Estrous cycle regulation of mammary epithelial cell proliferation, differentiation, and death in the Sprague-Dawley rat: A model for investigating the role of estrous cycling in mammary carcinogenesis

The Sprague-Dawley rat is highly regarded for studies designed to investiga te the effects of endocrine modulation on mammary carcinogenesis. In this s tudy, we further evaluate the validity of the Sprague-Dawley rat model for the study of human breast cancer by evaluating the effects of normal 4-day estrous cycling on mammary epithelial cell proliferation, differentiation, and apoptotic death. Trends in mammary gland development with stage of 4-da y estrous cycle were evident. Mammary glands isolated from follicular and e arly luteal stages had predominantly ductal histoarchitecture, whereas glan ds isolated from mid-late luteal were predominantly lobuloalveolar. Quantit ation of BrdU incorporation revealed that epithelial cell proliferation was eight-fold higher in metestrus and diestrus-l than in proestrus. Expressio n of beta -casein and whey acidic protein (WAP)(4) mRNA was also highly dep endent on stage of estrous, with detection restricted to midcycle. Apoptoti c cell death of mammary epithelium was found to be suppressed during the pe ak in cell proliferation. TRPM-2/clusterin mRNA was elevated when apoptosis was low and milk protein mRNA levels were high, consistent with putative r oles for TRPM-2/clusterin in inhibiting cell death in regressing tissues an d inducing mammary epithelial cell differentiation. Cell proliferation, dif ferentiation, and death occurred only in a subset of epithelial cells per e strous cycle, and these cells appeared randomly distributed throughout mult iple ductules and alveoli. These observations suggest that cellular respons e(s) to ovarian hormone-dependent signals is asynchronous. Cumulatively, th ese observations demonstrate that rat mammary epithelial cell proliferation , differentiation, and death are under the control of cycling ovarian hormo nes, similarly to the human mammary epithelium during the menstrual cycle.