces showing Ca2+ currents triggered by a series of pulses (from –70 to +70 mV, 500 msec) from a holding potential of –80 mV in a control β-cell and a β-cell treated with 10 µM NB598, indicating a significant inhibition of CaV currents by 10 µM NB598. B, Current-voltage relationship of CaV channels under different concentrations of squalene epoxidase inhibitor NB598; 2 µM (P < 0.001) and 10 µM NB598 (P < 0.001) dose-dependently inhibited CaV currents at depolarizing voltages from –40 mV to +50 mV. C, Peak CaV currents at –10 mV for β-cells treated without or with 10 µM NB598 alone (NB) or after 1 h incubation with 10 mM soluble cholesterol (NB+Chol) before recording. Cholesterol repletion significantly restored the decreased CaV currents (*, P < 0.05; **P < 0.001).

 
NB598 increases KV channel inactivation
KV channels regulate membrane potential repolarization and finely tune insulin secretion (2). We reported that KV2.1 channels are associated with lipid raft domains in β-cells, and the currents are regulated by the surrounding lipid environment (22). NB598 at concentrations up to 10 µM did not affect peak outward KV currents or the voltage dependence of activation (data not shown) but increased current inactivation (Fig. 5AGo). A longer (12 sec) depolarization at +70 mV was performed to clearly display this inactivation effect on KV channels. Increasing concentrations of NB598 accelerated the inactivation rate of KV channels (Fig. 5BGo). Steady-state inactivation was measured after a 5-sec conditioning depolarization from –120 mV to +20 mV followed by 500-msec test steps at +60 mV. No hyperpolarizing shift in the steady-state inactivation curve was observed, but the slope factor decreased from 15.4 ± 1.2 mV (control) to 9.2 ± 0.7 mV (10 µM NB598 treated cells; n = 4, P < 0.01) (Fig. 5CGo). Furthermore, the relative amount of noninactivating current was significantly different. Control cells displayed 45 ± 1% noninactivating KV current at 0 mV (n = 4), whereas cells cultured with 10 µM NB598 displayed only 11 ± 1% noninactivating KV currents (n = 4; P < 0.05) (Fig. 5CGo). Cholesterol repletion fully restored the inactivated KV currents (Fig. 5Go, B and C), confirming that the effect of NB598 on KV channels was a result of reduced membrane cholesterol levels. Besides KV channels, 10 µM NB598 also decreased KATP current density at –140 mV by 57 ± 4% (n = 7, P < 0.01, data not shown). However, we do not believe the changes in either KV or KATP channels played a role on the reduced insulin release observed above because it has been well established that reductions in either KV or KATP currents enhance insulin secretion (2, 34, 35).


Figure 5
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  		FIG. 5. NB598 increases the steady-state inactivation of KV channels in mouse β-cells. Pancreatic β-cells from MIP-GFP mice were held at –80 mV, and whole-cell KV currents were measured after depolarization from –80 to +60 mV in 10-mV increments using 500-msec step pulses. A, Representative traces of KV currents from a control β-cell and a β-cell cultured for 48 h in the presence of 10 µM NB598, demonstrating that NB598 does not affect peak KV currents but enhances current inactivation. B, Longer depolarizations (12 sec) at +70 mV were performed to display this inactivation effect. Increasing concentrations of NB598 accelerated the inactivation rate of KV channels. The effect of 10 µM NB598 treatment on KV current inactivation was fully reversed after 1 h cholesterol repletion with 10 mM soluble cholesterol (10 µM NB + Chol). C, Steady-state inactivation of KV channels was measured after 5-sec conditioning depolarization pulses at voltages from –120 mV to +20 mV. No left shift in the steady-state inactivation curve was observed, but the slope factors for all NB598 doses decreased (P < 0.01). The relative amount of noninactivating current measured at 0 mV was also significantly decreased from cells cultured with NB598 (P < 0.05). The inactivation curve fully recovered after cholesterol repletion (10 µM NB + Chol).

