Bromodomain protein BRD4 promotes cell proliferation in skin squamous cell carcinoma

The present study examined the expression and biological functions of bromodomain-containing protein 4 (BRD4) in skin squamous cell carcinoma (SCC) cells. Our results show that BRD4 mRNA and protein expression was upregulated in human skin SCC cells, as compared to its level in the normal skin keratinocytes and fibroblasts. Treatment with BRD4 inhibitors, JQ1 and CPI203, resulted in proliferation inhibition, apoptosis and cell cycle arrest in both established (A431 cell line) and primary skin SCC cells. Furthermore, BRD4 knockdown (by targeted shRNAs) or knockout (by CRISPR/Cas9) largely inhibited A431 cell proliferation. Reversely, forced-overexpression of BRD4 in A431 cells facilitated cell proliferation. We show that BRD4 is required for the expression of several oncogenes, including cyclin D1, Bcl-2 and MYC. BRD4 inhibition, knockdown or knockout significantly decreased above oncogene expression in SCC cells. In vivo, CRISPR/Cas9-mediated BRD4 knockout significantly suppressed A431 xenograft tumor growth in severe combined immunodeficient (SCID) mice. Together, our results suggest that BRD4 could be a novel and pivotal oncogenic protein of skin SCC.