ErbB2/HER2 expression level determines CDK4-inhibitor sensitivity and cyclin D1 and c-Myc dependency at the G1/S transition

Abstract

AbstractCell-cycle entry and subsequent G1 to S phase transition are crucial for mammalian cell proliferation. The pathway mediated by members of the ErbB receptor family (ErbB1/2/3/4) plays an important role in the proliferation of several types of human cancer cells, and increased ErbB2/HER2 activity is often observed in malignant breast cancer. However, the underlying mechanisms, such as the transcriptional regulation of the G1/S transition via ErbB2 activation, remain unclear. Here, we performed a comprehensive time-course analysis of growth factor-stimulated MCF-7 breast cancer cells to identify the regulatory mechanisms underlying G1/S transition. Live-cell imaging using cell-cycle reporters enabled the estimation of the timing and duration of G1/S transition under conditions of ErbB2 activation, which was found to be mainly driven by cyclin D1/CDK4 under conditions of Hsp90-mediated ErbB2 stabilization. Although most cells exhibited a cell-cycle arrest upon CDK4-inhibitor treatment, a subpopulation of cells exhibited persistent G1/S transition, which was attributed to the activation of the transcriptional activity of c-Myc through epigenetic modifications. Conversely, in cells with high ErbB2-expression, CDK4 inhibition led to an excessive increase in c-Myc transcriptional activity, rapid decrease in cyclin D1 expression, and a reversible cell-cycle arrest. Overall, our results demonstrated that ErbB2 receptor levels alter the dependency of the G1/S regulatory network on cyclin D1 and c-Myc via epigenetic regulation, ultimately providing two routes for cell-cycle progression. These results might be extrapolated to suggest that the differences in ErbB2 levels within breast cancer tissues are associated with heterogeneity in CDK4-inhibitor sensitivity, potentially complicating treatment strategies.One Sentence SummaryCDK4 inhibition in the background of high ErbB2 expression caused c-Myc activation and reversible cell-cycle arrest.Bullet pointsAfter ErbB2 activation, G1/S transition was driven by CDK4-cyclin D1 activity.Most cells were arrested in the G1 phase after CDK4 inhibition, but cells with increased c-Myc transcriptional activity were insensitive to CDK4 inhibition.Hsp90-mediated ErbB2 stabilization was required for the G1/S transition.High levels of ErbB2 activate AKT and c-Myc and suppress p27 expression, resulting in the accumulation of cyclin D1-CDK4 and cell-cycle arrest.Cells with high ErbB2 expression maintained c-Myc transcriptional activity after CDK4-inhibitor treatment, exhibiting reversible cell-cycle arrest and reduced sensitivity to the inhibitor.