Palbociclib-mediated cell cycle arrest can occur in the absence of the CDK inhibitors p21 and p27

Abstract

The use of CDK4/6 inhibitors in the treatment of a wide range of cancers is an area of ongoing investigation. Despite their increasing clinical use, there is limited understanding of the determinants of sensitivity and resistance to these drugs. Recent data have cast doubt on how CDK4/6 inhibitors arrest proliferation, provoking renewed interest in the role(s) of CDK4/6 in driving cell proliferation. As the use of CDK4/6 inhibitors in cancer therapies becomes more prominent, an understanding of their effect on the cell cycle becomes more urgent. Here, we investigate the mechanism of action of CDK4/6 inhibitors in promoting cell cycle arrest. Two main models explain how CDK4/6 inhibitors cause G1 cell cycle arrest, which differ in their dependence on the CDK inhibitor proteins p21 and p27. We have used live and fixed single-cell quantitative imaging, with inducible degradation systems, to address the roles of p21 and p27 in the mechanism of action of CDK4/6 inhibitors. We find that CDK4/6 inhibitors can initiate and maintain a cell cycle arrest without p21 or p27. This work clarifies our current understanding of the mechanism of action of CDK4/6 inhibitors and has implications for cancer treatment and patient stratification.

Cell overgrowth during G1 arrest triggers an osmotic stress response and chronic p38 activation to promote cell cycle exit

“…We previously showed that p21 protein starts to increase after two days CDK4/6i treatment (Pennycook and Barr, 2022) and p53, a key transcription factor for p21, was shown to be required for long-term cell cycle arrest after washout of prolonged CDK4/6i treatment (Crozier et al, 2022; Wang et al, 2022). The increase in p21 protein correlates with the loss of the ability of cells to re-enter S-phase after CDK4/6i washout between two and three days of CDK4/6i treatment ( Figure 1E ) and p21 induction is p53-dependent ( Figure 2A ).…”

Cellular mechanisms underlying response and resistance to CDK4/6 inhibitors in the treatment of hormone receptor-positive breast cancer

“…It has been assumed that these agents directly inhibit CDK4/6 enzymatic activity, but this has recently been called into question by work suggesting that by binding to CDK4/6, they in fact operate as indirect CDK2 inhibitors [50]. This claim has, in turn, been refuted by other studies, and it is critical to resolve this issue, the answer to which forms the foundation for all research using these compounds [103]. Indeed, the CDK2 inhibition hypothesis is built upon the notion that CDK4/6 activity requires the formation of trimers which also contain cyclin D and p21/p27, a concept which is also controversial [103][104][105][106].…”

“…This claim has, in turn, been refuted by other studies, and it is critical to resolve this issue, the answer to which forms the foundation for all research using these compounds [103]. Indeed, the CDK2 inhibition hypothesis is built upon the notion that CDK4/6 activity requires the formation of trimers which also contain cyclin D and p21/p27, a concept which is also controversial [103][104][105][106].…”

The Combination of the CDK4/6 Inhibitor, Palbociclib, With the Vitamin D3 Analog, Inecalcitol, Has Potent In Vitro and In Vivo Anticancer Effects in Hormone-Sensitive Breast Cancer, But Has a More Limited Effect in Triple-Negative Breast Cancer

“…Also, p21 levels were not affected by palbociclib monotherapy or after combination therapy, while other studies in which higher palbociclib concentrations were applied, detected upregulation of p21 (35,36). However, a recent study demonstrated that the CDKinhibitors p21 and p27 are not required for palbociclibmediated cell cycle arrest (39).…”