INTRODUCTION
The effects of progesterone in the breast are controversial due to the structural and developmental complexity of this organ. Depending on the experimental model system, the cell context, and the duration of treatment, progesterone can elicit either proliferative or antiproliferative effects on breast epithelial cell growth. The multiple secondary hormonal factors, which regulate breast cell growth and development in combination with progesterone, contribute to this conundrum. We propose that these seemingly contradictory effects of progesterone can be explained by virtue of its ability to act as a priming factor for the actions of secondary agents. The purpose of this minireview is to highlight recent data suggesting that the strength and duration of progesterone signaling determine whether it acts as a proliferative or inhibitory agent and that progestin pretreatment sets the stage for enhanced activity of locally acting cytokines and growth factors through synergy with their signaling pathways. Our intention is not to review the extensive literature regarding the growth-altering properties of progesterone; we refer the reader to several reviews on the subject (1–4). Rather, here we would like to develop the concept that progesterone is a priming factor in the context of breast cancer progression; progesterone pretreatment promotes a switch from growth driven primarily by steroid hormones to growth driven primarily by peptide growth factors. Thus, the priming effects of progesterone may contribute, in part, to the development of steroid hormone resistance during breast cancer progression. It is our hope that these ideas will invite further discussion regarding the role of progesterone in breast cancer growth and progression, thus complementing the much better understood role of estradiol in this process.
PROGESTERONE AND BREAST CELL PROLIFERATION
The requirement for progesterone in normal mammary gland lobular-alveolar development is well established (5). However, its role in the mature, precancerous, and cancerous breast remains poorly defined. During the cyclical hormonal changes that characterize the normal menstrual cycle, breast epithelium undergoes increased DNA synthesis associated with mitosis. This peaks in the late luteal phase, at a time when circulating levels of progesterone are highest, providing evidence that progesterone is a proliferative hormone in the breast. Consistent with this observation, the high progesterone levels during pregnancy induce further lobular-alveolar development in preparation for lactation, indicating both a proliferative and differentiative function for this hormone. Other studies of the effects of progesterone on normal breast epithelial cell growth have, however, produced equivocal results (reviewed in Refs. 2, 4). Progesterone increases DNA synthesis in organ culture, decreases or has no effect on the growth of explants in nude mice, and decreases proliferation in primary cell cultures of normal breast epithelium and cultured breast cancer cells (reviewed in Ref. 1). In vivo studies involving treatment of patients with high doses of progesterone before breast surgery show fewer mitotic figures when compared with estrogen alone, or estrogen plus progesterone, and high-dose progestins are effective second-line therapies for patients whose tumors are hormone responsive. Apparently, progesterone can be both proliferative and antiproliferative. Is there a unifying hypothesis that can reconcile this paradox?
PROGESTERONE EFFECTS ON THE CELL CYCLE: ROLE OF TREATMENT TIME AND DOSE
Studies using human breast cancer cell lines have provided valuable insight into the paradoxical effects of progestins on cell proliferation, with the demonstration of clear biphasic effects on cell cycle progression (4, 6, 7). Hissom and Moore (8) first reported the proliferative effect of progesterone in T47D cells. Indeed, the immediate response of asynchronous cultured T47D human breast cancer cells to a single pulse of progesterone is proliferative (Fig. 1); there is transient induction of genes associated with cell cycle progression, with acceleration of cells through one mitotic cycle. Early (0–12 h) changes include increased hyperphosphorylated pRb, increased expression of cyclins D1, D3, E, A, and B, activation of cyclin-CDK2 and -CDK4 complexes, and induction of c-myc and c-fos mRNAs (9). Levels of the cyclin-dependent kinase (CDK) inhibitors, p21 and p27, gradually rise during the proliferative phase of progestin treatment, peaking after the end of the first G1/S transition (6). Interestingly, induction of cyclin D1 alone, using an inducible promoter expression system, mimics the effects of progestins and accelerates G1-phase progression with kinetics similar to those of progesterone (10). Studies using transgenic and knockout mice have further demonstrated the absolute requirement for cyclin D1 activation in the mitogenic response of the mammary gland to progestins (11, 12). Progestins have also been reported to decrease expression of the p53 tumor suppressor protein, possibly contributing further to their proliferative effects (13). In sum, a single pulse of progesterone is transiently growth stimulatory.
