INCREASED EXPRESSION OF THE P27(KIP1) PROTEIN IN HUMAN ESOPHAGEAL CANCER CELL-LINES THAT OVER-EXPRESS CYCLIN D1

In the present study we have characterized eight human esophageal squa mous carcinoma cell lines for levels of expression of cyclins D1, E, A and B1; CDKs 1, 2 and 4; the CDK inhibitors p16(INK4), p21(WAF1) and p27(KIP1); th, retinoblastoma (Rb) protein; and in vitro CDK2- and CDK 4-associated kinase activity; and also compared the growth properties of these cell lines, The level of the cyclin D1 protein varied by over 30-fold amongst the eight cell lines, The high level in two cell line s was associated with amplification of this gene, but in three cell Li nes it was due to post-transcriptional events. Amongst the eight cell lines there was a significant correlation between the levels of cyclin D1, Rb and p27(KIP1) proteins, and CDK4-associated kinase activity, F urthermore, when an exogenous cyclin D1 cDNA was over-expressed in the EC109 cell line by transfection, this led to increased expression of both Rb and p27(KIP1). There was, however, no correlation between the level of cyclin D1 expression and the cell doubling times, duration of the G1 phase, or colony-forming efficiency in agar, Two of the cell l ines displayed a high level of the cyclin E protein, low levels of cyc lin D1, lacked expression of the Rb protein and expressed high levels of the p16(INK4) protein, One of these cell lines displayed amplificat ion of the latter gene, There was no correlation between the levels of cyclins E or A and in vitro CDK2 kinase activity, but CDK2 kinase act ivity was inversely correlated with the duration of the G1 phase of th e cell cycle, Taken together, these studies indicate marked heterogene ity in the expression of cell cycle-related proteins amongst a series of esophageal carcinoma cell lines, The correlation between the levels of the cyclin D1, Rb and p27(Kip1) proteins suggest the existence of a homeostatic feedback loop between positive and negative acting compo nents of the cell cycle machinery.