Functional modulation of nuclear steroid receptors by tauroursodeoxycholic acid reduces amyloid beta-peptide-induced apoptosis.

Tauroursodeoxycholic acid (TUDCA) prevents amyloid beta-peptide (Abeta)-induced neuronal apoptosis, by modulating both classical mitochondrial pathways and specific upstream targets. In addition, activation of nuclear steroid receptors (NSRs), such as the mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR) differentially regulates apoptosis in the brain. In this study we investigated whether TUDCA, a cholesterol-derived endogenous molecule, requires NSRs for inhibiting Abeta-induced apoptosis in primary neurons. Our results confirmed that TUDCA significantly reduced Abeta-induced apoptosis; in addition, the fluorescently labeled bile acid molecule was detected diffusely in both cytoplasm and nucleus of rat cortical neurons. Interestingly, experiments using small interfering RNAs (siRNAs) revealed that, in contrast to GR siRNA, MR siRNA abolished the antiapoptotic effect of TUDCA. Abeta incubation reduced MR nuclear translocation while increasing nuclear GR levels. Notably, pretreatment with TUDCA markedly altered Abeta-induced changes in NSRs, including MR dissociation from its cytosolic chaperone, heat shock protein 90, and subsequent translocation to the nucleus. Furthermore, when a carboxy terminus-deleted form of MR was used, nuclear trafficking of both MR and the bile acid was abrogated, suggesting that they translocate to the nucleus as a steroid-receptor complex. Transfection experiments with wild-type or mutant MR confirmed that this interaction was required for TUDCA protection against Abeta-induced apoptosis. Finally, in cotransfection experiments with NSR response element reporter and overexpression constructs, pretreatment with TUDCA significantly modulated Abeta-induced changes in MR and GR transactivation. In conclusion, these results provide novel insights into the specific cellular mechanism of TUDCA antiapoptotic function against Abeta-induced apoptosis and suggest targets for potential therapeutic intervention.

Nuclear translocation of UDCA by the glucocorticoid receptor is required to reduce TGF-beta1-induced apoptosis in rat hepatocytes.

Ursodeoxycholic acid (UDCA) inhibits classical mitochondrial pathways of apoptosis by either directly stabilizing mitochondrial membranes or modulating specific upstream targets. Furthermore, UDCA regulates apoptosis-related genes from transforming growth factor beta1 (TGF-beta1)-induced hepatocyte apoptosis by a nuclear steroid receptor (NSR)-dependent mechanism. In this study, we further investigated the potential role of the glucocorticoid receptor (GR) in the anti-apoptotic function of UDCA. Our results with short interference RNA (siRNA) technology confirmed that UDCA significantly reduces TGF-beta1-induced apoptosis of primary rat hepatocytes through a GR-dependent effect. Immunoprecipitation assays and confocal microscopy showed that UDCA enhanced free GR levels with subsequent GR nuclear translocation. Interestingly, when a carboxy-terminus deleted form of GR was used, UDCA no longer increased free GR and/or GR translocation, nor did it protect against TGF-beta1-induced apoptosis. In co-transfection experiments with GR response element reporter and overexpression constructs, UDCA did not enhance the transactivation of GR with TGF-beta1. Finally, using a fluorescently labeled UDCA molecule, the bile acid appeared diffuse in the cytosol but was aggregated in the nucleus of hepatocytes. Both siRNA assays and transfection experiments with either wild-type or mutant forms of GR showed that nuclear trafficking occurs through a GR-dependent mechanism. In conclusion, these results further clarify the anti-apoptotic mechanism(s) of UDCA and suggest that GR is crucial for the nuclear translocation of this bile acid for reducing apoptosis.