Biphasic Regulation of Breast Cancer Cell Growth by Progesterone: Role of the Cyclin-Dependent Kinase Inhibitors, p21 and p27Kip1

Abstract

Depending on the tissue, progesterone is classified as a proliferative or a differentiative hormone. To explain this paradox, and to simplify analysis of its effects, we used a breast cancer cell line (T47D-YB) that constitutively expresses the B isoform of progesterone receptors. These cells are resistant to the proliferative effects of epidermal growth factor (EGF). Progesterone treatment accelerates T47D-YB cells through the first mitotic cell cycle, but arrests them in late G1 of the second cycle. This arrest is accompanied by decreased levels of cyclins D1, D3, and E, disappearance of cyclins A and B, and sequential induction of the cyclin-dependent kinase (cdk) inhibitors p21 and p27(Kip1). The retinoblastoma protein is hypophosphorylated and extensively down-regulated. The activity of the cell cycle-dependent protein kinase, cdk2, is regulated biphasically by progesterone: it increases initially, then decreases. This is consistent with the biphasic proliferative increase followed by arrest produced by one pulse of progesterone. A second treatment with progesterone cannot restart proliferation despite adequate levels of transcriptionally competent PR. Instead, a second progesterone dose delays the fall of p21 and enhances the rise of p27(Kip1), thereby intensifying the progesterone resistance in an autoinhibitory loop. However, during the progesterone-induced arrest, the cell cycling machinery is poised to restart. The first dose of progesterone increases the levels of EGF receptors and transiently sensitizes the cells to the proliferative effects of EGF. We conclude that progesterone is neither inherently proliferative nor antiproliferative, but that it is capable of stimulating or inhibiting cell growth depending on whether treatment is transient or continuous. We also suggest that the G1 arrest after progesterone treatment is accompanied by cellular changes that permit other, possibly tissue-specific, factors to influence the final proliferative or differentiative state.

Progesterone Stimulates Proliferation and Promotes Cytoplasmic Localization of the Cell Cycle Inhibitor p27 in Steroid Receptor Positive Breast Cancers

“…In agreement with studies in the rat, it has been shown that decreased nuclear p27 levels may facilitate proliferation in human breast cancer cell lines [18]. According to previous reports, prolonged treatment with PS (for more than 36 h) blocks proliferation in T47D cells due to accumulation of p27 protein [38]. In contrast, present observations in the rat mammary tumors in vivo showed that total cellular p27 protein was not increased by chronic E + P treatment.…”

Linkage of progestin and epidermal growth factor signaling: Phosphorylation of progesterone receptors mediates transcriptional hypersensitivity and increased ligand-independent breast cancer cell growth

“…5B). However, breast cancer cells are often insensitive to mitogenic stimulation by EGF alone (38). Thus, we were surprised to find significant ligand-independent actions of PR that also map to its transcriptional activity with regard to EGF-induced colony growth in soft agar (Fig.…”

“…T47D cells containing wt PR-B were not further stimulated to proliferate in response to EGF alone. This is not surprising, as breast cancer cell lines growing in vitro generally appear to be unresponsive to EGF alone (38), perhaps due to their relatively high basal growth rates (25-30% S phase cells) in control (starved) conditions. In support of this interpretation, EGF stimulated a weak response in S294A PR-containing cells.…”

“…Progestins have biphasic effects on breast cancer cell growth in vitro, with an initial peak of S phase entry (18 hrs) and proliferation that may last from one (38,39) to multiple rounds (40) of cell division followed by cell growth inhibition (38,39). The response to progestins is influenced by the presence of additional hormones, such as estrogens (40) or by growth factors, including EGF (38).…”

“…The response to progestins is influenced by the presence of additional hormones, such as estrogens (40) or by growth factors, including EGF (38). Additionally, long-term exposure to progestins may increase cell survival (41).…”