Epidermal homeostasis relies on the progenitors to continuously proliferate or initiate differentiation. The molecular mechanisms underlying the earliest events of differentiation initiation remain incompletely understood. Here we show that the kinase activity of CDK9, in association with the Super Elongation Complex (SEC), plays a crucial role in the process. Progenitor-state keratinocytes are associated with inactive CDK9 as part of the SEC. Maintenance of this inactive state of CDK9 relies on the cooperation between HEXIM1 and the SEC-scaffold protein AFF1 but not AFF4. Using a combination of RNAi and RNA-seq, we identified that both HEXIM1 and AFF1 are essential for repressing genes related to “epidermis development” and “keratinization”. Using ChIP-seq, we found that AFF1 and HEXIM1 co-bind near the promoter regions of 2112 genes, including a group of genes with highly enriched Pol II accumulating in the promoter genes (known as “paused” Pol II). CDK9 kinase activation, triggered by SEC disruption or Protein kinase C (PKC) signaling, rapidly upregulates the group of AFF1-HEXIM1 target genes with “paused” Pol II within as short as 1 hour. These rapidly upregulated genes include the transcription factor ATF3, and we found that ATF3 overexpression is sufficient to upregulate other differentiation-activating transcription factors, including OVOL1, PRDM1, ZNF750, and GRHL3. ATF3 overexpression in primary human keratinocytes diminished clonogenicity and led to epidermal tissue hypoplasia. Mechanistically, we found that SEC disruption by peptidomimetic inhibitors, or PKC signaling, disrupts CDK9 from HEXIM1 binding near the promoters, and releases active CDK9 to promote Pol II elongation in the gene bodies. Using co-immunoprecipitation, we further found that the interaction between AFF1 and HEXIM1 is specific to the progenitor state but not in differentiated keratinocytes. Thus, our data suggest that the activity switch of CDK9, in association with AFF1 and HEXIM1, underlies an early step of epidermal progenitor differentiation initiation, in rapid response to cellular signaling such as PKC activation.

ESDR547 - CDK9 Kinase Activation in Association with AFF1-SEC Initiate Epidermal Progenitor differentiation

The scientific meeting of the European Society for Dermatological Research (ESDR) is held yearly, usually in September. With an international attendance of more than 1000 skin scientists from around the globe, the four-day event includes short oral presentations from peer-reviewed abstracts, keynote lectures from international experts, sponsored symposia, satellite programs, educational events as well as an important focus on posters, networking and building collaborations amongst scientists. For more information about this year’s meeting visit our website.