Estrogen-mediated S-Phase Entry
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Estrogen-mediated S-Phase Entry

Estrogens are a class of steroid hormones that play a central role in reproduction, and are regarded as the powerful female hormones that make a girl develop into a woman capable of reproduction. Estrogenic steroids, that include: E1 (Estrone), E2 (Estradiol/17-beta Estradiol) and E3 (Estriol), regulate cellular functions in a wide variety of tissues and influence proliferation in the female reproductive tract and mammary gland. Estrogens play a key role in the development of the mammary gland. In the normal gland, proliferating cells do not express estrogen receptors. In contrast, estrogen receptor-a (ERa) which acts as a ligand (estrogen)-dependent transcription factor is expressed in the majority of mammary tumors (70%).The control of cell proliferation by estrogens such as 17-ß estradiol (E2) is a complex process. Estrogens bound to ERa regulate target genes implicated in proliferation including CDK2, CDK4,cyclin D1 (CCND1) or the proto-oncogene c-Myc (MYC). In addition, several genes which negatively control cell proliferation such as cyclin G2 (CCNG2), caspase 9 (CASP9) or p21 are repressed by estrogens (Ref.1 and 2).

cyclin D1 (CCND1) belongs to the family of D-type cyclins, which regulate G1–S cell cycle progression. CCND1 acts through activation of cyclin-dependent kinases (CDKs) that phosphorylate and inactivate the retinoblastoma protein. CCND1 could also promote cell cycle progression through CDKs-independent mechanisms, such as interaction with and modulation of transcription factor activities. Because CDKs control cell division, dysregulation of their regulatory partners, the cyclins, has been implicated in the initiation and promotion of hyperplasia and oncogenesis. Two different cyclins, Cyclin-Aand CCND1, have been identified as ER activators. cyclin D1 activation of ER does not involve a phosphorylation, while ER activation of Cyclin-Ais triggered by a phosphorylation in the AF1 (Activation Function-1) domain (Ref.3). D-type cyclins play an essential role in recognition of extracellular growth stimuli and initiation of G1 transit. Estrogen regulation of cellular proliferation has been attributed to cyclin D1 expression. Estrogen-induced proliferation of uterine and breast epithelium in vivo is associated with formation of active cyclin D1-CDK4 complexes and increased expression of Cyclin-D1 mRNA and protein (Ref.4).

Transcriptional activation of MYC and cyclin D1 expression by ER in early G1-phase facilitates Cyclin-E-CDK2 activation in mid-to-late G1- and S-phase entry.CDK4 is activated by binding to cyclin D1 and acts early in G1-phase, while CDK2 kinase functions in conjunction with Cyclin-E and Cyclin-A and is necessary for progression through late G1 and entry into S-phase. A primary target of CDKs action in G1-phase is the RB, which mediates G1 arrest through sequestration of transcriptional factors of the E2F-DP1 family. When dephosphorylated in G1, RB complexes with and blocks transcriptional activation by E2Ftranscription factors. But phosphorylation of RB by Cyclin-CDKs complexes, leads to its dissociation from E2F, allowing E2Fto activate the transcription of genes required for S-phase. E2Factivity consists of a heterodimeric complex of an E2Fpolypeptide and a DP1 protein (Ref.5). Phosphorylation of other members of the pocket protein family (p107 and p130) by active CDKs-CDKs complexes leads to release of E2Fand DP1 transcription factors and transcription of requisite genes for S-phase entry. One of the genes activated by E2Fis Cyclin-E itself, leading to a positive feedback cycle as Cyclin-E accumulates.

MYC participates in Cyclin-E-CDK2 activation by eliciting CDC25A (Cell Division Cycle-25A) expression. CDC25A expression is required for S-phase entry and is induced in G1 by MYC and E2F. CDC25A is active from mid-G1 through S-phase and participates in activation of CDK2. Ultimately, active Cyclin-E-CDK2 elicits S-phase entry both through contribution to pocket protein phosphorylation and E2Frelease and through phosphorylation of additional, unknown mediators of S-phase entry. Upon inactivation of pocket proteins, derepression at E2F-dependent promoters and consequent induction of Cyclin-A, CDC25A, and E2F1 provides further reinforcement for G1/S transition and progression through the S-phase entry and hence, cell cycle progression (Ref.1). Expression of cyclin D1 and complex formation with CDK4 leads to sequestration of p21Cip1 and p27Kip1 CDK inhibitors and initiates phosphorylation and inactivation of pocket proteins, including RB. Estrogens elicit down-regulation of both p21Cip1 and p27Kip1 independent of cyclin D1-CDK4 function and at least in part through the proteasome promoted by CDK2 and the F-box protein SKP2 (S-phase Kinase-associated Protein-2) at the G1/S transition, and independent of SKP2 in G1-phase (Ref.6 and 8).

Although estrogen does not appear to directly cause the DNA mutations that trigger the development of human cancer, estrogen does stimulate the proliferation of mutant cells. In breast and uterine cancers that are ER-positive, this protein, the ER, combines with normal circulating estrogen in the woman's body. It's this combination of estrogen and ER that is necessary for the breast cancer to continue to grow (Ref.2). If we interrupt that combination, that binding, we can actually stop the progress of cancerous growth. Multiple mechanisms participate in the regulation of estrogen-controlled genes, providing a wide spectrum of possibilities for development of drugs, including pure/mixed agonists or antagonists, known as: SERM (Selective Estrogen Receptor Modulators). Since ERs of different target tissues vary in chemical structure, drugs that block the action of estrogen are selective in nature. Antiestrogens, such as Tamoxifen, are used as therapeutic agents for the treatment and possible prevention of breast cancer. Tamoxifen is believed to function as an antitumor agent by inhibiting the action of the ER in breast tissue (Ref.7).