LACK OF RELATIONSHIP BETWEEN CDK ACTIVITY AND G(1) CYCLIN EXPRESSION IN BREAST-CANCER CELLS

The G(1) cyclins, cyclin D1 and E, are rate limiting for progression t hrough G(1) phase of the cell cycle in breast epithelial cells and are oncogenic when expressed in the mammary epithelium of transgenic mice . These genes are frequently overexpressed in clinical breast cancer w here overexpression appears to be associated with specific disease phe notypes, altered responsiveness to therapeutic intervention and patien t survival. In order to investigate the functional correlates of cycli n D1 and cyclin E overexpression we employed a panel of normal, immort alized and neoplastic breast epithelial cell lines to examine the rela tionships between cyclin gene expression, cyclin-CDK complex formation and CDK activity. In agreement with earlier studies cyclin D1 and E e xpression varied over an approximately tenfold range among the 18 cell lines studied. There was no apparent relationship, however, between c yclin D1 expression and the in vitro activity of its major kinase part ner, Cdk4, although MDA-MB-134 cells displayed the highest level of bo th cyclin D1 expression and Cdk4 activity. Similarly, there was no sig nificant relationship between cyclin E expression and cyclin E-Cdk2 ac tivity. Fractionation of whole cell lysates by gel filtration chromato graphy revealed that similar to 90% of the cyclin E protein was presen t in inactive complexes containing the CDK inhibitors p21 and p27. Muc h of the small fraction of active cyclin E protein was of very high ap parent molecular mass, >400 kDa, suggesting that formation of these co mplexes is a more important determinant of cyclin E-Cdk2 activity than cyclin E abundance. These data suggest that properties of cyclins D1 and E in addition to their ability to activate Cdk4 and Cdk2 may contr ibute to the effects of overexpression on the breast cancer phenotype.