Submitted to: Domestic Animal Endocrinology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/3/2008
Publication Date: 8/1/2008
Citation: Zhou, Y., Capuco, A.V., Jiang, H. 2008. Involvement of Connective Tissue Growth Factor (CTGF) in Insulin-like Growth Factor-I (IGF1) Stimulation of Proliferation of a Bovine Mammary Epithelial Cell Line. Domestic Animal Endocrinology. 35(2):180-189. Interpretive Summary: Insulin-like growth factor I (IGF1) plays an important role in mammary gland development and lactation, in part by stimulating proliferation of the milk-producing epithelial cells. We used the bovine mammary epithelial cell line, MAC-T cells, as a model to understand the mechanism by which IGF1 stimulates proliferation of the , mammary epithelial (secretory) cells. A global analysis of gene expression showed that expression of 115 genes was significantly altered by IGF1 treatment of MAC-T cells. Among the most significantly down-regulated genes was connective tissue growth factor (CTGF). Administration of growth hormone (GH), a major stimulator of IGF1 production decreased expression of CTGF in the mammary glands of cows. Data show that that CTGF attenuates IGF1-stimulated proliferation of MAC-T cells. Thus, reduced expression of CTGF promotes proliferation of bovine mammary cells. The biochemical pathways involved in this regulation were evaluated and results of this study suggest that a novel relationship between CTGF and IGF1 exists in the bovine mammary gland for regulation of cell proliferation.
Technical Abstract: Insulin-like growth factor I (IGF1) plays an important role in mammary gland development and lactation in part by stimulating proliferation of the milk-producing epithelial cells. In this study, we used the bovine mammary epithelial cell line MAC-T cells as a model to understand the mechanism by which IGF1 stimulates proliferation of the bovine mammary epithelial cells. A microarray analysis revealed 155 transcripts that were significantly up- or down-regulated by IGF1 in MAC-T cells. Among the most significantly down-regulated genes was connective tissue growth factor (CTGF), a secretory protein that has both proliferative and apoptotic effects, depending on cell type, and also a low-affinity binding protein of IGF1. Quantitative real-time PCR confirmed IGF1 regulation of expression of CTGF and eight other mRNAs in MAC-T cells. Using selective inhibitors of signaling pathways from the IGF1 receptor (IGF1R), it was found that IGF1 suppressed CTGF mRNA expression in MAC-T cells through the phosphatidylinositol 3-kinase pathway. Administration of growth hormone (GH), a major stimulator of IGF1 production in vivo, decreased mammary expression of CTGF mRNA in cows. However, GH had no direct effect on CTGF mRNA expression in MAC-T cells, suggesting that IGF1 mediates the reduced expression of CTGF mRNA in the mammary gland. In the absence of IGF1, CTGF stimulated proliferation of MAC-T cells, but in combination with IGF1 it attenuated the stimulatory effect of IGF1 on proliferation of MAC-T cells. CTGF’s attenuation of IGF1-stimulated proliferation could be reversed by excess IGF1. Western blotting analyses indicated that despite being an IGF1 binding protein, CTGF did not affect IGF1-induced phosphorylation of IGF1R or total IGF1R expression in MAC-T cells, indicating that CTGF attenuation of IGF1-stimulated proliferation of MAC-T cells is not mediated by decreasing the ability of IGF1 to bind to and phosphorylate IGF1R or by decreasing IGF1R expression by the cells. Overall, the results of this study suggest a novel biochemical and functional relationship between CTGF and IGF1 in the bovine mammary gland, where IGF1 may inhibit CTGF expression to reduce the attenuating effect of CTGF on IGF1 stimulation of epithelial cell proliferation.