Estrogen regulation of cell cycle progression in breast cancer cells¹

Abstract

Estrogens are potent mitogens in a number of target tissues including the mammary gland where they play a pivotal role in the development and progression of mammary carcinoma. The demonstration that estrogen-induced mitogenesis is associated with the recruitment of non-cycling, G₀, cells into the cell cycle and an increased rate of progression through G₁ phase, has focused attention on the estrogenic regulation of molecules with a known role in the control of G₁–S phase progression. These experiments provide compelling evidence that estrogens regulate the expression and function of c-Myc and cyclin D1 and activate cyclin E-Cdk2 complexes, all of which are rate limiting for progression from G₁ to S phase. Furthermore, these studies reveal a novel mechanism of activation of cyclin E-Cdk2 complexes whereby estrogens promote the formation of high molecular weight complexes lacking the CDK inhibitor p21. Inducible expression of either c-Myc or cyclin D1 can mimic the effects of estrogen in activating the cyclin E-Cdk2 complexes and promoting S phase entry, providing evidence for distinct c-Myc and cyclin D1 pathways in estrogen-induced mitogenesis which converge on the activation of cyclin E-Cdk2. These data provide new mechanistic insights into the known mitogenic effects of estrogens and identify potential downstream targets that contribute to their role in oncogenesis.

Introduction

Sex steroid hormones have a major role in the growth and development of target tissues including the mammary gland where they interact with other hormones, growth factors and cytokines in the precise regulation of proliferation and differentiation. Although the molecular basis of these interactions remains poorly understood increasing insight into the steroidal regulation of proliferation has been gained using steroid responsive human breast cancer cell lines as models. In these cells estrogens are mitogenic[1]as a result of accelerated progression through G₁ phase of the cell cycle[2]. Similar mechanisms have been identified in vivo where an additional effect of estrogen in recruiting non-cycling, G₀, cells into the cell cycle has been documented[3]. These cell cycle phase-specific effects of estrogens have focused attention on the role of estrogens and their receptors in the control of key regulatory processes controlling the entry into, progression through, and exit from G₁ phase of the cell cycle.

Progress through G₁ phase requires phosphorylation and inactivation of the retinoblastoma protein (pRB) with the consequent release of E2F transcription factors essential for the activation of genes required for S phase progression4, 5. Phosphorylation of pRB is mediated by the action of holoenzyme complexes comprising a cyclin regulatory subunit and a catalytic cyclin-dependent kinase (CDK)6, 7. Control of CDK activity in G₁ phase and subsequent progression to S phase is achieved by several mechanisms including: transcriptional activation of D-type cyclins and cyclin E; activation and inactivation of cyclin/CDK enzyme complexes by phosphorylation/dephosphorylation events, mediated predominantly by the CDK activating kinase (CAK) and Cdc25 phosphatases[7]; and by interactions with members of two distinct families of CDK inhibitors of which p16^INK4A and p21^{WAF1,CIP1,SDI1} are prototypic[8].

Another well studied target of estrogen action with a role in the control of cell cycle progression is the proto-oncogene c-myc. c-myc is a central regulator of cell proliferation and apoptosis and encodes a nuclear phosphoprotein (c-Myc) of the basic helix-loop-helix family of transcription factors which is required for mitogenic signaling by growth factor receptors. In fibroblasts inhibition of c-Myc can cause G₁ cell cycle arrest (reviewed in[9]), whereas the activation of conditional alleles of c-myc in these cells results in activation of cyclin D1-Cdk4 and cyclin E-Cdk210, 11, 12. c-Myc is rapidly upregulated by estrogen in breast cancer cells, suggesting a similar importance in estrogen-regulated cell cycle progression.

The recent expansion of knowledge on the molecular mechanisms regulating rates of cell cycle progression has provided a framework within which to develop deeper insight into the mechanistic basis of estrogen-induced mitogenesis. This brief review summarizes recent developments in this area.