Model for the Priming Effects of Progesterone on Breast Cancer Cell Growth A single dose of progesterone accelerates breast cancer cells through one round of cell division, followed by growth arrest at the G1/S boundary of the second cell cycle. Panel 1, A single dose of progesterone (P1) has biphasic effects, first stimulating growth (0–24 h) and then inhibiting it (48 h). Eventually the cells recover (not shown). Continuous treatment with multiple doses of progesterone (P2−4) chronically inhibits cell growth. Panel 2, Progesterone-induced arrest at the G1/S boundary defines a decision point at which secondary factors, including growth factors and cytokines, can induce breast cells to undergo proliferation, differentiation, or involution. This decision is influenced by cross-talk between progesterone and growth factor/cytokine-mediated signaling pathways outlined in this review. A growth-stimulatory secondary factor is depicted, but alternatively cells may, in theory, be primed for the actions of differentiative or apoptotic stimuli, depending on the cell context and hormonal milieu.
However, despite the initial proliferative burst, subsequent effects of a single pulse of progesterone are growth inhibitory (Fig. 1), characterized by arrest in the late G1-phase of the second cell cycle (4, 6, 7). This growth arrest is accompanied by a reversal of the above cell cycle parameters. Thus, 24–48 h after the start of progesterone treatment, cyclins D1, D3, and E decrease, cyclins A and B plummet, and pRb and p107 (14) become hypophosphorylated and eventually disappear (4). Most importantly, p21 and p27 are sequentially induced by progestins, and reach peak levels by 24–48 h, with concomitant decreases in cyclin-CDK activities (6). As a result, 48 h after a single pulse of progesterone, cell growth stops. Interestingly, a second dose of progesterone, administered 48 h after the first, fails to restart proliferation, despite the presence of high levels of transcriptionally functional progesterone receptors (PR). Rather, sequential doses of progesterone actually prolong the growth arrest by further increasing the levels of p21 and p27 (6). We believe that continuous progestin exposure, from either multiple sequential treatments with natural progesterone, or from treatment with high doses of nonmetabolizable synthetic progestins, produces sustained autoinhibition, due in part to the fact that transcriptionally competent progestin-occupied PR induce high levels of the inhibitors, p21 and p27. On the other hand, if a second dose of progesterone does not follow rapidly on the heels of the first, the cells recover, resume growing, and reacquire sensitivity to the proliferative effects of progesterone.
Based on this biphasic response to a pulse of progesterone, we propose that transient or intermittent doses of progesterone are growth stimulatory, while continuous or sustained high-dose progesterone is growth inhibitory (Fig. 1). Such a model has far-reaching implications for the timing of progestin treatments in clinical settings and predicts that the effects of continuously administered progestins differ significantly from those of episodically or cyclically administered progestins. It may also explain why physiological levels of progesterone may have different proliferative effects than high-dose progestins. This model also suggests that the endogenous cyclical progesterone of the menstrual cycle may have different physiological consequences than the continuous progesterone of pregnancy. Clearly then, the dose and timing of progestin treatments may prove to be critical in determining the experimental or clinical outcome.
PROGESTERONE PRIMES CELLS FOR THE ACTIONS OF GROWTH FACTORS
The sequential stimulatory, followed by inhibitory, effects of progestins on breast cancer cell growth suggest that the biphasic response is part of a single signaling cascade. Sutherland et al. (4) suggest that one round of cell division followed by acute growth inhibition at the G1/S boundary reflects progestin-induced differentiation, a one-process model. However, they also recognize the possibility that the biphasic effects of progestins on breast cell growth are mediated by distinct mechanisms that, under differing conditions, lead to predominance of either a stimulatory or inhibitory response pathway. Recent results from our laboratory appear to blend these two possibilities (6, 15, 16). We propose that progesterone acts as a priming agent. In this scenario, progesterone treatment brings about a dual response: first, it drives cells to a decision point at the G1/S boundary, and second, it induces cellular changes that permit other factors, possibly tissue-specific, to influence the ultimate proliferative or differentiative state of the cells. Whether to grow, differentiate, or die (as in the case of involution at the end of lactation), is thus determined by the cell context and the endogenous hormonal milieu. This model suggests that breast cells can be directed toward one path or another by cross-talk between various signaling pathways and the progesterone/PR complex (Fig. 1). We outline below some evidence for such cross-talk.
