460 Rapid activation of epidermal progenitor differentiation via CDK9 activity modulated by AFF1 and HEXIM1

Epidermal barrier function requires differentiation initiation from a sustainable pool of progenitors. The mechanisms underlying the earliest events initiating remain unclear. Here we show that RNA Polymerase II (Pol II) pause release, controlled by CDK9 kinase activity, plays crucial role in rapid induction differentiation. Using RNA-seq combination with Pol ChIP-seq, identified cluster upregulated genes featuring robust release keratinocyte A subset these showed within 3 hours Peptidomimetic inhibitors targeting super elongation complex (SEC), keratinocytes cultured undifferentiation condition. longer-term (24 hour) treatment led to significant changes (fold change >2, p<0.05) 1286 keratinocyte-differentiation-signature genes. Mechanistically, found SEC scaffold protein AFF1, but not AFF4, is essential for repressing progenitor state. AFF1 knockdown induced epidermal hypoplasia and impaired regenerative capacity. We further represses progenitors through sequestering inactive state via its interaction HEXIM1. HEXIM1 directly bind near transcription start sites 92 rapid-response genes, sustaining pausing These response include ATF3, whose overexpression sufficient drive activation other differentiation-activating factors, including ZNF750, PRDM1, OVOL1, GRHL3. In addition, activity are both involved immediate PKC-signaling activation. Taken together, our findings suggest model mediated switch underlies initial steps differentiation, cellular signaling such as PKC