| Abstract: | Retinoids are potent inhibitors of growth and tumor progression in many mammary carcinoma cell lines, though regulation of growth in nontumorigenic mammary epithelial cells by retinoids is less clear. Here, we have characterized the inhibition of MAC-T (a nontransformed bovine mammary epithelial cell line) cellular proliferation by retinoids and their role in regulating insulin-like growth factor binding proteins (IGFBPs). Retinoic acid (RA) (100 nM) was a potent inhibitor of MAC-T cell proliferation. Retinol was 10–100 times less effective. Neither retinoid could completely arrest growth at noncytotoxic concentrations. Retinoic acid inhibited cellular proliferation by 1 h (P < .05), but inhibition was fivefold greater by 24 h (P < .01). This second stage of growth inhibition (after 12 h) was dependent upon protein synthesis. However, RA-induced inhibition of cellular proliferation did not persist, with thymidine incorporation increasing toward control levels by 4 days in culture. Retinoic acid was less effective in inhibiting thymidine incorporation when cells were stimulated with insulin, des(1–3) IGF-I, or Long(R3) IGF-I when compared to cells stimulated with native IGF-I or serum. Inhibition of proliferation by RA was associated with increased levels of IGFBP-2 in conditioned media and in plasma membrane preparations. Treatment with insulin or des(1–3) IGF-I resulted in the appearance of IGFBP-3 in conditioned media and on the cell surface. However, RA significantly reduced IGFBP-3 levels in conditioned media and eliminated IGFBP-3 associated with the plasma membrane. Thus, RA is a potent but transient inhibitor of bovine mammary epithelial cell proliferation, and this growth inhibition is correlated with increased IGFBP-2 accumulation and inhibition of IGF-I stimulated IGFBP-3 protein secretion. © 1996 Wiley-Liss, Inc. |