A clear demonstration of the concept that progesterone can function as a priming factor comes from studies involving pretreatment of breast cells with progesterone, followed by treatment with growth factors or cytokines. For example, despite the fact that epidermal growth factor (EGF) receptors are present and functional in T47D human breast cancer cells, EGF is not mitogenic in progestin-naive cells. However, after priming by progestins for approximately 48 h, the cells become highly sensitive to the proliferative effects of EGF, despite loss of pRb protein and high levels of p21 (6). This response is perhaps not surprising, since progestin-mediated up-regulation of EGF receptors and other type I tyrosine kinase receptor family members (c-erbB2 and c-erbB3) is well documented; overexpression of these receptors is predictive of a poor prognosis in breast cancer patients (reviewed in Ref. 17). Progestins also up-regulate PRL receptors, insulin-like growth factor receptors, transforming growth factor-α receptors, fibroblast growth factors, and insulin receptors (reviewed in Ref. 17), thereby perhaps pushing cells toward a proliferative path in the presence of the appropriate growth-stimulatory ligands, which can bypass the G1/S block produced by progesterone alone (Fig. 1). For example, while progesterone alone inhibits the growth of T47D cells, cotreatment with insulin leads to synergistic induction of cell growth (18, 19). Estrogen and progesterone augment growth responsiveness of mammary tissue to cAMP, via increases in cAMP-dependent protein kinase activity (20). Thus, progesterone can greatly increase the sensitivity of breast cells to locally acting mitogens. Furthermore, in addition to increasing the sensitivity of cells to growth factors by increasing the levels of their receptors, progestins also enhance the sensitivity of their downstream signaling pathways (see below).
CONVERGENCE OF PROGESTERONE SIGNALING WITH GROWTH FACTOR AND CYTOKINE SIGNALING
Progestin/PR-mediated sensitization of breast cancer cells to growth factors and cytokines occurs at multiple levels in mitogenic signaling cascades and contributes to a biochemical switch in growth regulation (15, 16). We will discuss two distinct pathways through which progesterone/PR amplify epidermal growth factor and cytokine signaling: one involves mitogen- activated protein kinase (MAPK), the other involves signal transducers and activators of transcription (STATs).
MAPKs
How does EGF push progestin-arrested cells past the G1/S boundary in the face of low hypophosphorylated pRb and rising p21/p27 levels? We recently reported that pretreatment with progestins can selectively potentiate EGF-stimulated MAP kinase activities, leading to synergistic up-regulation of cell cycle proteins required for transition past the G1/S boundary (Fig. 2A). Interestingly, EGF has no effect on cyclin E expression in progestin-naive cells, but up-regulates cyclin E after progestin pretreatment. Cyclin E is required for entry into S-phase and, in contrast to cyclin D1, cyclin E can promote G1/S transition even in cells lacking functional pRb protein (21). Accumulation of active cyclin E/CDK2 complexes is controlled by Myc and Ras (22). Progestins are known to increase the expression of c-myc (4, 9), and the human c-myc promoter contains a progesterone response element (PRE) (4, 23). This may explain how EGF, via Ras/MAPKK/MAPK activation, can regulate cyclin E only in progestin-pretreated cells (Fig. 2), where c-myc is also up-regulated. Similarly, only progesterone-pretreated T47D breast cancer cells undergo proliferation in response to EGF, despite low levels of hypophosphorylated pRb (6). We speculate that EGF-stimulated cell-cycle reentry of cells that are growth arrested after progestin treatment may be mediated by a MAPK- and cyclin E-dependent, pRb-independent, pathway. Consistent with this idea is the finding that inhibition of MAPK by the MEK inhibitor (PD98059) in the presence of progestin and EGF results in levels of p21, cyclin D1, and cyclin E that fall below the control levels seen with EGF alone (Fig. 2B). Thus, while the levels of cyclin D1, cyclin E, and p21 are up-regulated by progestins alone in a MAPK-independent manner, pretreatment with progestin followed by EGF synergistically up-regulates these proteins via a MAP kinase-dependent mechanism. These findings support the concept that, as a consequence of progestin pretreatment or priming, the regulation of key cell cycle proteins switches from MAPK-independent to MAPK-dependent mechanisms.
