CDK4 inactivation balances resistance to apoptosis with heightened metabolic sensitivity in triple negative breast cancer cells

The shift in the energetic demands of proliferating cells during tumorigenesis requires intense crosstalk between the cell cycle and metabolism. Beyond their role in cell proliferation, cell cycle regulators also modulate intracellular metabolism in normal tissues. However, in the context of cancer, where CDK4 is upregulated or stabilized, the metabolic role of CDK4 is barely understood. Using both genetic and pharmacological approaches, we aimed to determine the metabolic role of CDK4 in TNBC cells. Unexpectedly, deletion of CDK4 only slightly reduced triple-negative breast cancer (TNBC) cell proliferation and allowed tumor formation in vivo . Furthermore, proapoptotic stimuli failed to induce appropriate cell death in TNBC cells with CDK4 depletion or long-term CDK4/6 inhibitor treatment. Mechanistically, CDK4 enhances mitochondria-ER contact (MERC) formation, thus promoting mitochondrial fission and ER-mitochondrial calcium signaling. Phosphoproteomic analysis also revealed a role for CDK4 in regulating PKA activity at MERCs to sustain ER-mitochondrial calcium signaling. This proper CDK4-mediated mitochondrial calcium signaling is then required for metabolic flexibility of TNBC cells. Taken together, these results demonstrate that CDK4 inhibition leads to cell death resistance, inhibiting mitochondrial apoptosis and functions through attenuated MERCs formation and ER-mitochondrial calcium signaling in TNBC. Overall, this study provides new insights into the mechanisms of TNBC resistance to CDK4/6i therapy and paves the way to explore potential synergistic therapeutic targeting MERCs-associated metabolic shifts.