Proliferation, Invasion and p16 Localisation in Malignancies

Abstract: Uncontrolled proliferation and invasion are two characteristics in tumour development and progression. Previous studies have shown that invading tumour cells in colorectal cancer have low proliferation in parallel to upregulation of the tumour suppressor p16. We set out to detail the p16 expression and proliferation in basal cell carcinoma and squamous cell carcinoma of the skin, tumours that are locally infiltrative and aggressive. p16 was upregulated at the invasive margin compared to central parts in both tumours, and in basal cell carcinoma there was also a decreased proliferation in infiltrating p16 high tumour cells. In squamous cell carcinoma the Rb-pathway was non-functional, probably due to inactivation of Rb or a de-localisation of p16 to the cytoplasm, and p16 could consequently not inhibit proliferation in infiltrating cells. In contrast to the malignant skin tumours, p16 was commonly expressed in central parts of the tumour islands in cylindroma, and in this benign slow growing tumour p16 seemed important in restraining the proliferative activity of the tumour cells. Further, by subcellular fractionation we confirmed that p16 was expressed in both the nucleus and in the cytoplasm in tumour cell lines, and the protein could bind to cdk4/6 in both subcellular compartments, even if it seemed incapable of inhibiting proliferation. Investigating post-translational modifications of p16 revealed at least two forms of p16 in cell lines, as well as in skin tumours. We further investigated the relation between tumour cell proliferation and migration in a breast cancer cell line and observed that resting cells were more migratory than actively cycling cells. This was also confirmed in a cohort of primary breast tumours, where proliferation and infiltrative growth pattern were inversely correlated. However, the actively cycling breast tumour cells could be rendered more migratory upon exposure to macrophage like stimuli. Thus, in some tumour cells, proliferation and invasion/migration seem to be contrasting events. However, in other tumours, potentially with more severe aberrations in cell cycle regulatory pathways, the cells can proliferate and migrate concurrently.