Do two wrongs make a right? For cancer cells, they just might.

From the Sullivan Lab, Human Biology Division

You may know the humble mitochondrion as the ‘powerhouse of the cell,’ a kidney bean-shaped organelle which provides our cells with the energy (in the form of ATP) to perform biochemistry, respond to the environment, and—in the case of cancer—proliferate voraciously. Mitochondria are central hubs of cellular metabolism, and mitochondrial dysfunction is thought to underlie processes as diverse as aging, neurodegeneration, and even cancer. The Sullivan Lab in the Human Biology Division at Fred Hutchinson Cancer Center devotes their time to studying the metabolism of proliferating cancer cells—unsurprisingly, mitochondria play a key role here. Ask a member of the lab about this organelle, however, and their answer might catch you off guard. “Although mitochondria are classically thought of as cellular ‘ATP factories,’ their anabolic roles in metabolism are much less appreciated,” Dr. Sullivan notes.

Indeed, although most cancers can get their ATP from sources other than mitochondria, they still rely on functional mitochondria to survive and grow. So, if ATP isn’t the growth-limiting output of mitochondria in proliferating cancer cells, then what is? This is the question that Dr. Sullivan asked and answered during his postdoctoral work, and where our story begins. In what was a surprising finding, he identified the amino acid aspartate as the most important metabolic output of mitochondria in proliferating cells—so important, in fact, that providing enough aspartate to cancer cells sidestepped their reliance on functional mitochondria to proliferate. However—in classic science fashion—solving one discrepancy revealed another, this time to do with one remaining player in the story: oncogenic mutations in mitochondrial components.

More specifically, it’s been known for over twenty years that inactivating mutations in an enzyme called succinate dehydrogenase (SDH), a core component of mitochondrial machinery and part of the aspartate synthesis pathway, drive some human cancers including several neuroendocrine and renal types. This apparent paradox revealed a new question: if the main role of mitochondria in proliferating cells is to produce aspartate, then how do tumors cope with SDH mutations? More specifically, how are these cancer cells meeting their aspartate demands? This is where the project was picked up by Madeleine Hart, a graduate student in the Sullivan Lab and first author of a recent preprint reporting the study findings.

To model SDH-deficient tumors, Hart and colleagues used pharmacological and CRISPR-based approaches to inhibit or knock out SDH in various cancer cell lines. They first confirmed that SDH-deficient cancer cells proliferate slower than their wild-type counterparts, and that this proliferation defect is rescuable by supplementing the cells with aspartate. While nailing down a potential alternative synthesis pathway for aspartate in these cells proved challenging, they did discover one important clue: after several months of passaging, SDH-deficient cells slowly improved their proliferation rates to approach wild-type levels. This result suggested that—whatever the mechanism—these cells were slowly adapting to SDH deficiency, presumably by increasing their aspartate production to enable faster growth. By combining several other experimental findings with clues from the literature, the researchers made another unexpected finding: it appeared that cells with SDH deficiencies responded by downregulating another important mitochondrial component, Complex I (CI) of the electron transport chain, as they ‘adapted’ in culture. This was surprising, considering that CI is normally a proliferation-promoting component of mitochondria, so the team hypothesized that CI loss was perhaps beneficial for SDH-deficient cells. To put this to the test, they studied the effects of inhibitors of CI and SDH in several cancer cell lines. As expected, treatment with either inhibitor alone impaired cell proliferation; however, treating cells with both inhibitors resulted in a near-complete rescue of growth!

A cartoon of metabolic networks with an outline of a mitochondrion in the center
A mitochondria emerging from the metabolic milieu Image made by the author

To discount the possibility that this was all an artifact of their cell types or culture conditions, the team reported one more shocking finding: they cultured cancer cells derived from a patient with an SDH-mutant cancer (which had already ‘adapted’ by downregulating CI) and genetically restored CI activity to these cells. Consistent with their hypothesis—and contrary to what one might predict—restoring CI activity hindered the proliferation of these cells. So, why would cells respond to one mitochondrial deficiency by inducing another? A few more genetic experiments provided convincing evidence that CI loss was beneficial to SDH-deficient cells by altering the mitochondrial NAD+/NADH ratio—a key metabolic feature which was crucial for the alternative aspartate synthesis pathway in these cells.

“As most scientists who culture cells can attest, it’s fairly easy to make cells grow worse; it’s much harder to make them grow better. We’ve accomplished just that by feeding them two substances which on their own are poisonous.” Dr. Sullivan comments. “It really highlights the complexity of metabolism, which makes it a beautiful but difficult process to study.” Hart, meanwhile, is excited about the implications of the findings: “It’s been over 22 years since SDH was identified as a tumor suppressor, and I think this study gives some important insight into how these tumors might operate in humans. The fact that we can improve the growth of SDH-deficient cancer cells by further inhibiting normally-crucial mitochondrial processes—and the fact that the cells themselves select for this impairment over time—convinces me that the result is noteworthy and worth pursuing further.”

The spotlighted research was funded by the National Institutes of Health.

Fred Hutch/University of Washington/Seattle Children’s Cancer Consortium member Dr. Lucas Sullivan contributed to this study.

Hart, M. L., Quon, E., Vigil, A.-L. B. G., Engstrom, I. A., Newsom, O. J., Davidsen, K., Hoellerbauer, P., Carlisle, S. M., & Sullivan, L. B. (2022). Mitochondrial Redox Adaptations Enable Aspartate Synthesis in SDH-deficient Cells. Biorxiv.