Abstract
The genes encoding MDM2 and CDK4 are frequently co-amplified in sarcomas, and inhibitors to both targets are approved or clinically tested for therapy. However, we show that inhibitors of MDM2 and CDK4 antagonize each other in their cytotoxicity towards sarcoma cells. CDK4 inhibition attenuates the induction of p53-responsive genes upon MDM2 inhibition. Moreover, the p53 response was also attenuated when co-depleting MDM2 and CDK4 with siRNA, compared to MDM2 single knockdown. The complexes of p53 and MDM2, as well as CDK4 and Cyclin D1, physically associated with each other, suggesting direct regulation of p53 by CDK4. Interestingly, CDK4 inhibition did not reduce p53 binding or histone acetylation at promoters, but rather attenuated the subsequent recruitment of RNA Polymerase II. Taken together, our results suggest that caution must be used when considering combined CDK4 and MDM2 inhibition for patient treatment. Moreover, they uncover a hitherto unknown role for CDK4 and Cyclin D1 in sustaining p53 activity.
Highlights
Cyclin-dependent kinase 4 (CDK4) is a key promoter of cell proliferation
(see figure on previous page) Fig. 1 CDK4 inhibitors and MDM2 antagonists fail to synergize with regard to cytotoxicity towards sarcoma cells. a SJSA cells were treated with dimethyl sulfoxide (DMSO), Nutlin, PD0332991 and its combination at the indicated concentrations adhering to the depicted schedule. b Cell viability was measured by quantification of the ATP content
The CDK4–Cyclin D1 complex associates with p53, and its activity is required for gene induction and RNA Polymerase II recruitment by p53
Summary
Cyclin-dependent kinase 4 (CDK4) is a key promoter of cell proliferation. It enables the transition through the G1 phase of the cell cycle, a prerequisite for subsequent entry to S phase and cell division. Tumor cells often activate CDK4 to ensure proliferation, either by silencing genes that encode CDK4 antagonists or by enhancing CDK4 expression, e.g., through gene amplification. Pharmacological inhibitors of CDK4 have proven to be effective in cancer treatment, leading to the Food and Drug Administration (FDA) approval of Palbociclib (PD0332991), Ribociclib (LEE011) and Abemaciclib (LY2835319)[1]. The MDM2 oncoprotein has been extensively evaluated as a drug target. MDM2 antagonizes the tumor suppressor p53 by physical interaction and subsequent ubiquitination of p53. The interaction of p53 and MDM2 is amenable to targeting by small compounds, with
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