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Endocrinology Vol. 141, No. 8 2938-2944
Copyright © 2000 by The Endocrine Society

ARTICLES

Endocrine Disruptive Effects of Polychlorinated Aromatic Hydrocarbons on Intestinal Cholecystokinin in Rats1

Heung-Man Lee, Qin He, Ella W. Englander and George H. Greeley, Jr.

Department of Surgery, University of Texas Medical Branch, Galveston, Texas 77555

Address all correspondence and requests for reprints to: George H. Greeley, Jr., Ph.D., Department of Surgery, University of Texas Medical Branch, 301 University Boulevard, Galveston, Texas 77555-0725. E-mail: ggreeley{at}utmb.edu


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
The ubiquitous and persistent nature of polychlorinated aromatic hydrocarbons (PCAHs) in our environment and the risk of exposure to PCAHs have provoked concern over their potential toxicity. In humans, exposure to PCAHs is aimed chiefly at epithelial cells residing in the intestinal mucosa, because oral intake of contaminated food is a major source of PCAHs. The purpose of this study, therefore, was to examine the effects of chronic exposure to various PCAHs [i.e. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,4,7,8-pentachlorodibenzofuran (PeCDF), 3,3',4,4',5-pentachlorobiphenyl (PCB-126), and 2,2'4,4'5,5'-hexachlorobiphenyl (PCB-153)], given alone or as mixtures, on intestinal cholecystokinin (CCK) peptide and messenger RNA levels. We show that chronic PCAH treatment significantly lowers intestinal levels of stored CCK peptide. Intestinal CCK messenger RNA levels are not affected. In addition, 3,3',4,4',5pentachlorobiphenyl treatment increased intestinal insulin-like growth factor-binding protein-3 levels in a dose-related manner. Acute 2,3,7,8-tetrachlorodibenzo-p-dioxin treatment of intestinal CCK cells lowered levels of CCK-processing enzymes (i.e. prohormone convertase-1 and -2). Together, these data indicate that PCAHs may decrease intestinal levels of stored CCK peptide by affecting the intestinal insulin-like growth factor system and CCK processing.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
POLYCHLORINATED AROMATIC hydrocarbons (PCAHs) are a group of widespread and persistent environmental contaminants produced by bleaching of paper pulp, inappropriate incineration of trash, and synthesis of chlorinated phenoxy herbicides (1, 2, 3). In humans, ingestion of contaminated food is a major source of PCAH exposure (4). Therefore, potential targets in the intestinal lumen for the toxic action of PCAHs are the luminally exposed enteroendocrine cells that produce and secrete intestinal peptide hormones (5).

In the proximal gastrointestinal tract, several peptide hormones are expressed that exert a variety of metabolic effects on the gastrointestinal system and brain (5). One of the major intestinal hormones is cholecystokinin (CCK). CCK is expressed in enteroendocrine cells, called I cells, which are distributed throughout the proximal intestine epithelium. Systemic CCK participates in the regulation of gallbladder contraction, pancreatic secretion, stomach emptying, and intestinal motility (6). CCK can also inhibit food intake (7). Our laboratory and others have shown that the insulin-like growth factor (IGF) system may play a major role in the regulation of intestinal hormone gene expression, including CCK (8, 9). Treatment of mice in vivo or of enteroendocrine cells in vitro with IGF-I can up-regulate gut peptide messenger RNA (mRNA) and peptide levels. In vivo, IGF-I activity is modulated in part by IGF-binding proteins (IGFBPs) (10, 11). IGFBPs can bind and inhibit the stimulatory actions of IGF-I.

Gastrointestinal peptide hormones are synthesized initially as biologically inactive proforms (12). Proforms are then processed into mature, biologically active peptides by processing enzymes called prohormone convertases (PCs). Intestinal pro-CCK is processed to mature CCK by PC-1 and PC-2 (13, 14, 15, 16).

