<scp>CDK</scp> 11 inhibition induces cytoplasmic p21 ^{<scp>WAF</scp> 1} splice variant by p53 stabilisation and <scp>SF</scp> 3 <scp>B</scp> 1 inactivation

CDK11 is a cyclin-dependent kinase with a role in transcription and RNA splicing and represents a potential target for cancer treatment. We show that blocking CDK11 activity with the OTS964 inhibitor causes p53 stabilisation through MDM2 downregulation. Under these conditions, p53 activates the expression of its downstream effector CDKN1A (p21^WAF1), produced in two isoforms, the canonical p21^C and the recently described p21^L. We compared the ability of both isoforms to block proliferation and showed that p21^L partially lost its inhibitory potential, likely due to the missing cyclin-binding Cy2 and PCNA-interacting motifs and its cytoplasmic localisation. We identified the epitopes of four p21^WAF1 antibodies using phage display to determine isoform specificity. Moreover, we show that the trigger for p21^L induction is inhibition of the spliceosomal protein SF3B1. CDK11 activates SF3B1 by phosphorylation, and inhibition of either SF3B1 or CDK11 induces p21^L. We discovered an isoform similar to human p21^L in murine cells, suggesting evolutionary conservation of CDKN1A alternative splicing. Our results uncover an unknown link between RNA splicing and proliferation control involving a novel isoform of a key cell cycle inhibitor.