Endocrine Abstracts

Mitochondria-endoplasmic reticulum contact sites (MERCS) play a key role in cholesterol metabolism and steroidogenesis by facilitating the efficient transfer and processing of cholesterol for steroid hormone synthesis. However, whether MERCS play a role in bile acid synthesis is completely unexplored. Here we begin to investigate a role for MERCS in bile acid synthesis using two alternative models of MERCS disruption. First, to reduce the total number of MERCS we performed siRNA knockdown of Ribosome binding protein 1 (RRBP1). Second, to increase the distance between ER and mitochondria at the contact sites we overexpressed the testis-specific protein (FATE1). The two models were validated by qPCR, western blot and electron microscopy. Gene expression studies were then conducted to assess the effects on bile acid synthesis. RRBP1 knockdown (KD) in human hepatoma cells decreased mRNA expression of CYP7A1 (1.66±0.29 [SC] vs. 0.95±0.09 [KD], P = 0.002), CYP8B1 (1.31±0.16 [SC] vs. 0.68±0.17 [KD], P = 0.008), and AKR1D1 (1.37±0.23 [SC] vs. 0.80±0.38 [KD], P = 0.022) and increased the expression of HSD3B7 (0.76±0.16 [SC] vs. 1.20±0.34 [KD], P = 0.032), while CYP27A1 expression remained unchanged. The expression of FXR and its target SHP were unchanged, suggesting these changes are not a result of altered bile acid levels or signalling via FXR. However, LXRβ expression was increased (0.925±0.15 [SC] vs. 1.24±0.26 [KD], P = 0.047), consistent with altered oxysterol signalling. In contrast, FATE1 overexpression had no effect on the expression of genes involved in bile acid synthesis or signalling. In conclusion, these data highlight the critical role of MERCS in BA synthesis. Our data would suggest that the regulation of the total number of MERCS, but not the distance between the ER and mitochondria, is associated with changes in the expression of genes involved in bile acid synthesis and signalling.