Cross-Talk between Progesterone and EGF A, R5020 pretreatment amplifies the up-regulation of cell cycle proteins by EGF. T47D-YB breast cancer cells were pretreated without (C, EtOH vehicle) or with the progestin R5020 (R, 10 nm) for 48 h, followed by EGF (E, 30 ng/ml), for an additional 8 h. Regulation of p21, cyclin D1, and cyclin E protein levels was monitored by immunoblotting using specific antisera. Four data points were quantified by densitometric analysis; each bar represents the relative induction of protein levels using an arbitrary scale. B, Progestin pretreatment induces a biochemical switch in the regulation of cell-cycle proteins from MAPK-independent to MAPK-dependent pathways. The above experiments (panel A) were repeated in the presence or absence of the MEK inhibitor, PD98159 (20 μm). Solid bars received dimethyl sulfoxide vehicle control; striped bars received the MEK inhibitor, PD98059. Each bar represents the induction of protein levels in EGF-treated cells (above that of untreated controls), in the absence (EtOH vehicle) or presence of R5020. These data show induction of cell-cycle proteins in the absence of the MEK inhibitor (solid bars), under conditions in which MAPKs are active. However, when MAPK activation is blocked (striped bars), cell cycle protein induction by EGF alone (without R5020) can proceed, but further induction with R5020 is completely blocked. In fact, there is no MAPK-independent regulation of these proteins in the presence of R5020. For details, see original immunoblots showing regulated changes in cell cycle proteins (15 ).
STATs and Other Transcription Factors
Additional key downstream mediators of growth factor and cytokine action are also regulated by progesterone/PR, including members of the STAT family (16). STATs, as their name implies, have dual functions. They reside in the cytoplasm and are recruited to, and activated by, type I tyrosine kinase receptors after binding of growth factors or by soluble tyrosine kinases of the Janus kinase (JAK) family after binding of cytokines to their receptors. Activated STATs then translocate to the nucleus, where they bind DNA and regulate gene transcription (24). STATs lack an identifiable nuclear localization signal, and the mechanism of their translocation is unclear. In the absence of growth factors, we have found that progesterone treatment promotes the nuclear translocation of Stat5 (16). Since Stat5 and PR interact, as measured by their association in coimmunoprecipitates, we speculated that Stat5 is translocated to the nucleus as a companion to PR (16). In fact, Wyszomierski et al. (25) recently reported that ligand-bound GR translocate Stat5 into the nucleus and enhance Stat5 binding to DNA. In breast cancer cells, progesterone has other interesting effects on STAT signaling; Stat5 and JAK2 are phosphorylated and/or recruited to phosphotyrosine-containing proteins after a single dose of progestin (15, 16). Additionally, Stat5a, Stat5b, and Stat3 proteins are up-regulated, while Stat1 is down-regulated, by progestin treatment in a PR-dependent manner (16).
Consistent with the concept of progesterone priming, Stat5 proteins and mRNAs are up-regulated during pregnancy, when progesterone levels are high (discussed in Ref. 16). Indeed, EGF and PRL can only induce tyrosine phosphorylation of Stat5 in T47D breast cancer cells after progestin pretreatment (16). Like PR and cyclin D1, Stat5a is required for complete mammary gland lobular alveolar growth and lactation (26). Other transcription factors, such as members of the CCAAT/enhancer-binding protein family (C/EBP) are also up-regulated by progesterone in a PR-dependent manner (J. K. Richer and K. B. Horwitz, in preparation). C/EBP-β is involved in lobular alveolar outgrowth during differentiation of the mammary gland (27, 28). The remarkable temporal importance of PR, cyclin D1, Stat5, and C/EBP-β in normal growth and development of the mammary gland suggests the intriguing possibility that disregulation of these factors contributes to breast pathology.
Of note, the progestin-induced changes in growth factor/cytokine receptors, and STAT and C/EBP levels, occur after induction of cell-cycle proteins and the transient proliferative burst caused by a single dose of progesterone (see above). These changes are therefore unlikely to be involved in the early growth response. Rather, progesterone may sensitize normal or malignant breast cells to subsequent actions of growth factors and/or cytokines, with the potential for synergistic responses. Thus, in addition to cross-talk at the level of cell-surface receptors, and at the level of their signaling intermediates, this model suggests a third level of regulation – directly on the transcription complex.
