The cyclin D-cyclin dependent kinase 4/6 (CDK4/6)-Rb axis is frequently dysregulated in hormone receptor (HR)-positive, HER2-negative breast cancer and causes uncontrolled cell proliferation. Consequently, CDK4/6 inhibitors are currently used in combi...
The cyclin D-cyclin dependent kinase 4/6 (CDK4/6)-Rb axis is frequently dysregulated in hormone receptor (HR)-positive, HER2-negative breast cancer and causes uncontrolled cell proliferation. Consequently, CDK4/6 inhibitors are currently used in combination with endocrine therapy to treat advanced HR-positive breast cancer. Although the therapeutic benefit of CDK4/6 inhibitors is clear, acquired resistance is frequently observed in clinical cases. Therefore, understanding the molecular mechanisms of resistance to CDK4/6 inhibitors is required to develop more effective treatment strategies.
To investigate the mechanisms underpinning resistance to CDK4/6 inhibitors, palbociclib-resistant breast cancer cell line was established by continuous treatment of HR-positive breast cancer cells with palbociclib. In palbociclib-resistant cells, the loss of estrogen receptor (ER) α and increased expression of interleukin-6 (IL6) were observed. The downregulation of forkhead box protein A1 (FOXA1), transcriptional partner of ERα, led to the induction of IL6, and to subsequent IL6-mediated suppression of ERα expression in resistant cells. In addition, the secretion of IL6 from palbociclib-resistant cells not only promoted the proliferation of the resistant cells themselves but also stimulated the migration of parental nonresistant cells. Furthermore, the inhibition of IL6/STAT3 signaling effectively suppressed the growth of palbociclib-resistant cells. Taken together, these results highlight the crucial role of IL6 in resistance to palbociclib and indicate that the induction of IL6 is linked to the dysregulation of ERα-related signaling in palbociclib-resistant breast cancer cells. Therapeutically, a STAT3 inhibitor proves to be a potential candidate to control acquired resistance to palbociclib.