Abstract
The bioenergetics of cancer cells is characterized by a high rate of aerobic glycolysis and suppression of mitochondrial metabolism (Warburg phenomenon). Mitochondrial metabolism requires inward and outward flux of hydrophilic metabolites, including ATP, ADP and respiratory substrates, through voltage-dependent anion channels (VDACs) in the mitochondrial outer membrane. Although VDACs were once considered to be constitutively open, closure of the VDAC is emerging as an adjustable limiter (governator) of mitochondrial metabolism. Studies of VDACs reconstituted into planar lipid bilayers show that tubulin at nanomolar concentrations decreases VDAC conductance. In tumor cell lines, microtubule-destabilizing agents increase cytoplasmic free tubulin and decrease mitochondrial membrane potential (ΔΨm), whereas microtubule stabilization increases ΔΨm. Tubulin-dependent suppression of ΔΨm is further potentiated by protein kinase A activation and glycogen synthase kinase-3β inhibition. Knockdown of different VDAC isoforms, especially of the least abundant isoform, VDAC3, also decreases ΔΨm, cellular ATP, and NADH/NAD+, suggesting that VDAC1 and VDAC2 are most inhibited by free tubulin. The brake on mitochondrial metabolism imposed by the VDAC governator probably is released when spindles form and free tubulin decreases as cells enter mitosis, which better provides for the high ATP demands of chromosome separation and cytokinesis. In conclusion, tubulin-dependent closure of VDACs represents a new mechanism contributing to the suppression of mitochondrial metabolism in the Warburg phenomenon.
Footnotes
This work was supported, in part, by the National Institutes of Health National Institute of Diabetes and Digestive and Kidney Diseases [Grants DK073336, DK37034]. E.N.M. is a recipient of a Specialized Program of Research Excellence Career Development Award [Grant P50 CA058187]. Imaging facilities for this research were supported, in part, by the Hollings Cancer Center, Medical University of South Carolina [Grant P30 CA138313].
Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
ABBREVIATIONS:
- ΔΨm
- mitochondrial membrane potential
- MIM
- mitochondrial inner membrane
- MOM
- mitochondrial outer membrane
- VDAC
- voltage-dependent anion channel
- PKA
- protein kinase A
- GSK3β
- glycogen synthase kinase-3β
- TMRM
- tetramethylrhodamine methylester
- siRNA
- short interfering RNA
- ANT
- adenine nucleotide transporter
- db-cAMP
- dibutyryl-cAMP
- H89
- N-[2-[[3-(4-bromophenyl)-2-propenyl]amino]ethyl]-5-isoquinolinesulfonamide dihydrate dihydrochloride.
- Received March 29, 2012.
- Accepted April 30, 2012.
- Copyright © 2012 by The American Society for Pharmacology and Experimental Therapeutics
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