| Summary: | 博士 === 中山醫學大學 === 微生物免疫研究所 === 102 === part1
In mammary epithelial cells (MECs), prolactin-induced signaling and gene expression requires integrin-mediated cell adhesion to basement membrane (BM). In the absence of proper cell–BM interactions, for example, culturing cells on collagen-coated plastic dishes, signal propagation is substantially impaired. Here we demonstrate that the RhoA-Rok-myosin II pathway accounts for the ineffectiveness of prolactin signaling in MECs cultured on collagen I. Under these culture conditions, the RhoA pathway is activated, leading to downregulation of prolactin receptor expression and reduced prolactin signaling. Enforced activation of RhoA in MECs cultured on BM suppresses prolactin receptor levels, and prevents prolactin-induced Stat5 tyrosine phosphorylation and β-casein expression. Overexpression of dominant negative RhoA in MECs cultured on collagen I, or inhibiting Rok activity, increases prolactin receptor expression, and enhances prolactin signaling. In addition, inhibition of myosin II ATPase activity by blebbistatin also exerts a beneficial effect on prolactin receptor expression and prolactin signaling, suggesting that tension exerted by the collagen substratum, in collaboration with the RhoA-Rok-myosin II pathway, contributes to the failure of prolactin signaling. Furthermore, MECs cultured on laminin-coated plastic have similar morphology and response to prolactin as those cultured on collagen I. They display high levels of RhoA activity and are inefficient in prolactin signaling, stressing the importance of matrix stiffness in signal transduction. Our results reveal that RhoA has a central role in determining the fate decisions of MECs in response to cell–matrix interactions.
part 2
Insulin-like growth factor-I (IGF-I) and estrogens play important roles in the development of mammary glands. However, the abnormal increase of these hormones has been implicated in the development, progression and adverse outcome of breast cancer. Accumulating evidence reveals a crosstalk between IGF-I and estrogens-triggered signaling pathways, resulting in a synergistic effect on cell proliferation. The underlying mechanism is not completely elucidated, but upregulation of IGF-I receptor (IGF-IR) and insulin receptor substrate-1 (IRS-1) by estrogens might have a part in it. Here we found that overexpression of estrogen receptor a (ERa)in primary mouse mammary epithelial cells did not increase IRS-1 levels; instead, it decreased them in a dose-dependent manner. In addition, IGF-I-induced IRS-1 tyrosine phosphorylation was hampered. Similar results were obtained in primary human mammary epithelial cells and a normal mouse mammary cell line NMuMG. In ERa-negative human breast cancer cell lines, overexpression of ERa increased IRS-1 levels in Hs578T but decreased them in MDA-MB-231. Surprisingly, IRS-1 tyrosine phosphorylation was inhibited in both cell lines. This might be due to an increase in levels of IGF-binding proteins (IGFBPs). Although enhanced ERa expression in primary cells and cancer cells lead to the same result, primary cells are far more sensitive than cancer cells. We speculate that normal mammary cells possess safe-guard mechanisms to prevent over-stimulation when they are exposed to improper insults, such as overexpression of ERa. This protective mechanism might be still retained in certain cancer cells but the sensitivity is decreased.
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