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Differential regulation of p27Kip1 depending on culture conditions and its correlation with status of p14ARF and p53

p27Kip1 is known as a major cyclin‐dependent kinase inhibitor and a tumor suppressor, and often functionally hampered at protein level. p27 protein expression levels are frequently low in various cancers and negatively correlated with malignancy of cancer. However, in our previous study, we discovered that p27 overexpression does not inhibit the proliferation of two cancer cell lines due to a functional suppression of p27 by nucleophosmin isoform 1 (NPM1); that is, a qualitative, not quantitative, suppression of p27 function occurs in these cancer cell lines. To clarify the regulation of p27 in several types of cancer, we investigated p27 function in other cancer cell lines, based on proliferation assays in those cell lines carrying doxycycline‐inducible p27, and found that MDAH041 cells which express p14ARF, an antagonist of NPM1, show growth inhibition depending on p27 induction. Moreover, to investigate p27 function under anchorage‐independent culture conditions, we performed soft agar colony formation assay and observed that the colony formation of some cell lines carrying wild‐type p53, a major tumor suppressor, was inhibited depending on p27 induction. These results suggest that p27 function is regulated differentially among cancer cell types under anchorage‐dependent and anchorage‐independent culture conditions.