Lapatinib induces p27^Kip1-dependent G1 arrest through both transcriptional and post-translational mechanisms

Lapatinib, a dual EGFR/HER 2 tyrosine kinase inhibitor, has been shown to have potent antitumor effects against human breast cancer. Recent studies have shown that lapatinib upregulates p27Kip1 (here after referred to as p27) expression and induces G1 cell cycle arrest in various types of cancer cells. However, the regulation of p27 in lapatinib-induced cell cycle arrest is not well studied. Here we demonstrate that lapatinib-induced cell growth inhibition and G1 cell cycle arrest in HER 2-overexpressing human breast cancer cells were dependent on p27. We also show that lapatinib-induced upregulation of p27 expression was mediated through both transcriptional and post-translational mechanisms. On the one hand, lapatinib treatment led to increased FOXO ₃a expression and enhanced p27 transcription. On the other hand, lapatinib treatment resulted in increased DYR K1B expression, which correlated with increased p27 phosphorylation at Ser10 and decreased p27 degradation. Interestingly, we found that ER β1 but not ER β2 expression also upregulated p27 and enhanced lapatinib-induced cell proliferation inhibition and G1 cell cycle arrest in HER 2-overexpressing breast cancer cells. Taken together, our results suggest that lapatinib induces p27 expression via both transcriptional and post-translational upregulations, leading to cell cycle arrest and cell proliferation inhibition, and that its effect on breast cancer cells may be modified by ER expression status.