The Opioid Growth Factor (OGF)-OGF receptor axis uses the p16 pathway to inhibit head and neck cancer

Head and neck squamous cell carcinoma (HNSCC) represents 5.5% of malignancies worldwide, with ∼ 30,000 new cases and ∼ 11,000 deaths reported in the United States annually. The opioid growth factor (OGF; [Met ⁵]-enkephalin) and the OGF receptor (OGFr) form an endogenous growth regulating system; the OGF-OGFr axis influences the G₀-G₁ phase of the cell cycle in HNSCC. Cells treated with small interfering RNA (siRNA) for OGFr no longer responded to the growth inhibitory effects of OGF or the growth stimulatory effects of naltrexone, indicating that these activities are entirely mediated by OGFr. In this investigation, we examined the precise target of OGF in the cell cycle. Using SCC-1 cells, OGF decreased the phosphorylation of retinoblastoma protein. This change was correlated with reduced Cdk4, but not Cdk2, kinase activity. OGF treatment increased cyclin-dependent kinase inhibitor p16 protein expression. Importantly, p16 complexed with Cdk4 was increased by OGF treatment at all time points, consistent with the hypothesis that OGF mediated growth inhibition through p16. Blockade of OGF-OGFr interactions with naloxone abolished the increased expression of p16 protein by OGF. Inhibition of p16 (INK4a) activation by p16-specific siRNA blocked OGF's repressive action on proliferation of SCC-1, CAL-27, and SCC-4 HNSCC cells. These data are the first to reveal that the target of cell proliferative inhibitory action of OGF in human HNSCC is a cyclin-dependent kinase inhibitory pathway, and this may be useful in the diagnosis and treatment of HNSCC.