Cellular factors may enable squamous carcinoma cells to overcome TGFβ-mediated repression of CDK2 activity

Cell lines developed from head and neck squamous cell carcinomas exhibit variable responses to the negative regulatory effects of transforming growth factor β (TGFβ) on cell growth. To analyse the effects of TGFβ on regulators of cell cycle progression, we characterised cell lines derived from head and neck squamous cell carcinoma (HNSCC) for their biological sensitivities to TGFβ, growth inhibition, then examined the effects of TGFβ treatment on the expression and activity of cyclin dependent kinases (CDKs) and inhibitors of these kinases. Western blot analysis of cell lysates from untreated or TGFβ-treated cultures showed no alterations in expression of CDK2, CDK4, CDK6 or cyclin E in cell lines which were either sensitive (HaCaT, HN6) or refractory (HN12, HN30) to the growth-inhibitory effects of TGFβ. However, treatment of cells with TGFβ resulted in a several fold increase in cellular levels of p21 (WAF1/Cip1), irrespective of biological response. Immune complex in vitro kinase assays demonstrated that the activity of CDK2 was inhibited by exposure to ligand in each case, confirming that a TGFβ signalling pathway which regulates kinase activity was intact in these cell lines. The data suggest that cellular factors expressed in HN12 and HN30 enable these cells to override TGFβ-mediated inhibition of CDK2 activity and allow cell cycle progression. This may represent an important mechanism which allows cells to evade growth arrest during malignant progression.