Heat shock protein 90, a potential biomarker for type I diabetes: mechanisms of release from pancreatic beta cells

Abstract

Heat shock protein (HSP) 90 is a molecular chaperone that regulates diverse cellular processes by facilitating activities of various protein clients. Recent studies have shown serum levels of the alpha cytoplasmic HSP90 isoform are elevated in newly diagnosed type I diabetic patients, thus distinguishing this protein as a potential biomarker for pre-clinical type I diabetes mellitus (TIDM). This phase of disease is known to be associated with various forms of beta cell stress, including endoplasmic reticulum stress, insulitis, and hyperglycemia. Therefore, to test the hypothesis that HSP90 is released by these cells in response to stress, human pancreatic beta cells were subjected to various forms of stress in vitro. Beta cells released HSP90 in response to stimulation with a combination of cytokines that included IL-1β, TNF-α, and IFN-γ, as well as an agonist of toll-like receptor 3. HSP90 release was not found to result from cellular increases in HSP90AA1 gene or HSP90 protein expression levels. Rather, cell stress and ensuing cytotoxicity mediated by c-Jun N-terminal kinase (JNK) appeared to play a role in HSP90 release. Beta cell HSP90 release was attenuated by pre-treatment with tauroursodeoxycholic acid (TUDCA), which has been shown previously to protect beta cells against JNK-mediated, cytokine-induced apoptosis. Experiments here confirmed TUDCA reduced beta cell JNK phosphorylation in response to cytokine stress. Furthermore pharmacological inhibition and siRNA-mediated knockdown of JNK in beta cells also attenuated HSP90 release in response to cytokine stress. Pharmacological inhibition of HSP90 chaperone function exacerbated islet cell stress during the development of TIDM in vivo; however, it did not affect the overall incidence of disease. Together, these data suggest extracellular HSP90 could serve as a biomarker for preclinical TIDM. This knowledge may be clinically relevant in optimizing treatments aimed at restoring beta cell mass. The goal of such treatments would be to halt the progression of at-risk patients to insulin dependence and lifelong TIDM.