The pro-tumorigenic activity of p38 gamma overexpression in nasopharyngeal carcinoma

It is urgent to identify and validate biomarkers for early diagnosis and efficient treatment of nasopharyngeal carcinoma (NPC). Recent studies have proposed p38 gamma (p38 gamma) as a cyclin-dependent kinase (CDK)-like kinase that phosphorylates retinoblastoma (Rb) to promote cyclins expression and tumorigenesis. Here the Gene Expression Profiling Interactive Analysis (GEPIA) database and results from the local NPC tissues demonstrate that p38 gamma is significantly upregulated in NPC tissues, correlating with poor overall survival. Furthermore, p38 gamma mRNA and protein expression is elevated in established NPC cell lines (CNE-1 HONE-1 and CNE-2) and primary human NPC cells, but low expression detected in human nasal epithelial cells. In established and primary NPC cells, p38 gamma depletion, using the shRNA strategy or the CRISPR/Cas9 gene-editing method, largely inhibited cell growth, proliferation and migration, and induced significant apoptosis activation. Contrarily, ectopic p38 gamma overexpression exerted opposite activity and promoted NPC cell proliferation and migration. Retinoblastoma (Rb) phosphorylation and cyclin E1/A expression were decreased in NPC cells with p38 gamma silencing or knockout, but increased after p38 gamma overexpression. Moreover, mitochondrial subcellular p38 gamma localization was detected in NPC cells. Significantly, p38 gamma depletion disrupted mitochondrial functions, causing mitochondrial depolarization, reactive oxygen species production, oxidative injury and ATP depletion in NPC cells. In vivo, intratumoral injection of adeno-associated virus-packed p38 gamma shRNA potently inhibited primary human NPC xenograft growth in nude mice. In p38 gamma shRNA virus-injected NPC xenograft tissues, p38 gamma expression, Rb phosphorylation, cyclin E1/A expression and ATP levels were dramatically decreased. Taken together, we conclude that p38 gamma overexpression is required for NPC cell growth, acting as a promising therapeutic target of NPC.