KREJČÍŘ, R., ARCIMOWICZ, L., MARTÍNKOVÁ, L., HRABAL, V., ZAVADIL KOKÁŠ, F., HENEK, T., KUCERIKOVA, M., BONCZEK, O., ZATLOUKALOVÁ, P., HERNYCHOVÁ, L., COATES, P.J., VOJTĚŠEK, Bořivoj, LANE, D. P. CDK11 inhibition induces cytoplasmic p21WAF1 splice variant by p53 stabilisation and SF3B1 inactivation. Molecular Oncology. 2026, 20(3), 804-822. ISSN 1574-7891. E-ISSN 1878-0261

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 (p21WAF1), produced in two isoforms, the canonical p21C and the recently described p21L. We compared the ability of both isoforms to block proliferation and showed that p21L 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 p21WAF1 antibodies using phage display to determine isoform specificity. Moreover, we show that the trigger for p21L induction is inhibition of the spliceosomal protein SF3B1. CDK11 activates SF3B1 by phosphorylation, and inhibition of either SF3B1 or CDK11 induces p21L. We discovered an isoform similar to human p21L 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.