PCAHs are exceptionally potent anorexic and leptotenic agents (1, 2, 17, 18, 19, 20, 21, 22). Earlier studies have shown that PCAHs cause a dose-dependent reduction in food intake that is paralleled by a reduced body weight gain and increased mortality in rats and mink. Very little information is available on the effects of PCAHs on the gastrointestinal system. The purpose of this study, therefore, was to investigate the effects of chronic exposure to various PCAHs [i.e. 3,3',4,4',5-pentachlorobiphenyl (PCB-126), 2,2',4,4'5,5'-hexachlorobiphenyl (PCB-153), 2,3,4,7,8-pentachlorodibenzofuran (PeCDF), 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)] given alone or as mixtures on intestinal CCK peptide and mRNA levels. The effects of PCB-126 on intestinal IGFBP-3 protein levels were also examined. In addition, the acute effects of TCDD on PC-1 and PC-2 protein levels in a CCK-producing cell line were examined.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Animals and tissue preparation
Animals were treated with the various PCAHs and tissues harvested by Battelle Laboratories (Columbus, OH) as part of a contract with the NIEHS (Research Triangle Park, NC). The following PCAHs were tested: 1) PCB-126 (cas no. 57465–28-8), 2) 2,2',4,4',5,5'-hexachlorobiphenyl (PCB-153; cas no. 35065–27-1), 3) PeCDF (cas no. 57117–31-4), and 4) TCDD (cas no. 74601–6). Adult female Sprague-Dawley rats were given PCAHs either alone or in combination at the following doses; PCB-126 (0, 550, 1000 ng/kg BW/day; PeCDF (0, 92, 200 ng/kg BW/day; and TCDD alone (0, 46, 1000 ng/kg BW/day); a mixture of PCB-126 and PCB-153 (0, 0; 300 ng/kg BW/day, 300 µg/kg BW/day; 300 ng/kg BW/day, 300 µg/kg BW/day; 1000 ng/kg BW/day, 1000 µg/kg BW/day) or a mixture of PCB-126, PeCDF and TCDD (0, 0, 73, 15, 7, 153, 30, 15; 333, 66, 33 ng/kg BW/day). The PCAHs were obtained from the National Cancer Institute Repository (TCDD), Cambridge Isotope Laboratory (PeCDF) (Andover, MA), AccuStandard (New Haven, CT) (PCB-126), and Radian International (PCB-153) (Austin, TX). Their purities ranged from 98–99.5%. All PCAHs were given orally by gastric lavage in corn oil, 5 days/week for 13 weeks. Rats were killed in the ad libitum fed condition. Upon death, the intestine was removed immediately; a piece of proximal intestine was placed in a vial containing RNAlater (Ambion, Inc., Austin, TX) and stored at 4 C for 2–3 days maximally for subsequent shipment on wet ice to G. Greeley’s laboratory. A second piece of full thickness duodenum was frozen in liquid nitrogen and stored at -80 C until it was shipped on dry ice. Upon arrival to our laboratory, RNAlater specimens were stored at 4 C (1–2 days) until total cellular RNA was extracted. All other specimens were stored at -80 C until they were extracted for gut peptide hormones, chromogranin A (CGA) protein, or total cellular protein. In a majority of the cases, treatment groups consisted of 8–10 rats/group.

Gut peptide and CGA: extractions and RIAs
Duodenal pieces (~0.5 mg) were homogenized in water (1:10, wt/vol) and then immediately boiled for 10 min. Boiled homogenates were clarified by centrifugation, and supernatants were harvested. The pellets were resuspended in 2 N acetic acid and left overnight at 4 C. Acidified homogenates were centrifuged and combined with the aqueous supernatants. Supernatants were lyophilized and resuspended in appropriate assay buffers. Duodenal CCK levels were measured using a CCK antibody that detects all forms of biologically active CCK; it is specific for the C-terminal amide (23). Duodenal secretin levels were measured by RIA using a secretin antiserum generated against porcine secretin (24). Duodenal CGA levels were measured using an antiserum generated against a synthetic fragment of rat CGA [CGA-(351–381)] (25).

Northern blotting analyses
RNAlater samples were homogenized in 4 M guanidine, and total cellular RNA was prepared by ultracentrifugation in a CsCl gradient as described previously (8, 26). Total cellular RNA (30 µg) samples were denatured twice at 65 C for 5 min (i.e. separated by vortexing) in a formamide/formaldehyde-containing loading buffer before separation by gel electrophoresis. Electrophoresis was performed on a 1.0% denaturing agarose gel containing 1 x MOPS (20 mM 3-[N-morpholino]propanesulfonic acid, pH 7.0, containing 8 mM sodium acetate and 1 mM EDTA, pH 8.0, and 5% formaldehyde). Transcript sizes were determined by comparison with migration of a RNA ladder (Ambion, Inc., Millennium marker, 0.5–9 kb). The total cellular RNA or polyadenylated mRNA specimens and ladder were blotted onto nylon filters (Ambion, Inc., Brightstar nylon membranes) and cross-linked by means of a UV cross-linker.