 
Inhibition of cholesterol biosynthesis by NB598 impairs β-cell exocytosis
We next investigated the effects of NB598 pretreatment on β-cell exocytosis. To exclude the dependency of Ca2+ influx from Ca2+ channels, exocytosis was elicited by flash photolysis of caged Ca2+ (NP-EGTA) (9). Exocytosis was monitored as an increase in whole-cell Cm. In response to the step-like elevation in [Ca2+]i generated by the uncaging of Ca2+ by flash photolysis, capacitance traces displayed a rapid, burst-like increase within the first 0.5 sec after the flash followed by a slower sustained phase of exocytosis. Changes in Cm consisted of three components, which have been termed the fast burst, slow burst, and sustained components (12, 36). The fast and slow burst components are generally interpreted as the fusion of docked and primed vesicles from the rapidly and slowly releasable pools, respectively, whereas the sustained component represents refilling of the releasable pools from a large depot pool of vesicles (37). In our experiments, flash photolysis of NP-EGTA induced a step-like homogenous increase in [Ca2+]i from 200 to 300 nM to 5–10 µM. The averaged Cm traces were compared from control and NB598-treated cells responding to similar step-like [Ca2+]i elevations (Fig. 6Go). The amplitude of the fast burst in NB598-treated cells was reduced from 282 ± 30 femtofarad (fF) (control) to 212 ± 16 fF but was not significantly different. However, there was a dramatic reduction in the size of the slow burst component from 513 ± 42 fF (control) to 158 ± 3 fF (P < 0.01) in NB598-treated cells. Furthermore, we also observed a significant decrease in the sustained component. The amplitude of the sustained component from NB598-treated cells was 192 ± 27 vs. 365 ± 30 fF in control cells. These results suggest that inhibition of cholesterol synthesis with NB598 markedly impaired β-cell exocytosis from both fusion vesicle pool and refilling of releasable pool.


Figure 6
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  		FIG. 6. NB598 inhibits β-cell exocytosis independently of CaV channels. Exocytosis was elicited by flash photolysis and was monitored by whole-cell membrane capacitance measurement. Treatment with 10 µM NB598 powerfully reduced exocytosis from mouse pancreatic β-cells. A, Averaged [Ca2+]i and capacitance changes from control (black bars) and 10 µM NB598-treated (gray bars) cells. Arrow indicates the initial flash. Capacitance increase in the cells treated with 10 µM NB598 is significantly inhibited, compared with control cells, indicating an inhibition of NB598 on β-cell exocytosis independently from CaV channels. B, Mean amplitudes of the exocytotic burst and sustained components from control (gray bars) and 10 µM NB598-treated (white bars) cells, respectively. The Cm response was fitted with a triple exponential function, and the amplitudes of the two fast components were taken as the size of the fast burst and slow burst components, respectively. The slow component represents the sustained phase of secretion. NB598 inhibited both slow burst and sustained components of exocytosis, indicating the inhibition of cholesterol synthesis affects not only the release of docked and primed granules but also the refilling of granules. *, P < 0.01, compared with control.

 
Cholesterol inhibition causes redistribution of membrane raft-associated ion channels and SNARE proteins
To further determine the molecular mechanism of the impaired function of ion channels and insulin exocytosis, we examined whether the effects of NB598 were caused by changes in protein expression and/or membrane distribution of those corresponding ion channels and SNARE proteins. MIN6 β-cells were cultured for 48 h in the absence and presence of 10 µM NB598, and protein expression was determined by Western blot analysis. NB598 treatment did not have any significant effect on the protein expression of the channel proteins CaV1.2 or KV2.1 and the SNARE proteins syntaxin 1A, SNAP-25, and VAMP-2 (data not shown). Discontinuous sucrose gradient centrifugation of MIN6 cell lysate incubated with Triton X-100 at 4 C was used to detect protein targeting to membrane raft fractions (22, 23). We observed a redistribution of CaV1.2, KV2.1, syntaxin1A, SNAP-25, and VAMP-2 out of the cholesterol-rich membrane raft fraction located at the interface of 5 and 30% sucrose (Fig. 7Go).


Figure 7
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  		FIG. 7. Inhibition of cholesterol causes redistribution of ion channels and SNARE proteins from cholesterol-rich membrane raft microdomains. MIN6 β-cells were cultured for 48 h without and with 10 µM NB598. Localization of ion channels and SNARE proteins to membrane rafts was characterized by sucrose gradient ultracentrifugation and subsequent Western blot analysis. The interface between 5 and 30% sucrose gradient denotes the membrane raft fraction. CaV and KV channels and the SNARE proteins syntaxin 1A, SNAP-25, and VAMP-2 were found to be associated with cholesterol-rich membrane rafts in MIN6 cells. Inhibition of cholesterol synthesis with NB598 caused redistribution of those ion channels and SNARE proteins from the membrane rafts.

