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Hint2 Is Expressed in the Mitochondria of H295R Cells and Is Involved in Steroidogenesis

Services of Endocrinology & Diabetology (S.L., F.A., L.V., M.F.R.) and Laboratory Medicine (M.F.R.), University Hospital of Geneva, CH-1211 Geneva 14, Switzerland; and Institute for Clinical Pharmacology (J.-F.D.), University of Bern, CH-3012 Bern, Switzerland

Address all correspondence and requests for reprints to: Dr. Michel F. Rossier, Service of Endocrinology & Diabetology, University Hospital, 24 rue Micheli-du-Crest, CH-1211 Geneva 14, Switzerland. E-mail: michel.rossier{at}hcuge.ch.

Hint2 belongs to the superfamily of histidine triad hydrolase enzymes. Recently, it has been shown to influence the mitochondria-dependent apoptosis occurring in hepatocytes, but its mechanism of action is still obscure. Here, we demonstrate that Hint2 is expressed in the mitochondria of H295R cells and in normal adrenals, and that this protein is involved in steroidogenesis. The presence of Hint2 in H295R cells was revealed by RT-PCR and by immunoblot analysis of subcellular fractions. The protein appeared associated with mitochondrial membranes, probably facing the interior of the organelle. Hint2 overexpression in H295R cells had no effect on pregnenolone secretion elicited by angiotensin II or K⁺, whereas protein silencing with specific small interfering RNA resulted in a marked reduction of the steroidogenic response. The duration of the mitochondrial calcium signal induced by angiotensin II was also reduced upon Hint2 down-regulation with small interfering RNA, but not affected after its overexpression, suggesting that under basal conditions, Hint2 is optimally expressed, and not rate limiting in steroidogenesis. Moreover, Hint2 also appeared involved in Ca²⁺-independent pathways leading to steroid formation. Indeed, pregnenolone formation in response to either forskolin or a hydroxyl analog of cholesterol was markedly reduced after Hint2 silencing. Calcium-dependent and calcium-independent actions of Hint2 on steroidogenesis could be related to its ability to maintain a favorable mitochondrial potential. In conclusion, these data suggest that, in H295R cells, Hint2 is required for an optimal steroidogenic response, possibly because of a particular signalling function exerted within the mitochondria and that still remains to determine at the molecular level.