Membranes were then prehybridized and hybridized with Northern Max hybridization solution (Ambion, Inc.) according to the instructions supplied with the kits. Random primed complementary DNA (cDNA) probes were synthesized by the Strip-EZ DNA kit (Ambion, Inc.). After prehybridization for 16–24 h, membranes were hybridized with an [{alpha}-32P]deoxy-ATP-radiolabeled cDNA probe (5–6 x 106 cpm/ml) overnight at 45 C. Membranes were washed three times for 5–10 min each time at room temperature in 2 x SSC (standard saline citrate)-0.1% SDS, followed by three 20-min washes in 0.2 x SSC-0.1% SDS at 45 C. Membranes were then exposed to Kodak Biomax MR film (Eastman Kodak Co., Rochester, NY) at -80 C between intensifying screens. Signal intensity on x-ray films was quantified by densitometric analysis (LYNX Molecular Biology Workstation, Image Recognition Systems, an Applied Imaging company, Cheshire, UK).

To ensure the accuracy of the changes in mRNA abundance, and equal loading and transfer of RNA, mRNA levels were normalized to 18S rat ribosomal mRNA or GAPDH levels. The folowing probes were used: a rat CCK cDNA from J. Dixon (27) and a rat 18S ribosomal RNA probe from American Type Culture Collection (Manassas, VA).

Western blotting analyses
STC-1 cell and intestinal protein extracts were prepared by homogenizing cells or intestinal pieces (<100 mg) in RIPA buffer [50 mM Tris-HCl (pH 7.5) with 150 mM NaCl, 0.1% SDS, 1.0% Nonidet P-40, and 0.5% sodium deoxycholate] supplemented with a freshly prepared protease inhibitor cocktail (Sigma, St. Louis, MO). Supernatant protein concentrations were measured using the Bradford reagent (Bio-Rad Laboratories, Inc., Hercules, CA). Aliquots of intestinal extracts [30 µg protein diluted with Laemmli loading buffer (Bio-Rad Laboratories, Inc.)] were boiled for 5 min and then electrophoresed on 10% SDS-polyacrylamide gels, transferred onto a polyvinylidene difluoride membrane, and probed with appropriate antibodies. An antibody specific for rat IGFBP-3 [gift from S. Shimasaki (28)], rat PC-1 (gift from I. Lindberg, Louisiana State University Medical Center, New Orleans, LA), and PC-2 (Alexis Corp., San Diego, CA) were used. Signals were detected using the enhanced chemiluminescence system.

In vitro cell culture experiments
A murine intestinal enteroendocrine cell line (STC-1 cells) (29) was treated with TCDD (50 nM; AccuSTANDARD, New Haven, CT) for 24–48 h. Cells were harvested and extracted for total protein. PC-1 and PC-2 protein levels were examined by Western blotting analyses.

Statistics
Results are shown as the mean ± SE. Data were analyzed by a one- or two-way ANOVA followed by the Newman-Keuls test where pertinent. P < 0.05 was considered significant.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
PCAH treatment lowers duodenal CCK peptide levels: lack of effect of PCAHs on duodenal secretin and CGA levels
The purpose of this experiment was to examine the effects of various PCAHs, given chronically, on duodenal CCK mRNA and peptide levels in rats. Duodenal stores of CCK peptide were decreased by all PCAHs tested (i.e. PCB-126; PeCDF; TCDD; a mixture of PCB-126, PeCDF, and TCDD; and a mixture of PCB-126 and PCB-153; Fig. 1Go). In some cases, duodenal CCK peptide levels were decreased by the PCAH in a dose-response manner. In rats given PCB-126, at 550 and 1000 ng/kg BW·day, duodenal CCK peptide levels were 72 ± 7% and 44 ± 8% of control duodenal CCK levels, respectively. In rats given PeCDF at 200 ng/kg BW·day, duodenal CCK peptide levels were 58 ± 6% of control duodenal CCK levels. Duodenal CCK peptide levels were 67 ± 7% and 54 ± 7% of control duodenal CCK levels in rats given TCDD at 46 and 100 ng/kg BW·day, respectively. Duodenal CCK peptide levels also decreased in a dose-response manner in rats given a mixture of PCB-126, PeCDF, and TCDD. Duodenal CCK levels were 67 ± 7%, 54 ± 7%, and 36 ± 5% of control duodenal CCK levels in rats given graded doses of a mixture of PCB-126, PeCDF, and TCDD. In rats given graded doses of PCB-126 (0, 300, 300, and 1000 ng/kg BW·day) plus PCB-153 (0, 300, 3000, and 1000 µg/kg BW·day) as a mixture, duodenal CCK levels were 60 ± 12%, 70 ± 10%, and 33 ± 10% of control duodenal CCK levels.



