Troglitazone reduces heat shock protein 70 content in primary rat hepatocytes by a ubiquitin proteasome independent mechanism

• Published in: Pharmacological research Vol. 48; no. 1; pp. 119 - 126
• Main Authors: Davies, Gerald F., Roesler, William J., Ovsenek, Nick, Bharadwaj, Lalita A.
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier Ltd 01-07-2003
• Subjects: Acetylcysteine - analogs & derivatives; Acetylcysteine - pharmacology; Animals; Blotting, Northern; Blotting, Western; Cells, Cultured; Chemical and Drug Induced Liver Injury; Chromans - adverse effects; Chromans - pharmacology; Cysteine Endopeptidases - metabolism; Dose-Response Relationship, Drug; Gene Expression - drug effects; Heat shock protein 70; Heat-Shock Proteins - drug effects; Heat-Shock Proteins - genetics; Heat-Shock Proteins - metabolism; Hepatocytes - drug effects; Hepatocytes - enzymology; Hepatocytes - metabolism; Histones - drug effects; Histones - metabolism; Liver Diseases - metabolism; Male; Mice; Mice, Inbred C57BL; Multienzyme Complexes - metabolism; Peroxisome proliferator activated receptor (γ-isoform); Primary rat hepatocytes; Proteasome Endopeptidase Complex; Rats; Rats, Sprague-Dawley; RNA, Messenger - metabolism; Thiazolidinediones - adverse effects; Thiazolidinediones - pharmacology; Troglitazone; Ubiquitin - metabolism; Ubiquitin–proteasome system; Heat shock protein 70; Peroxisome proliferator activated receptor (γ-isoform); Troglitazone; Primary rat hepatocytes; Ubiquitin–proteasome system
• ISSN: 1043-6618; 1096-1186
• DOI: 10.1016/S1043-6618(03)00082-3

Troglitazone (TRG) is an antidiabetic agent that increases the insulin sensitivity of target tissues in non-insulin-dependent diabetes mellitus. Therapy with troglitazone has been associated with severe hepatic injury in a small percentage of patients and the mechanism of TRG-induced hepatotoxicity remains unclear. A family of highly conserved stress proteins identified as heat shock proteins (Hsps), are well-known to protect cells against a wide variety of toxic conditions such as extreme temperature changes, oxidative stress and toxic drugs. The stress-inducible Hsp 70 protein is one of the best-known endogenous factors protecting cells from injury under various stress conditions. Here we examined the effects of TRG on Hsp 70 mRNA and protein expression in primary cultures of rat hepatocytes. We also investigated the effects of TRG in an in vivo model by examining Hsp 70 protein levels in livers prepared from C57 mice fed a 0.2% dietary admixture of TRG. Levels of Hsp 70 mRNA increased in a concentration-dependent manner in rat hepatocytes treated for 8
h with increasing concentrations of TRG. However, Hsp 70 protein levels decreased significantly in cells treated with increasing concentrations of TRG. C57 mice fed a 0.2% admixture of TRG for 10 days, also demonstrated decreased liver Hsp 70 protein levels. To investigate whether TRG decreased Hsp 70 protein levels by activating the ubiquitin–proteasome pathway, cells were pretreated with 10
μM lactacystin, a potent and specific inhibitor of this pathway. Lactacystin pretreatment failed to prevent TRG-induced decrease in Hsp 70 protein. The data suggests that TRG-induced effects may be mediated through another system of regulated proteolysis or may involve a post-transcriptional regulator mechanism. The mechanism of TRG-induced hepatotoxicity remains unclear, however, the effects induced by TRG on Hsp 70 may, in part, play a role.