Cyclin D3–CDK6 inhibits the glycolytic enzymes PFK1 and PKM2 to prevent T-ALL cell apoptosis.
Major finding: Cyclin D3–CDK6 inhibits the glycolytic enzymes PFK1 and PKM2 to prevent T-ALL cell apoptosis.
Mechanism: Inhibition of PFK1 and PKM2 promotes pentose phosphate and serine pathway production of antioxidants.
Impact: High levels of cyclin D3 and CDK6 may predict response to CDK4/6 inhibitors in multiple tumor types.
Inhibitors of the cyclin-dependent kinases CDK4 and CDK6 induce cell-cycle arrest in RB1-proficient tumors and have had promising results in several tumor types. However, in T-cell acute lymphoblastic leukemia (T-ALL), which predominately expresses CDK6 and the activating cyclin, cyclin D3, inhibition of CDK6 or cyclin D3 induces apoptosis. The mechanisms underlying the prosurvival function of cyclin D3–CDK6 have not been elucidated, prompting Wang and colleagues to search for substrates that may promote cancer cell survival. 6-phosphofructokinase (PFK1) and pyruvate kinase M2 (PKM2), enzymes that catalyze irreversible, rate-limiting steps in glycolysis, were directly phosphorylated and inhibited by cyclin D3–CDK6, suggesting that cyclin D3–CDK6 may have a unique role in glucose metabolism. Mechanistically, suppression of the rate-limiting glycolytic enzymes shunted glycolytic intermediates into the pentose phosphate pathway and serine synthesis pathway, which produce NADPH and glutathione (GSH) to protect against reactive oxygen species (ROS). Thus, the CDK4/6 inhibitor palbociclib reduced NADPH and GSH levels in T-ALL cells, thereby increasing ROS levels to induce apoptosis, which could be rescued by the antioxidant N-acetyl-cysteine. In vivo, palbociclib induced apoptosis of human T-ALL cells, but did not induce apoptosis in T-ALL cells expressing phosphomimetic PFK1 and PKM2 mutants. Moreover, in breast cancer cells, which express CDK4 instead of CDK6, palbociclib induced cell-cycle arrest instead of apoptosis, further indicating that expression of cyclin D3 and CDK6 in T-ALL cells promotes apoptosis in response to palbociclib. Additionally, 16 of 18 nonleukemic cancer cell lines exhibiting high expression of cyclin D3 and CDK6 underwent apoptosis in response to palbociclib, and, in melanoma patient-derived xenografts, high cyclin D3 and CDK6 expression was associated with tumor regression after CDK4/6 inhibition. Together, these findings elucidate a prosurvival role for cyclin D3–CDK6 in metabolism, in addition to its role in cell-cycle progression, and suggest that high levels of cyclin D3 and CDK6 may predict response to CDK4/6 inhibitors.