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  		Figure 1. PCAH treatment decreases duodenal levels of stored CCK peptide. Intestinal levels of stored CCK peptide are shown as a percentage of control intestinal levels of stored CCK peptide. In control rats, the intestinal level of stored CCK peptide in duodenal extracts is 22 ± 2.6 ng/g duodenum (n = 40 rats). *, P < 0.05 vs. controls; **, P = 0.05 vs. controls and lower doses of respective PCAH. Intestinal CCK peptide levels were measured by means of a CCK RIA that uses an antiserum that specifically detects mature CCK peptide.

 
Duodenal CCK mRNA levels were unaffected by PCAH treatment. In rats given graded doses of PCB-126, PeCDF, or TCDD alone; a mixture of PCB-126 plus PCB-153; or a mixture of PCB-126, PeCDF, and TCDD, Northern blotting analysis showed that duodenal CCK expression was not affected significantly (P > 0.05 vs. controls). For example, in rats given graded doses of TCDD alone or a mixture of graded doses of PCB-126 plus PCB-153, duodenal CCK mRNA levels were not changed (Fig. 2Go).



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  		Figure 2. Northern blotting analysis of duodenal CCK mRNA levels in rats given TCDD alone (left panel) or a mixture of PCB-126 plus PCB-153 (right panel). Duodenal CCK mRNA levels are shown as a Northern blot and as a bar graph. For the Northern blot analysis, 30 µg total cellular RNA were electrophoresed in a formaldehyde-denaturing gel, transferred onto a nylon membrane, and hybridized with a 32P-labeled cDNA probe. Membranes are also hybridized with a rat 18S ribosomal RNA or GAPDH probe to correct for loading and transfer differences. Only three lanes of each group are shown, although there were 8–10 rats/group. Duodenal CCK mRNA levels of all PCAH treatment groups were analyzed; PCAHs did not affect intestinal CCK expression significantly. The bar graph shows the ratio of CCK mRNA over 18S or GAPDH mRNA densitometric readings for Northern blots. The mean ± SEM are shown for 8–10 rats/group.

 
To test the specificity of the inhibitory effect of PCAHs on stores of CCK peptide, the levels of another duodenal peptide hormone, secretin, were examined. Additionally, the duodenal levels of CGA, a marker for all enteroendocrine cells, were measured to assess whether PCAH treatment affects the density of duodenal enteroendocrine cells. Duodenal levels of secretin were unaffected by chronic TCDD treatment (Fig. 3Go). Although duodenal CGA protein levels appear higher in PCB-126-treated rats, duodenal CGA levels are not affected significantly (P > 0.05) by PCB-126 treatment (Fig. 3Go).



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  		Figure 3. Top, Duodenal levels of stored secretin peptide are unchanged by TCDD treatment in rats. Bottom, Duodenal levels of stored CGA protein are not altered significantly (P > 0.05) by PCB-126 treatment in rats. Although CGA levels appear higher in PCB-126-treated rats, the levels are not statistically significantly higher. Duodenal secretin and CGA levels for other PCAH treatment groups are not shown. n = 6–7 rats.

 
PCB-126 treatment increases duodenal IGFBP-3 protein levels
Western blotting analysis of duodenal protein extracts of rats treated with PCB-126 showed that treatment with graded doses of PCB-126 resulted in a significant doserelated increase in duodenal IGFBP-3 protein levels (Fig. 4Go). Compared with control IGFBP-3 protein levels, PCB-126 at 100, 300, 550, and 1000 ng/kg BW·day increased duodenal IGFBP-3 protein levels by approximately 500%, 720%, 1000%, and 1300%, respectively.



