C-terminally phosphorylated p27 drives cancer-promoting inflammation

Abstract

Abstract p27 normally inhibits cell cycle progression and cell growth. In cancers, deregulated p27 acts as an oncogene to promote metastasis when phosphorylated by PI3K-activated kinases at T157 and T198 (p27pTpT). We showed p27pTpT transcriptionally co-activates cJun and induces STAT3 activation. Here, we pursue the hypothesis that p27pTpT transcriptionally co-activates STAT3 target genes to expand cancer stem cells (CSC) and promote cancer inflammation. In triple negative breast cancer (TNBC) lines, we showed that p27pTpT increases stem cell properties, including sphere formation and expression of stem cell transcription factors MYC, OCT4, NANOG, and KLF4through STAT3. ChIP-seq/RNA-seq showed p27 recruits STAT3 to gene promoters to induce profiles of p27/STAT3 co-target genes, including MYCand JAG1, that increase tumor-initiating stem cells (TISC) in vivo. This work revealed a novel role for p27pTpT as a driver of CSC expansion. Since CSCs mediate treatment resistance, in part, by promoting inflammation, we next tested if p27pTpT drives cancer inflammation. p27/STAT3 co-target genes associate strongly with inflammatory pathways. ATAC-seq/ ChIP-seq showed p27pTpT increased chromatin accessibility to co-recruit p27/STAT3 to promoters of oncogenic, proinflammatory genes (IL-6, NF-κB/RELA, VEGFA)and to mediators of immune evasion. p27pTpT also increased STAT3 activation, p65 (NF-κB subunit), and IL-6 protein levels on WB, and increased CCL2 secretion on ELISA. Notably, both p27 knockdown and STAT3 inhibition decreased p27pTpT-driven CCL2 secretion. Together, these data indicate that when p27 is C-terminally phosphorylated, it acquires a novel oncogenic role to promote STAT3-driven CSC expansion and cancer inflammation.