The receptor tyrosine kinase ErbB2/HER2 governs CDK4 inhibitor sensitivity, timing, and irreversibility of the G1/S transition.

Cell cycle entry and the irreversible transition from the G1 to S phase are crucial for mammalian cell proliferation. Among the ErbB family, the ErbB2/HER2 receptor is a key driver of cancer growth. However, the quantitative mechanisms underlying the ErbB2-mediated G1/S transition remain unclarified. Here, we performed an extensive time-course analysis of high and low ErbB2-expressing breast cancer cells to describe the regulatory mechanisms of the ErbB2-mediated G1/S transition. Live-cell imaging using cell cycle reporters revealed that the G1/S transition occurs 20 h after ErbB2 activation, driven primarily by the cyclin D1/CDK4-RB axis. Hsp90 is regulated by CDK4 activity and controls the stability of ErbB2 protein in a time-dependent manner. CDK4 inhibitor treatment arrested the cell cycle in most cells; a subpopulation showed a 25-h delay in G1/S entry associated with enhanced c-Myc activation. In high ErbB2-expressing cells, CDK4 inhibition led to c-Myc overactivation, a rapid decrease in cyclin D1 expression, and cell cycle arrest. Overall, we demonstrate how ErbB2 receptor levels modulate the roles of cyclin D1 and c-Myc in the G1/S transition and suggest that variations in ErbB2 levels within breast cancer tissues confer heterogeneous sensitivity to CDK4 inhibitors, potentially complicating treatment.