# CDK11 inhibition induces cytoplasmic p21 WAF1 splice variant by p53 stabilisation and SF3B1 inactivation

## TL;DR

Inhibiting CDK11 leads to p53 stabilization and production of a less active cytoplasmic p21L variant, linking RNA splicing to cell cycle control.

## Contribution

Discovery of a novel cytoplasmic p21L isoform and its connection to RNA splicing via SF3B1 inactivation.

## Key findings

- CDK11 inhibition stabilizes p53 by downregulating MDM2 and activates p21L expression.
- p21L has reduced cell cycle inhibition due to cytoplasmic localization and missing functional motifs.
- SF3B1 inactivation is the trigger for p21L induction, and a similar isoform exists in murine cells.

## Abstract

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.

CDK11 inhibition stabilises the tumour suppressor p53 and triggers the production of an alternative p21WAF1 splice variant p21L, through the inactivation of the spliceosomal protein SF3B1. Unlike the canonical p21WAF1 protein, p21L is localised in the cytoplasm and has reduced cell cycle‐blocking activity. These findings reveal a new mechanism linking proliferation control and RNA splicing.