Retinoblastoma protein function and p16INK4a expression in actinic keratosis, squamous cell carcinoma in situ and invasive squamous cell carcinoma of the skin and links between p16INK4a expression and infiltrative behavior.

p16INK4a is involved in many important regulatory events in the cell and the expression and function is closely associated with the retinoblastoma protein (Rb). Earlier, we have in colorectal cancer and in basal cell carcinoma showed that p16INK4a is upregulated at the invasive front causing cell cycle arrest in infiltrative tumor cells via a functional Rb. This role for p16INK4a as a regulator of proliferation when tumor cells infiltrate might besides a general cyclin-dependent kinase (cdk) inhibitory effect explain why p16INK4a is deregulated in many tumor forms. The expression pattern of p16INK4a in relation to Rb-function in squamous cancer and precancerous forms of the skin has not been fully detailed. We therefore characterized the expression of p16INK4a, Rb-phosphorylation and proliferation in actinic keratosis, squamous cell carcinoma in situ and invasive squamous cell carcinoma with special reference to infiltrative behavior. The expression of p16INK4a varied between the lesions, with weak and cytoplasmic p16INK4a expression and functional Rb in actinic keratosis. Strong nuclear and cytoplasmic p16INK4a expression was observed in all carcinomas in situ in parallel with lack of Rb-phosphorylation but high proliferation indicating a nonfunctional Rb. Invasive squamous carcinoma showed a mixed p16INK4a expression pattern where some tumors had strong cytoplasmic p16INK4a expression, large fraction of Rb-phosphorylated cells and high proliferation. Interestingly, despite this disability of p16INK4a to inhibit proliferation there was an upregulation of cytoplasmic p16INK4a in infiltrative cells compared to tumor cells towards the tumor center. A similar scenario but strong and combined nuclear and cytoplasmic p16INK4a expression in infiltrative cells, was observed in other invasive squamous cancers. This suggests that the p16INK4a upregulation in infiltrative cells is governed independently of the subcellular localization or of the potential to affect proliferation via Rb, and suggests a potentially proliferation independent function for p16INK4a in infiltrative behavior.