Cytosolic malate dehydrogenase activity helps support glycolysis in actively proliferating cells and cancer.

Increased glucose consumption is a hallmark of cancer cells. The increased consumption and subsequent metabolism of glucose during proliferation creates the need for a constant supply of NAD, a co-factor in glycolysis. Regeneration of the NAD required to support enhanced glycolysis has been attributed to the terminal glycolytic enzyme, lactate dehydrogenase (LDH). However, loss of glucose carbons to biosynthetic pathways early in glycolysis reduces the carbon supply to LDH. Thus, alternative routes for NAD regeneration must exist to support the increased glycolytic rate while allowing for the diversion of glucose to generate biomass and support proliferation. Here we demonstrate, using a variety of cancer cell lines as well as activated primary T cells, that cytosolic malate dehydrogenase 1 (MDH1) is an alternative to LDH as a supplier of NAD. Moreover, our results indicate that MDH1 generates malate with carbons derived from glutamine, thus enabling utilization of glucose carbons for glycolysis and for biomass. Amplification of MDH1 occurs at an impressive frequency in human tumors and correlates with poor prognosis. Together, our findings suggest that proliferating cells rely on both MDH1 and LDH to replenish cytosolic NAD, and that therapies designed at targeting glycolysis must consider both dehydrogenases.

Tumor-associated mutant p53 promotes cancer cell survival upon glutamine deprivation through p21 induction.

Cancer cells depend on glutamine to sustain their increased proliferation and manage oxidative stress, yet glutamine is often depleted at tumor sites owing to excessive cellular consumption and poor vascularization. We have previously reported that p53 protein, although a well-known tumor suppressor, can contribute to cancer cell survival and adaptation to low-glutamine conditions. However, the TP53 gene is frequently mutated in tumors, and the role of mutant p53 (mutp53) in response to metabolic stress remains unclear. Here, we demonstrate that tumor-associated mutp53 promotes cancer cell survival upon glutamine deprivation both in vitro and in vivo. Interestingly, cancer cells expressing mutp53 proteins are more resistant to glutamine deprivation than cells with wild-type p53. Depletion of endogenous mutp53 protein in human lymphoma cells leads to cell sensitivity to glutamine withdrawal, whereas expression of mutp53 in p53 null cells results in resistance to glutamine deprivation. Furthermore, we found that mutp53 proteins hyper-transactivate p53-target gene CDKN1A upon glutamine deprivation, thus triggering cell cycle arrest and promoting cell survival. Together, our results reveal an unidentified mechanism by which mutp53 confers oncogenic functions by promoting cancer cell adaptation to metabolic stress.

Transcription factor C/EBP-ß induces tumor-suppressor phosphatase PHLPP2 through repression of the miR-17-92 cluster in differentiating AML cells.

PHLPP2, a member of the PH-domain leucine-rich repeat protein phosphatase (PHLPP) family, which targets oncogenic kinases, has been actively investigated as a tumor suppressor in solid tumors. Little is known, however, regarding its regulation in hematological malignancies. We observed that PHLPP2 protein expression, but not its mRNA, was suppressed in late differentiation stage acute myeloid leukemia (AML) subtypes. MicroRNAs (miR or miRNAs) from the miR-17-92 cluster, oncomir-1, were shown to inhibit PHLPP2 expression and these miRNAs were highly expressed in AML cells that lacked PHLPP2 protein. Studies showed that miR-17-92 cluster regulation was, surprisingly, independent of transcription factors c-MYC and E2F in these cells; instead all-trans-retinoic acid (ATRA), a drug used for terminally differentiating AML subtypes, markedly suppressed miR-17-92 expression and increased PHLPP2 protein levels and phosphatase activity. Finally, we demonstrate that the effect of ATRA on miR-17-92 expression is mediated through its target, transcription factor C/EBPß, which interacts with the intronic promoter of the miR-17-92 gene to inhibit transactivation of the cluster. These studies reveal a novel mechanism for upregulation of the phosphatase activity of PHLPP2 through C/EBPß-mediated repression of the miR-17-92 cluster in terminally differentiating myeloid cells.