Contribution of the cyclin-dependent kinase inhibitor p27(KIP1) to the confluence-dependent resistance of HT29 human colon carcinoma cells

We have previously shown that growth of HT29 human colorectal cancer cells at confluence increased their resistance to the cytotoxic agent cisplatin. This study further explores the mechanisms of this resistance phenotype. DNA platination induced by cisplatin exposure is slightly reduced by confluence. However, at an equivalent DNA platination level, non-confluent cells accumulate in the G₂/M phase of the cell cycle, demonstrate aberrant mitotic figures and die by apoptosis, while confluent cells progress slowly through the cell cycle, do not reach mitosis and are more resistant to drug- induced cell death. At a molecular level, cisplatin enhances cyclin B and p34(cdc2) levels and histone HI kinase activity in non-confluent, but not in confluent, cells. Furthermore, when HT29 cells reach confluence, expression of the cyclin-dependent kinase inhibitor p27(KipI) increases and cells accumulate in the G₀/G₁ phase of the cell cycle. Transfection-mediated over-expression of p27(KipI) in non-confluent HT29 cells decreases the cytotoxic activity of cisplatin as well as its ability to trigger apoptosis. Non-confluent HT29 cells over-expressing p27(KipI) are also more resistant to doxorubicin, etoposide and 5-fluorouracil. Our results suggest that p27(KipI) contributes to the confluence-dependent resistance phenotype.