Free Tubulin and cAMP-Dependent Phosphorylation Modulate Mitochondrial Membrane Potential in Hepg2 Cells: Possible Role of VDAC

BACKGROUND: Conductance of the voltage-dependent anion channel (VDAC) in the mitochondrial outer membrane has been proposed to limit mitochondrial metabolism in cancer cells and contribute to the Warburg effect. Since tubulin binding and phosphorylation promote VDAC closure, we hypothesized that free tubulin and cAMP-dependent phosphorylation by protein kinase A (PKA) modulate ΔΨ in cancer cells by regulating VDAC-dependent flux of substrates into mitochondria. Our AIM was to modulate VDAC closure and opening in intact cells by increasing and decreasing endogenous free tubulin and by promoting and blocking PKA activation. METHODS: HepG2 human hepatoma cells were incubated in Hank's solution with 5% CO2/air, and ΔΨ was assessed by confocal microscopy of TMRM. Free and polymerized tubulin was determined using a commercial kit. RESULTS: Myxothiazol (10 μM), a respiratory inhibitor, caused only a slight decrease of (TMRM fluorescence), but subsequent addition of oligomycin (10 μg/ml), a F1-F0-ATPase inhibitor, collapsed ΔΨ nearly completely, showing that inhibition of both respiration and ATPase are required to collapse ΔΨ. Stabilization of microtubules by paclitaxel (10 μM) increased ΔΨ by 60%, whereas disruption by colchicine (10 μM) or nocodazol (10 μM) decreased ΔΨ by 60-70%. Paclitaxel pretreatment prevented the depolarizing effect of colchicine and nocodazol. Dibutyryl cAMP (1 mM) decreased ΔΨ by 45% whereas H89 (1 μM), a specific inhibitor of PKA, increased ΔΨ by 94% and blocked the effect of dibutyryl cAMP. CONCLUSION: Free tubulin and cAMP/PKA-dependent phosphorylation modulate mitochondrial ΔΨ in HepG2 cells, most likely by regulating VDAC conductance. Up and down regulation of ΔΨ by tubulin polymerization/depolarization and PKA dependent phosphorylation/dephosphorylation is consistent with the hypothesis that VDAC is rate-limiting for mitochondrial metabolism in cancer cells and responsible, at least in part, for the Warburg effect.