CDK4 Inactivation Hinders Cell Death and Metabolism Remodeling Mitochondria-ER contacts in Triple-Negative Breast cancer cells

The shift in the energetic demands of proliferating cells during tumorigenesis requires extensive 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. Here, using both genetic and pharmacological approaches, we sought to determine the metabolic role of CDK4 in triple-negative breast cancer (TNBC) cells. Unexpectedly, the deletion of CDK4 only modestly reduced TNBC cell proliferation and did not hinder tumor formation in vivo. Furthermore, proapoptotic stimuli failed to induce appropriate cell death in TNBC cells upon CDK4 depletion or long-term CDK4/6 inhibitor treatment. Mechanistically, CDK4 enhances mitochondria-endoplasmic reticulum contact (MERC) formation, thereby 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. Such CDK4-mediated mitochondrial calcium signaling is required for the metabolic flexibility of TNBC cells. Taken together, these results demonstrate that CDK4 inhibition leads to cell death resistance by 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 of MERCs-associated metabolic shifts.