Prohibitin‐1 plays a multifaceted role in mediating luteinizing hormone‐induced steroidogenesis in Leydig cells.

Abstract

Leydig cells (LCs) present in the interstitium between seminiferous tubules in the testis are responsible for the biosynthesis of testosterone from cholesterol in response to luteinizing hormone (LH) from the pituitary. The principal pathways of testosterone biosynthesis, the identities of the main steroidogenic enzymes, and genetic disorders of most of these enzymes have been elucidated. However, many important questions related to LH signaling and cholesterol transport remain unknown. For example, the mechanisms of StAR‐mediated cholesterol transport from the outer mitochondrial membrane to cytochrome P450 side chain cleavage (P450scc or Cyp11a1) enzyme in the inner mitochondrial membrane, which is a critical step in steroidogenesis, remain elusive. It is conceivable that we are missing some important players involved in this process.Prohibitin‐1 (PHB1) is a pleiotropic protein with cell compartment‐specific functions, including the phosphorylation‐dependent membrane signaling and mitochondrial chaperone, which appears to require heterodimerization with its homologous protein prohibitin‐2 (PHB2) in the inner mitochondrial membrane. Previously, we have reported transgenic mouse models expressing PHB1 (PHB‐Tg) or Y114F‐PHB1 (mPHB‐Tg) from the fatty acid binding protein‐4 (Fabp4) gene promoter. During their phenotypic characterization, unexpectedly, we found that male mPHB‐Tg mice, but not PHB‐Tg mice, have high testosterone levels. Subsequent analysis revealed that it was due to leaky overexpression of m/PHB1 in LCs. This prompted us to investigate whether LH regulates PHB1 in LCs. A dose and time‐dependent effect of human chorionic gonadotropin, hCG (pituitary analog of LH) was found on PHB1 expression levels in LCs. To define the cell compartment‐specific role of PHB1 in LCs, we investigated the functional status of LH signaling in testes/LCs and ultrastructure of LCs from transgenic mice. A difference in phospho‐ERK levels was found between PHB‐Tg and mPHB‐Tg, which was higher in mPHB1 expressing cells. A similar effect was found in PHB1 manipulated MA‐10 cells (a model LC line). A parallel change in ultrastructural features of LCs (mitochondrial structure and lipid droplets) was found in PHB and mPHB overexpressing cells. Moreover, co‐immunoprecipitation experiments showed that PHB1 and PHB2 interact with P450scc and/or StAR. Analysis of PHBs protein sequences revealed the presence of multiple CRAC and/or CARC motifs (cholesterol recognition motifs) in both proteins. In aggregate, this finding suggests that PHB1 is a LH regulated protein in LCs and plays a multifaceted role in LH‐induced steroidogenesis, including ERK activation and potentially in the functional coupling of StAR and P450scc across mitochondrial membrane for steroidogenesis. The scope of our findings is broad because the basics of cholesterol transport across mitochondrial membranes are similar in different steroidogenic tissues.