This Article Full Text Full Text (PDF) Purchase Article View Shopping Cart Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Copyright Permission Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Medina, E. A. Articles by Goldkorn, T. Search for Related Content PubMed PubMed Citation Articles by Medina, E. A. Articles by Goldkorn, T.

Tumor Necrosis Factor- Decreases Akt Protein Levels in 3T3-L1 Adipocytes via the Caspase-Dependent Ubiquitination of Akt

Signal Transduction, Department of Internal Medicine (E.A.M., R.R.A., T.R., S.S.C., T.G.), and Department of Cell Biology and Human Anatomy (E.A.M., K.L.E.), University of California School of Medicine, Davis, California 95616

Address all correspondence and requests for reprints to: Dr. Tzipora Goldkorn, Signal Transduction, 6510 Genome and Bioscience Facility Building, 451 East Health Sciences Drive, Davis, California 95616. E-mail: ttgoldkorn{at}ucdavis.

TNF- is a mediator of insulin resistance in sepsis, obesity, and type 2 diabetes and is known to impair insulin signaling in adipocytes. Akt (protein kinase B) is a crucial signaling mediator for insulin. In the present study we examined the posttranslational mechanisms by which short-term (<6-h) exposure of 3T3-L1 adipocytes to TNF- decreases Akt levels. TNF- treatment both increased the ubiquitination of Akt and decreased its protein level. The decrease in protein was associated with the presence of an (immunoreactive) Akt fragment after TNF- treatment, indicative of Akt cleavage. The broad-spectrum caspase inhibitor t-butoxycarbonyl-Asp(O-Me)-fluoromethyl ketone markedly suppressed these effects of TNF-. The caspase-6 inhibitor Z-Val-Glu(OMe)-Ile-Asp(OMe)-CH₂F potently suppressed Akt ubiquitination, degradation, and fragment formation, whereas the proteasome inhibitor Z-Leu-Leu-Leu-CHO modestly attenuated the decline in Akt levels. Exposure to TNF- also enhanced the association of Akt with an E3 ligase activity. Adipocytes preexposed to TNF- for 5 h and then stimulated with insulin for 30 min exhibited decreased levels of Akt, phosphorylated Akt, as well as phosphorylated Mdm2, which is a known direct substrate of Akt, and glucose uptake. Caspase inhibition attenuated these inhibitory effects of TNF-. Collectively, our results suggest that TNF- induces the caspase-dependent degradation of Akt via the cleavage and ubiquitination of Akt, which results in its degradation through the 26S proteasome. Furthermore, the caspase- and proteasome-mediated degradation of Akt due to TNF- exposure leads to impaired Akt-dependent insulin signaling in adipocytes. These findings expand the mechanism by which TNF- impairs insulin signaling.

This article has been cited by other articles:

				H. Crossland, D. Constantin-Teodosiu, S. M. Gardiner, D. Constantin, and P. L. Greenhaff A potential role for Akt/FOXO signalling in both protein loss and the impairment of muscle carbohydrate oxidation during sepsis in rodent skeletal muscle J. Physiol., November 15, 2008; 586(22): 5589 - 5600. [Abstract] [Full Text] [PDF]

				C. Van Themsche, L. Lafontaine, and E. Asselin X-Linked Inhibitor of Apoptosis Protein Levels and Protein Kinase C Activity Regulate the Sensitivity of Human Endometrial Carcinoma Cells to Tumor Necrosis Factor{alpha}-Induced Apoptosis Endocrinology, August 1, 2008; 149(8): 3789 - 3798. [Abstract] [Full Text] [PDF]

				A. F. Fernandes, J. Zhou, X. Zhang, Q. Bian, J. Sparrow, A. Taylor, P. Pereira, and F. Shang Oxidative Inactivation of the Proteasome in Retinal Pigment Epithelial Cells: A POTENTIAL LINK BETWEEN OXIDATIVE STRESS AND UP-REGULATION OF INTERLEUKIN-8 J. Biol. Chem., July 25, 2008; 283(30): 20745 - 20753. [Abstract] [Full Text] [PDF]

				K. K. Mann, M. Colombo, and W. H. Miller Jr. Arsenic trioxide decreases AKT protein in a caspase-dependent manner Mol. Cancer Ther., June 1, 2008; 7(6): 1680 - 1687. [Abstract] [Full Text] [PDF]

				S. J. Crozier, M. D. Sans, C. H. Lang, L. G. D'Alecy, S. A. Ernst, and J. A. Williams CCK-induced pancreatic growth is not limited by mitogenic capacity in mice Am J Physiol Gastrointest Liver Physiol, May 1, 2008; 294(5): G1148 - G1157. [Abstract] [Full Text] [PDF]

				N. Alamdari, D. Constantin-Teodosiu, A. J. Murton, S. M. Gardiner, T. Bennett, R. Layfield, and P. L. Greenhaff Temporal changes in the involvement of pyruvate dehydrogenase complex in muscle lactate accumulation during lipopolysaccharide infusion in rats J. Physiol., March 15, 2008; 586(6): 1767 - 1775. [Abstract] [Full Text] [PDF]

				Z. E. Floyd, B. M. Segura, F. He, and J. M. Stephens Degradation of STAT5 proteins in 3T3-L1 adipocytes is induced by TNF-{alpha} and cycloheximide in a manner independent of STAT5A activation Am J Physiol Endocrinol Metab, February 1, 2007; 292(2): E461 - E468. [Abstract] [Full Text] [PDF]

				J. Ramos, M. Sirisawad, R. Miller, and L. Naumovski Motexafin gadolinium modulates levels of phosphorylated Akt and synergizes with inhibitors of Akt phosphorylation Mol. Cancer Ther., May 1, 2006; 5(5): 1176 - 1182. [Abstract] [Full Text] [PDF]