Osteopontin promotes a cancer stem cell-like phenotype in hepatocellular carcinoma cells via an integrin-NF-κB-HIF-1α pathway

There is increasing evidence to suggest that hepatocellular carcinomas (HCCs) are sustained by a distinct subpopulation of self-renewing cells known as cancer stem cells. However, the precise signals required for maintenance of stemness-like properties of these cells are yet to be elucidated. Here, we demonstrated that the level of oncoprotein osteopontin (OPN) in tumor cells of the edge of bulk tumors was significantly correlated with the clinical prognosis of patients with HCC. OPN was highly expressed in side population fractions of HCC cell lines, as well as in dormant cells, spheroids and chemo-resistant cancer cells, all of which are considered as having stemness-like cellular features. Depletion of OPN in HCC cell lines resulted in a reduction in the proportion of side population fractions, formation of hepatospheroids, expression of stem-cell-associated genes and decreased tumorigenecity in immunodeficient mice. Mechanistically, OPN was demonstrated to bind to integrin alpha(v)beta(3) and activate the transcription factor NF-kappa B, which resulted in upregulation of HIF-1 alpha transcription and its downstream gene, BMI1, to mediate maintenance of the stemness-like phenotype. Suppression of the av beta 3-NF-kappa B-HIF-1 alpha pathway decreased OPN-mediated self-renewal capabilities. Levels of OPN protein expression were significantly correlated with HIF-1 alpha protein levels in HCC tumor tissue samples. OPN might promote a cancer stem cell-like phenotype via the av beta 3-NF-kappa B-HIF-1 alpha pathway. Our findings offer strong support for OPN requirement in maintaining stem-like properties in HCC cells.