Cyclin-dependent kinase (cdk) inhibitors/cdk4/cdk2 complexes in early stages of mouse mammary preneoplasia

The level of circulating ovarian hormones (estrogen and progesterone) alone or in combination with pituitary hormones have a potent mitogenic impact i n the normal mammary gland, and they also play a pivotal role in the develo pment and progression of mammary carcinoma. The differential effects of hor mones on the molecular components of cyclin-dependent kinase (cdk) complexe s in mammary epithelium of the hormone-dependent ductal outgrowth line, EL1 1, and the hormone-independent alveolar outgrowth line, TM2L, were the focu s of this study. The two outgrowth lines, which represent early stages in m ammary hyperplasia, were compared with normal mammary gland at different ho rmonal conditions: control, hormone stimulated by pituitary isograft, and h ormone depleted by ovariectomy. Hormonal stimulation by a pituitary isograf t resulted in DNA synthesis and lobuloalveolar development of normal mammar y ducts, DNA synthesis but no lobuloalveolar development in the EL11 ductal outgrowth, and no changes either in DNA synthesis or in lobuloalveolar mor phology in the TM2L outgrowth. The levels of cdk4- and cyclin D1-associated kinase activities were correlated with cell proliferation in only the alve olar phenotypes (i.e., in only hormonally stimulated normal virgin gland an d in alveolar mammary outgrowth), whereas cyclin DP-dependent kinase activi ty was correlated with cell proliferation in only the alveolar preneoplasia . p16(INK4a) and p21(Cip1) protein levels were decreased at the earliest st ages of preneoplasia, i.e., at immortalization, and were independent from c hanges in cyclin D1, which occurred later in preneoplasia, Although all cdk inhibitors changed in concordance with hormonal status reflected by prolif eration levels, p27(Kip1) Was the only cdk inhibitor that was up-regulated at the earliest stages of preneoplasia and may have a unique role in blocki ng alveolar differentiation in response to the loss of one or more of the c ell cycle-negative regulators. We hypothesize that up-regulation of p27(Kip 1) prevents immortalized ductal outgrowths (EL11) from progressing to the n eoplastic state, even under hormonal stimulation.