Abstract
Dexamethasone (Dex), a glucocorticoid receptor agonist, is one of steroidal anti- inflammatory drugs and is approved for acute inflammation and autoimmune diseases. However, it is prohibited to treat psychotic patients with Dex, since Dex causes depression or mental illness as side effects. It remains unclear how Dex induces psychotic symptoms. Recently, autophagy-lysosome protein degradation system is reported to be disrupted in psychotic patients. Therefore, it is possible that impaired protein degradation is involved in the pathogenesis of Dex-induced psychotic symptoms. Removal of misfolded proteins by protein degradation systems is important for various neuronal functions and survival. There are two major protein degradation systems: the ubiquitin-proteasome system and the autophagy-lysosome system. The latter is further classified into macroautophagy (MA), microautophagy (mA) and chaperone-mediated autophagy (CMA). Recently, we established a novel method to separately assess mA and CMA activity using GAPDH, a mA and CMA substrate, fused with HaloTag (HT). In the present study, we investigated the effect of Dex on CMA/mA activities in AD293 cells and primary cultured neurons from rat cerebral cortex. Dex significantly decreased GAPDH-HT puncta in AD293 cells and primary cultured cortical neurons. This reduction was diminished by the pretreatment of RU-486, a glucocorticoid receptor antagonist. Moreover, cortisol, an endogenous glucocorticoid, also decreased GAPDH-HT puncta in AD293 cells. These findings suggest that the activation of glucocorticoid receptor negatively regulates CMA/mA activities. To elucidate which of CMA or mA is mainly affected by Dex, we examined punctate GAPDH-HT accumulation under siRNA-mediated knockdown of LAMP2A, a CMA-related protein, and TSG101, a mA-related protein. Dex significantly decreased GAPDH-HT puncta in the presence of both LAMP2A and TSG101 knockdown. These findings suggest that glucocorticoids reduces both CMA and mA activities via the activation of glucocorticoid receptors.