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  		Figure 4. Top, Western blotting analysis of intestinal IGFBP-3 levels in rats given PCB-126. Treatment of rats with graded doses of PCB-126 resulted in a significant dose-related increase in duodenal IGFBP-3 levels. Bottom, Densitometric analysis of Western blotting findings for IGFBP-3. *, P < 0.05 vs. controls; **, P < 0.05 vs. controls or lower doses.

 
TCDD treatment decreases PC-1 and PC-2 protein levels
We found that PCAH treatment of rats lowered duodenal levels of stored, mature CCK peptide. A possible explanation for the reduced stores of mature CCK is that processing of pro-CCK to mature CCK is decreased. Pro-CCK is processed to mature CCK by PC-1 and PC-2. The purpose of this experiment, therefore, was to examine the effects of TCDD on protein levels of PC-1 and PC-2 in a murine intestinal CCK-expressing line (STC-1). This cell line also expresses PC-1 and PC-2. Treatment of STC-1 cells with TCDD at 50 nM for 24–48 h decreased cellular levels of PC-1 and PC-2 protein (Fig. 5Go).



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  		Figure 5. Western blotting analyses show that TCDD treatment of STC-1 cells decreased PC-1 and PC-2 protein levels. These data suggest that PCAH treatment lowers intestinal levels of CCK by decreasing the amounts of enzymes involved in the posttranslational processing of pro-CCK to mature CCK.

 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
The primary finding of the present study is that chronic exposure to several different types of commonly encountered environmental PCAHs (i.e. PCBs, dioxin, and PeCDF) can affect the intestinal endocrine system. More specifically, our in vivo findings demonstrate that chronic ingestion of PCB-126, PeCDF, or TCDD alone; a mixture of these three chemicals; or a mixture of PCB-126 and PCB-153 decreases intestinal stores of CCK peptide in a specific manner in rats. In addition, our results demonstrate that chronic ingestion of a polychlorinated biphenyl, PCB-126, also increases duodenal IGFBP-3 protein levels in a dose-response manner. Our in vitro experiments show that acute TCDD treatment of a CCK-producing cell line can down-regulate protein levels of two prohormone convertases, PC-1 and PC-2, which are involved in processing pro-CCK to mature, biologically active CCK.

Few previous studies have addressed the potential adverse effects of PCAH exposure on the gastrointestinal system. TCDD is an exceptionally potent anorexic agent. The most significant effect described in animals to date for PCAH exposure that is possibly linked with gastrointestinal function is the wasting syndrome. Numerous studies have shown that PCAH exposure causes a dose-dependent reduction in food intake that is paralleled by reduced body weight and mortality in rats and mink (1, 2, 17, 18, 19, 20, 21, 22). A single, sublethal dose of dioxin given to rats inhibits body weight gain permanently, whereas a lethal dose is preceded by hypophagia and body weight loss of up to 50% (17). A recent report also indicates that chickens fed a diet of fat contaminated with excessive amounts of PCAHs experienced a reduced weight gain (30). Evidence associating PCAHs with a toxic action on the gastrointestinal tract was demonstrated by lesions in the stomach and proximal intestine in the mink study (21). Although not examined, these lesions may be directly linked to the reduced food intake and inadequate nutrient-induced secretion of protective gut peptide hormones. Gastrointestinal hormones such as gastrin and peptide YY have a proliferative action on the intestinal epithelium (26, 31). Apoptotic cell death was also reported for digestive tissues in a submammalian vertebrate, medaka fry, that was exposed to TCDD during embryonic development (32). Furthermore, recent studies show that growth retardation and depressed weight gain occur in children exposed to mixtures of PCAHs (4, 33, 34, 35, 36, 37, 38, 39, 40). The exact mechanisms underlying PCAHinduced appetite suppression and depressed weight gain are not known, but they most likely involve systems that regulate food intake, nutrient digestion and absorption, metabolism, and growth. The paucity of studies on the gastrointestinal effects of PCAHs seems startling in view of the fact that oral ingestion of PCAHs is a primary route of exposure in humans (4), and in neonates, mothers’ milk can contain exceptionally high levels of PCAHs (4, 33, 34, 35, 36, 37, 38, 40, 41).

