Free Tubulin Modulates Mitochondrial Membrane Potential in Cancer Cells

Formation of the mitochondrial membrane potential (Delta psi) depends on flux of respiratory substrates, ATP, ADP, and Pi through voltage-dependent anion channels (VDAC). As tubulin promotes single-channel closure of VDAC, we hypothesized that tubulin is a dynamic regulator of Delta psi, which in cultured cancer cells was assessed by confocal microscopy of the potential-indicating fluorophore tetramethylrhodamine methylester (TMRM). Microtubule destabilizers, rotenone, colchicine, and nocodazole, and the microtubule stabilizer paclitaxel increased and decreased cellular free tubulin, respectively, and in parallel decreased and increased Delta psi. Protein kinase A (PKA) activation by cAMP analogues and glycogen synthase kinase 3 beta (GSK-3 beta) inhibition decreased Delta psi, whereas PKA inhibition hyperpolarized, consistent with reports that PKA and GSK-3 beta decrease and increase VDAC conductance, respectively. Plasma membrane potential assessed by DiBAC(4)(3) was not altered by any of the treatments. We propose that inhibition of VDAC by free tubulin limits mitochondrial metabolism in cancer cells. Cancer Res; 70(24); 10192-201. (C) 2010 AACR.