 

   Discussion
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 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Cholesterol is important in the organization of lipid bilayers, and the disorders of cholesterol can cause severe consequences (38). The majority of studies examining the role of cholesterol-rich membrane rafts have been based on acute cholesterol depletion with MβCD, a commonly used chemical that can effectively sequester cholesterol from PMs (39). However, because MβCD is membrane impermeable, treatment of cells with MβCD is expected to only deplete cholesterol and disrupt membrane rafts on the outer leaflet of the PM. Alternatively, it has also been suggested that cholesterol depletion with MβCD may affect intracellular cholesterol stores due to the rapid trafficking and efflux of cholesterol to the PM (33). However, the short incubation (30 min) used in our previous study may not have been long enough to cause an efficient depletion of intracellular cholesterol. The present study illustrates that chronic inhibition of cholesterol biosynthesis reduces cholesterol at the PMs as well as the intracellularly ER and insulin secretory granular membranes. Therefore, inhibition of endogenous cholesterol could lead to a more profound interference of cellular structure and function, resulting in alterations in β-cell stimulus-secretion coupling.

Cholesterol is synthesized in the ER and transported to the PMs via two pathways (40). The major pathway is via energy-dependent nonvesicular transport against a concentration gradient (41). About 20% of the de novo synthesized cholesterol is transported to PMs through vesicular transport via the Golgi apparatus (42). Although this vesicular pathway is not a major route of cholesterol export, some cholesterol exported from the ER may become confined to lipid rafts before reaching the PMs (42). Therefore, the passage of cholesterol through the ER-Golgi apparatus might play an important role in raft-dependent sorting of proteins (43, 44). Inhibition of endogenous cholesterol biosynthesis may cause an inappropriate protein sorting of membrane proteins such as ion channels and SNARE proteins in pancreatic β-cells. Therefore, the impaired function of ion channels and exocytosis caused by the inhibition of endogenous cholesterol biosynthesis by NB598 could be a result of the disruption of cholesterol in not only the PMs but also the ER and insulin secretory granules.

Roles of endogenous cholesterol in regulating KV channel function
Inhibition of squalene epoxidase significantly increased the inactivation of KV channels in β-cells but did not affect current density or the voltage dependence of channel activation. KV channel inactivation is described by a ball-and-chain model, a process by which the N-terminal cytoplasmic domain of KV{alpha} or KVβ subunit occludes the inner open channel pore (45). This cytoplasmic domain of KV channels was found to interact strongly with membrane lipids. KV channels, as well as other ion channels, are regulated by the lipid composition of PMs (46). Modulation of membrane lipids have been shown to result in rapid inactivation (A-type currents) of noninactivating KV channels and, conversely, endow noninactivating delayed rectifying properties to A-type KV currents (47). Inhibiting endogenous cholesterol biosynthesis could have profoundly altered the membrane lipid composition, channel-lipid interaction, and conformational changes of the channel proteins, all of which could have contributed to the observed enhancement of KV channel inactivation by NB598. The consequence of enhanced inactivation of KV channels on β-cell function remains to be further investigated. However, we speculate these effects do not contribute to the observed inhibition on insulin secretion because inhibition of KV channels are known to enhance insulin secretion (2, 35).

Role of endogenous cholesterol on β-cell exocytotic machinery
Chronic cholesterol synthesis inhibition markedly reduced CaV currents and insulin secretion. The link between the inhibition of cholesterol synthesis and the impairment in CaV channel function is not clear. No effect of NB598 on the protein expression of CaV channels was observed. The decreased CaV currents could be caused by a possible inappropriate membrane localization of the CaV channels out of membrane rafts or changes in the interactions of the different auxiliary CaV channel subunits. Second, reduced membrane cholesterol may lead to conformational changes of the channel protein due to disruption of the channel-lipid interaction as we suggest may be occurring with KV channels.