Interestingly, our studies demonstrate that a polychlorinated biphenyl, PCB-126, increases intestinal IGFBP-3 protein levels in a dose-response manner. Although not examined in the present study, the significance of this finding may be that the increased intestinal levels of IGFBP-3 can decrease the local amounts of bioactive IGF-I. The cellular actions of IGF-I are regulated in part by circulating and tissue levels of IGFBPs (10, 11). IGFBPs, by binding IGF-I, can decrease IGF-I activity. We have reported that IGF-I can exert a potent up-regulatory influence on intestinal hormone mRNA and peptide levels (8). Our laboratory and others have shown that IGF-I increases mRNA and peptide levels of intestinal CCK, neurotensin, and peptide YY in vivo and in vitro (8, 9). Furthermore, IGF-I may play a major role in the regulation of the intestinal absorptive surface, as the intestinal epithelium is the most sensitive target of IGF-I (42). The increased intestinal levels of IGFBP-3, therefore, may be an underlying mechanism in part for the PCAH-induced reduction in duodenal stores of CCK peptide. Alternatively, the increased duodenal levels of IGFBP-3 during PCB treatment may be a mechanism for sequestering IGF-I and preventing its degradation, in an attempt to restore intestinal CCK levels. As IGFBP and IGF-I expression is sensitive to nutritional status (43), another plausible explanation for the elevation in intestinal IGFBP-3 protein levels is that it represents a specific adaptation to the PCAH-induced reduction in food intake. It is designed to reduce the stimulatory actions of IGF-I on the intestinal epithelium and CCK peptide levels. A reduction in intestinal stores of CCK peptide may reduce food-induced CCK release and the acute inhibitory effects of intestinal CCK on food intake. Such effects seem appropriate in view of the already reduced food intake and weight gain in PCAH-treated animals.

We also found that another PCB, PCB-153, increases duodenal IGFBP-3 protein levels, but less potently compared with PCB-126 (data not shown), and that colonic IGFBP-3 protein levels are unaffected by PCB-126 (data not shown), suggesting that the effects of ingested PCAHs on IGFBP-3 are specific for the proximal intestine or simply that the colon is less exposed to ingested PCAHs compared with the proximal intestine.

Our in vitro findings indicate that acute TCDD treatment of a CCK-producing cell line (STC-1 cells) decreases PC-1 and PC-2 protein levels. PC-1 and PC-2 are involved in processing pro-CCK to biologically active, mature CCK (13, 14, 15, 16). The TCDD-induced reduction in PC protein levels in STC-1 cells may explain in part at least the in vivo reduction in duodenal CCK peptide stores. This finding suggests that one of the actions of PCAHs is to decrease duodenal CCK stores by exerting a posttranslational inhibitory action on its processing enzymes. We would expect to detect a build-up of pro-CCK-processing intermediates; however, we have not measured these forms. It is interesting that we did not detect an elevation in intestinal CCK mRNA levels in view of the reduced intestinal CCK peptide stores. It is reasonable to suggest that PCAHs might suppress intestinal CCK mRNA levels. We have shown previously that stomach gastrin mRNA levels increase when processing of progastrin is blocked (44).

Although speculative, the decrease in duodenal stores of CCK peptide during PCAH treatment may be due in part to an exaggerated secretion of CCK from the intestine. We have found in a separate study that diversion of bile and pancreatic exocrine secretions surgical away from the intestinal lumen in rats causes exaggerated CCK secretion and depletes intestinal CCK peptide stores substantially (>90%; Greeley et al., unpublished findings). Although we did not investigate whether circulating CCK levels are increased in PCAH-treated rats, we found in a preliminary study that acute TCDD treatment can enhance basal secretion of CCK from a CCK-producing cell line (Greeley et al., unpublished findings). An exaggerated secretion of CCK would agree with the reduced food intake of PCAH-treated rats. Intestinal CCK has an inhibitory action on food intake in humans and laboratory animals (7).

It should be mentioned that the dramatic decrease in intestinal stores of CCK peptide might be explained by a reduction in food intake caused by PCAH treatment. It is unlikely, however, that the diminished intestinal stores of CCK peptide are due to PCAH-induced reductions in food intake, as only starvation or complete fasting depletes intestinal stores of CCK peptide and mRNA significantly (45). PCAHS would be expected to reduce food intake maximally by 20% in this study (22).

In conclusion, this study shows that chronic PCAH treatment of rats lowers intestinal stores of CCK peptide. This reduction in intestinal CCK peptide may be due to a PCAH-induced down-regulation of IGF-I action and CCK-processing enzymes that serve to replenish depleted levels of CCK.


   Footnotes
 
1 This work was supported by NIH Grants ES-09450 and ES-10160. Back

Received March 9, 2000.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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