Mitochondrial binding of hexokinase II inhibits Bax-induced cytochrome c release and apoptosis.

Proapoptotic proteins such as Bax, undergo translocation to the mitochondria during apoptosis, where they mediate the release of intermembrane space proteins including cytochrome c. Bax binds to the voltage-dependent anion channel (VDAC). VDAC is a beta-barrel protein located in the outer mitochondrial membrane. In planar lipid bilayers, Bax and VDAC form a channel through which cytochrome c can pass. Hexokinase II (HXK II) also binds to VDAC. HXK II catalyzes the first step of glycolysis and is highly expressed in transformed cells, where over 70% of it is bound to the mitochondria. The present study demonstrates that HXK II interferes with the ability of Bax to bind to mitochondria and release cytochrome c. Detachment of HXK II from the mitochondria-enriched fraction isolated from HeLa cells promoted the binding of recombinant Bax-Delta19 and subsequent cytochrome c release. Similarly, the addition of recombinant HXK II to the mitochondria-enriched fraction isolated from hepatocytes, cells that do not express HXK II endogenously, prevented the ability of recombinant Bax-Delta19 to bind to the mitochondria and promote cytochrome c release. Similar results were found in intact cells, in which the detachment of mitochondrial bound HXK II or its overexpression potentiated and inhibited, respectively, Bax-induced mitochondrial dysfunction and cell death.