ErbB2/Neu-Induced, Cyclin D1-Dependent Transformation Is Accelerated in p27 -Haploinsufficient Mammary Epithelial Cells but Impaired in p27 -Null Cells

Abstract

ABSTRACT ErbB2/Neu destabilizes the cyclin-dependent kinase (Cdk) inhibitor p27 and increases expression of cyclin D1. Therefore, we studied the roles of p27 and cyclin D1 in ErbB2-mediated mammary epithelial cell transformation. Overexpression of ErbB2 or cyclin D1 in p27 +/− primary murine mammary epithelial cells resulted in increased proliferation, cyclin D1 nuclear localization, and colony formation in soft agar compared to those in p27 +/+ cells. In contrast, ErbB2- or cyclin D1-overexpressing p27 −/− cells displayed reduced proliferation, anchorage-independent growth, Cdk4 activity, cyclin D1 expression, and cyclin D1 nuclear localization compared to wild-type cells. A cyclin D1 mutation in its nuclear export sequence (T286A) partially rescued nuclear localization of cyclin D1 in p27 −/− cells but did not increase proliferation or Cdk4 kinase activity. Overexpression of E2F1, however, increased proliferation to the same degree in p27 +/+ , p27 +/− , and p27 −/− cells. Mammary glands from MMTV (mouse mammary tumor virus)- neu/p27 +/− mice exhibited alveolar hyperplasia, enhanced proliferation, decreased apoptosis, and accelerated tumor formation compared to MMTV - neu/p27 +/+ glands. However, MMTV - neu/p27 −/− glands showed decreased proliferation, cyclin D1 expression, and Cdk4 activity, as well as markedly prolonged tumor latency, compared to MMTV - neu/p27 +/+ glands. These results suggest that p27 +/− mammary epithelium may be more susceptible to oncogene-induced tumorigenesis, whereas p27 -null glands, due to severely impaired cyclin D1/Cdk4 function, are more resistant to transformation.