SNARE proteins constitute the core of exocytotic machinery in neuroendocrine cells and are critical for the release of neurotransmitter and hormone. Recent studies implicate that cholesterol-rich membrane rafts could play an important role in regulated exocytosis through compartmentalizing SNARE proteins at defined sites on the plasma membrane. We and others have previously shown that the SNARE proteins syntaxin1A, SNAP-25, and VAMP-2 are associated with cholesterol-rich membrane rafts in pancreatic β- and {alpha}-cells (22, 23, 48, 49). The t-SNAREs syntaxin 1 and SNAP-25 were found both to cluster in plasma membrane in β-cells and PC12 cells, and their integrity is dependent on membrane cholesterol (48, 49, 50, 51). Therefore, cholesterol being a major constituent of membrane rafts could play an essential role in regulating exocytosis through maintaining the function of secretory machinery. Single-cell membrane capacitance measurement indicated that NB958 treatment impaired exocytosis independently from the dysfunction of CaV channels. Cholesterol could regulate exocytosis through protein accumulation or exclusion in the membrane raft domains or reduce the energetic barrier for vesicular-plasma membrane lipid fusion (52).

We were initially surprised by our observations in this study because they are in contrast to our previous work (22), in which we observed acute membrane cholesterol depletion with MβCD did not affect CaV currents and enhanced insulin secretion. We concluded in the previous study that the enhanced insulin secretion could be partially mediated by the strong inhibition of the amplitude of the KV channels and effects on the exocytic machinery. However, it is not unexpected that acute and chronic manipulations in membrane cholesterol could elicit markedly different cellular changes. Inhibiting cholesterol synthesis would affect membrane cholesterol, as well as cholesterol-mediated processes. Cholesterol and cholesterol-interacting proteins (e.g. caveolin) regulate the trafficking and targeting of proteins, including ion channels, to membrane rafts (53, 54) and are important in coordinating the assembly of calcium channels with SNARE proteins in the exocytotic domains (55, 56). Given these possible explanations, we were even more astonished that acute cholesterol repletion restored much of the defects in CaV channel activity and insulin secretion. Therefore, further investigations are warranted into determining the precise mechanisms mediating the alterations in channel activity and exocytosis after chronic cholesterol depletion.

In summary, we have demonstrated a critical role for endogenous cholesterol in the normal function of pancreatic β-cells. Using NB598, a cholesterol biosynthesis inhibitor, we found there are two major roles that endogenous cholesterol may play in β-cell exocytosis. First, endogenous cholesterol maintains normal function of CaV channels. Second, cholesterol is critical in the mobilization and fusion of insulin granules with plasma membranes. Dysregulation of cellular cholesterol may cause impairment in β-cell function, a possible pathogenesis leading to the development of type 2 diabetes.


   Footnotes
 
This work was supported by grants and fellowships from the Canadian Institutes of Health Research (MOP 77638, to R.G.T.; MOP-69083, to H.Y.G. and R.G.T.), and equipment grants from James H. Cummings Foundation, J.P. Bickell Foundation, the Banting and Best Diabetes Centre (BBDC). F.X. was supported by an Ontario Graduate Scholarship, BBDC Studentship, and Canadian Diabetes Association Doctoral Studentship Research Award.

Current address for Y.C.: Central Laboratory, Guangzhou Children’s Hospital, Guangzhou 510120, Guangdong, China.

Current address for R.G.T.: Department of Biology, York University, Toronto, Ontario M3J 1P3, Canada.

Disclosure Statement: F.X., L.X., A.M., X.G., Y.C., H.Y.G., and R.G.T. have nothing to declare.

First Published Online July 3, 2008

Abbreviations: [Ca2+]i, Intracellular Ca2+ concentration; CaV, voltage-gated Ca2+ channel; Cm, membrane capacitance; ER, endoplasmic reticulum; FBS, fetal bovine serum; GFP, green fluorescence protein; KATP, ATP-sensitive K+; KRB, Krebs-Ringer bicarbonate; KV, voltage-gated K+; MBS, MES-buffered saline; MβCD, methyl-β-cyclodextrin; MES, 2-(N-morpholine) ethane sulfonic acid; MIP, mouse insulin promoter; PM, plasma membrane; R, ratio; SG, secretory granule; SNAP-25, synaptosomal-associated protein of 25 kDa; SNARE, soluble N-ethylmaleimide-sensitive factor attachment protein receptor; t-SNARE, SNARE proteins located on target PMs; VAMP, vesicle-associated membrane protein.

Received February 5, 2008.

Accepted for publication June 25, 2008.


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