SYNERGISTIC ACTIVATION OF GROWTH-REGULATORY GENES BY PROGESTERONE AND GROWTH FACTORS
Although growth factors/cytokines function at the cell surface and steroid hormones act primarily in the nucleus, triggering different signal transduction pathways, the resulting signals often converge on the same subset of genes. Transcriptional synergy on the β-casein promoter in the presence of PRL and glucocorticoids is mediated via an interaction between glucocorticoid receptors (GR) and Stat5 (29). Transcriptional synergy between EGF and progesterone has been reported using a minimal promoter containing a PRE as well as on the complex PREs of the mouse mammary tumor virus (MMTV) promoter (30). We recently reported transcriptional synergy between EGF and progesterone on the promoters that drive the growth-regulatory genes, c-fos and p21 (Fig. 3 and Ref. 16), neither of which contains a PRE. However, both STAT and C/EBP sites are present in these promoters. When transfected into breast cancer cells, the c-fos and p21 promoters are modestly responsive to progesterone or EGF alone, while both hormones added simultaneously elicit a profound synergistic response (Fig. 3). We showed that the progesterone response of the p21 promoter is mediated by tethering of PR to transcription factor Sp1 and CBP (31). Similarly, glucocorticoid regulation of p21 (32, 33) and other promoters (34) lacking glucocorticoid response elements maps to C/EBPβ sites. Thus, evidence is accumulating that PR and GR can regulate the activities of genes that are simultaneously regulated by growth factors, even when the promoters lack classical DNA binding sites for the steroid receptors. The ability of GR and the AP-1 and NFκB transcription factors to interact on target promoters lacking direct binding sites for at least one of the factors has recently been reviewed (35). Transcriptional cross-talk on promoters that lack DNA- binding sites for steroid receptors may be a more common mechanism of steroid hormone action than previously thought. Of note is the additional observation that cross-talk does not always imply transcriptional synergy; frequently, cross-talk results in interference between the actions of two hormonal agents, as in the case of glucocorticoid repression of the inflammatory response mediated by interleukins and interferons (reviewed in Ref. 35). Clearly, further studies are required to define the consequences of such complex interactions.
![Progesterone and EGF Synergize on the p21 and c-fos Promoters Triplicate cultures of T47D-YB cells were transfected either with the c-fos promoter (−357 to −276)-luciferase reporter construct, or the p21 promoter (−2320 to +1)-luciferase reporter construct, and treated with EtOH vehicle, progesterone alone (10 nm), EGF alone (10 nm), or simultaneously with both progesterone plus EGF. Twenty-four hours later, luciferase activity was measured in cell lysates. Fold increases in luciferase activity are indicated above the bars. Error bars represent the sem, and results are representative of five independent experiments. [This figure was modified from Ref. 16.] Note that simultaneous treatment with progesterone plus EGF produces strong transcriptional synergism, as compared with the modest synergism observed upon progestin pretreatment at the protein level (Fig. 2).](https://oup.silverchair-cdn.com/oup/backfile/Content_public/Journal/mend/13/6/10.1210_mend.13.6.0290/2/m_mg0690290003.jpeg?Expires=1724445546&Signature=MnFUKSHIn1pcMLNKnCUJeu8Q38UD3mcKnRqhfjmSieUS0iLZIK9VOLw8-G6x4S8tfqcHaTKLf~XB8lCAP1W-F3Q2aENLlDWSYep1ZDNZDBzKnmiNFBjriracXl2jDaGzLI~qcXnNAQPVqotbNLEiIA6fRV7kIO7ZG1UDkt6oGLxkQPQGXATGmFg7d~ZC6752Bvgd5JYMMAYai9vEMUX358~FJyjQHabIv0qL3Afe4i~aBaYb3VjhywihcN1rPQi1hT8OnLYb3yjOd762o7f7WwxSMVJhtNdAxYXzudIAAjL8YoWGTAosxS46N94EEJJeEFaP8Qm53nF0RL7S9VjM8A__&Key-Pair-Id=APKAIE5G5CRDK6RD3PGA)
Progesterone and EGF Synergize on the p21 and c-fos Promoters Triplicate cultures of T47D-YB cells were transfected either with the c-fos promoter (−357 to −276)-luciferase reporter construct, or the p21 promoter (−2320 to +1)-luciferase reporter construct, and treated with EtOH vehicle, progesterone alone (10 nm), EGF alone (10 nm), or simultaneously with both progesterone plus EGF. Twenty-four hours later, luciferase activity was measured in cell lysates. Fold increases in luciferase activity are indicated above the bars. Error bars represent the sem, and results are representative of five independent experiments. [This figure was modified from Ref. 16.] Note that simultaneous treatment with progesterone plus EGF produces strong transcriptional synergism, as compared with the modest synergism observed upon progestin pretreatment at the protein level (Fig. 2).
