The mitochondrial electron-transport chain consists of four major components, which are designated as complexes I, II, III and IV.[31
] Ubiquinone (coenzyme Q10) and cytochrome c are responsible for the communication between the four trans-membrane complexes enabling the electron flow through the chain from NADH dehydrogenase (complex I) to cytochrome c oxidase (complex IV).[31
] Cytochrome bc1
(complex III) is a transmembrane protein complex located in the inner mitochondrial membrane.[32
] This multi-subunit enzyme is a central component of the mitochondrial respiratory electron transport chain.[35
] The function of cytochrome bc1
is to reduce cytochrome c(Fe3+
) into cytochrome c(Fe2+
) using the membrane localized ubiquinol. The free energy of the proton-coupled electron transfer reaction, which is often referred as the protonmotive Q cycle,[36
] contributes to the proton gradient across the mitochondrial membrane to enable subsequent ATP production by ATP synthase.
Our work demonstrated that practical chemical synthesis of leucascandrolide A (1) and neopeltolide (2), in the absence of any other means of accessing such compounds, enabled a comprehensive genetic and biochemical investigation of the molecular mechanism of action. This effort resulted in identification of cytochrome bc1 complex as the cellular target of the two marine natural products. Highly potent cell-based antiproliferative activity of leucascandrolide A and neopeltolide compares favorably to the most potent inhibitors of cytochrome bc1 complex known today, identifying such compounds as a new class of highly useful biochemical tools for investigation of eukaryotic energy metabolism.
It is noteworthy that leucascandrolide A (1
) and neopeltolide (2
) do not share any significant degree of structural homology with previously reported small-molecule inhibitors of cytochrome bc1
complex, including P-type inhibitors [i.e., myxothiazol (30
) and stigmatellin (31
] ubiquinone mimetics,[41
] and N-type inhibitors [i.e., antimycin A (32
] Our studies demonstrated that leucascandrolide-based analog 3
and neopeltolide (2
) were exceedingly potent at inhibiting the activity of cytochrome bc1
complex. Such potent inhibition of isolated cytochrome bc1
complex by 2
was fully consistent with the ability of the two compounds to inhibit cell growth, ATP production, respiration and complex III activity in yeast mitochondria at similarly low concentrations. Our preliminary investigation revealed that the effect of compound 3
on reduction of cytochrome b in yeast mitochondria appeared to be similar to that observed for P-type inhibitors of complex III, i.e. myxothiazol and stigmatellin. The detailed biophysical and biochemical investigation of the mechanism of inhibition of cytochrome bc1
complex by leucascandrolide A and neopeltolide is currently in progress.
Oxidative phosphorylation and glycolysis are tightly coupled in eukaryotic cells. Due to the increased dependency of tumor cells on glycolysis, known as the Warburg effect,[44
] and its likely role in promoting cell proliferation, survival and invasion,[45
] disruption of energy metabolism in cancer cells is of significant current interest for the development of a new class of anticancer agents.[46
] Several recent studies demonstrated that the induction of hypoxia-like conditions using inhibitors of oxidative phosphorylation hypersensitized tumor cells to treatment with glycolytic inhibitors.[47
] Thus, a combination of suppressors of oxidative phosphorylation with inhibitors of glycolysis could represent a strategy for metabolic sensitization of tumor cells to cytotoxic and targeted therapies.