High Cks1 expression in transgenic and carcinogen-initiated mammary tumors is not always accompanied by reduction in p27Kip1.

Cks1 plays an essential role in SCF Skp2 -mediated ubiquitination, and consequently turnover, of the cdk2 inhibitor and tumor supressor p27 Kip1 . High Cksl expression is associated with aggressive breast tumors and correlates with low p27 Kip1 levels in some cases, although it is also an independent prognostic marker for survival, and provides predictive information in addition to that provided by p27 Kip1 alone. In this report we demonstrate that Cksl protein and mRNA are elevated to very high levels in mammary tumors initiated by erbB2, c-myc and polyoma middle-T (PyMT) in transgenic mice, whereas Cksl protein is hardly detectable in the normal mammary epithelium. Cksl is also highly up-regulated in rat mammary tumors initiated by methylnitrosourea (MNU). Despite high levels of Cksl expression, p27 Kip1 levels were not reduced, and were in fact slightly higher in mammary tumors initiated by erbB2, PyMT and MNU. In contrast mammary tumors from MMTV-c-myc mice did exhibit low p27 Kip1 and higher levels of Skp2. Together, these data suggest that deregulated Cksl expression might play roles in oncogene and carcinogen-initiated mammary tumorigenesis independent of p27 Kip1 turnover in certain tumors. Stable overexpression of Cksl in human breast carcinoma MCF-7 cells did not significantly reduce p27 Kip1 expression, although it conferred resistance to Faslodex (ICI 182780)-mediated inhibition of colony outgrowth in these cells. In contrast, Cksl-depleted MCF-7 cells formed fewer colonies in estrogen-containing medium. Therefore, our studies also suggest that Cksl levels regulate the responsiveness of ER + breast cancers to estrogens and anti-estrogens.