RECONCILING THE PRIMING EFFECTS OF PROGESTINS WITH p21 REGULATION
The role of p21 in the dual control of breast cancer cell growth by progestins and EGF is of interest. In T47D cells, increased p21 expression is associated with cell growth arrest in response to progestins alone (discussed above). However, after release from this arrest by EGF, p21 levels increase even further (Fig. 2), and progesterone plus EGF synergize to enhance p21 promoter-driven transcription (Fig. 3). This effect of progesterone plus EGF appears to be paradoxical, given the classical CDK-inhibitory/differentiative properties of p21. It is now recognized, however, that p21 has multifunctional actions that can contribute to antidifferentiative, antiapoptotic, or proliferative cellular responses. For example, forced expression of p21 inhibits terminal differentiation of primary mouse keratinocyctes, a function that is separable from the ability of p21 to inhibit cyclin-CDK complexes (36). Furthermore, p21 binds to and inactivates JNK (37), a mediator of apoptosis in certain systems; elevated levels of p21 may contribute to androgen-independent growth of prostatic carcinoma cells via an antiapoptotic mechanism (38). Additional proliferative actions of p21 may be explained by the finding that at low concentrations, p21 acts as a nucleation factor that assembles and activates cyclin D-CDK kinase complexes, while higher concentrations of p21 inhibit a constant level of cyclin-associated CDK activity (39). Since progestin plus EGF treatment up-regulates G1/S-phase cyclins, concomitantly up-regulated p21 may participate in cyclin-CDK activation (39). Thus, in breast cancer cells, multiple complex functions of p21 may contribute to restarting the cell cycle machinery in response to secondary growth signals initiated by EGF or cytokines, after progestin pretreatment and cell cycle arrest.
SUMMARY
In the breast, data from numerous laboratories suggest that cross-talk exists between PR and growth factor and cytokine signaling pathways at multiple levels (Fig. 4). At the cell surface (level 1), progestins up-regulate growth factor and cytokine receptors. We have expanded this observation by examining the effects of progestins in the cytoplasm (level 2) where progestins regulate several intracellular effectors by increasing the levels and altering the subcellular compartmentalization of Stat5, increasing the association of Stat5 with phosphotyrosine-containing proteins and tyrosine phosphorylation of JAK2, Cbl, and Shc, and potentiating EGF-stimulated p42/p44 MAPKs, p38 MAP kinase, and JNK activities. Together, these events lead to sensitization of downstream signaling pathways to the actions of locally acting secondary factors. Finally, inside the nucleus (level 3), agonist-occupied PR synergize with nuclear transcription factors that are growth-factor regulated, to control the activity of key genes involved in breast cell fate (Figs. 1 and 4). We speculate that after progesterone treatment, orchestrated combinations of steroid hormones and growth factors or cytokines can fine tune the timing and degree of expression of a subset of genes that determine whether progestin-primed cells undergo proliferation, differentiation, or programmed cell death.
Cross-Talk between Progesterone and Growth Factors/Cytokine-Signaling Pathways Occurs at Multiple Levels Level 1, Progesterone up-regulates growth factor and cytokine receptors at the cell surface (blue). Level 2, Progesterone regulates the levels, phoshorylation status, and/or subcellular localization of several intracellular effectors of growth factors and cytokines (green). Level 3, Progesterone-occupied PR and nuclear transcription factors synergize to control the activity of key genes involved in breast cell proliferation, differentiation, or involution (red) (see text for further discussion).
The paradoxical effects of progesterone have presented a longstanding conundrum to the scientist and clinician. Why are physiological levels of progesterone proliferative in the breast but antiproliferative and protective in the uterus? If progesterone is proliferative in the breast, why is high-dose progestin therapy successful in treating breast cancer? Our intent here has been to open a dialogue addressing these questions. Our data and that of others are beginning to show that one cannot approach the question of progestin actions in isolation. Other important regulatory proteins, whose expression may vary in tissue-specific ways, work in concert with progesterone to decide cell fate. The timing and dose of progesterone may also influence the biological response. Since progestins are widely used in oral contraception, in hormone replacement therapy, and in cancer treatments, it is becoming critically important that the subtleties of their mechanisms of action be clearly understood.
Acknowledgments
We gratefully acknowledge J. Dinny Graham, Ph.D. (University of Colorado Health Sciences Center, Department of Medicine, Division of Endocrinology) and Margaret C. Neville, Ph.D. (University of Colorado Health Sciences Center, Department of Physiology and Biophysics) for helpful comments on this manuscript.
This work was supported by NIH Grants DK-53825 (to C.A.L.), CA-26869 (to K.B.H.), and DK-48238 (to K.B.H.).
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