Ca²⁺-mediated regulation of VDAC1 expression levels is associated with cell death induction

VDAC1, an outer mitochondrial membrane (OMM) protein, is crucial for regulating mitochondrial metabolic and energetic functions and acts as a convergence point for various cell survival and death signals. VDAC1 is also a key player in apoptosis, involved in cytochrome c (Cyto c) release and interactions with anti-apoptotic proteins. Recently, we demonstrated that various pro-apoptotic agents induce VDAC1 oligomerization and proposed that a channel formed by VDAC1 oligomers mediates cytochrome c release. As VDAC1 transports Ca²⁺ across the OMM and because Ca²⁺ has been implicated in apoptosis induction, we addressed the relationship between cytosolic Ca²⁺ levels ([Ca²⁺]i), VDAC1 oligomerization and apoptosis induction. We demonstrate that different apoptosis inducers elevate cytosolic Ca²⁺ and induce VDAC1 over-expression. Direct elevation of [Ca²⁺]i by the Ca²⁺-mobilizing agents A23187, ionomycin and thapsigargin also resulted in VDAC1 over-expression, VDAC1 oligomerization and apoptosis. In contrast, decreasing [Ca²⁺]i using the cell-permeable Ca²⁺-chelating reagent BAPTA-AM inhibited VDAC1 over-expression, VDAC1 oligomerization and apoptosis. Correlation between the increase in VDAC1 levels and oligomerization, [Ca²⁺]i levels and apoptosis induction, as induced by H₂O₂ or As₂O₃, was also obtained. On the other hand, cells transfected to overexpress VDAC1 presented Ca²⁺-independent VDAC1 oligomerization, cytochrome c release and apoptosis, suggesting that [Ca²⁺]i elevation is not a pre-requisite for apoptosis induction when VDAC1 is over-expressed. The results suggest that Ca²⁺ promotes VDAC1 over-expression by an as yet unknown signaling pathway, leading to VDAC1 oligomerization, ultimately resulting in apoptosis. These findings provide a new insight into the mechanism of action of existing anti-cancer drugs involving induction of VDAC1 over-expression as a mechanism for inducing apoptosis. This article is part of a Special Issue entitled: Calcium Signaling in Health and Disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.