Browsing by Author "Weber, Miriam S."

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    Overexpression of ovine insulin-like growth factor-I (IGF-I) in the mammary glands of transgenic mice
    Weber, Miriam S. (Virginia Tech, 1995)
    Evidence indicates that the mammary gland may contribute to its own development by the production of insulin-like growth factor-I (IGF-I). To generate a model for investigation of the effects of enhanced IGF-I synthesis in the mammary gland, six transgenic founder mice were produced by microinjection of a cDNA sequence encoding ovine IGF-I (oIGF-I) under the control of the glucocorticoid-responsive mouse mammary tumor virus (MMTV) promoter. Transgenic lines were established from each MMTV-IGF-I transgenic founder, and four virgin and four lactating mice from each transgenic line were analyzed for transgene expression following administration of dexamethasone or vehicle. Virgin mice from three transgenic lines (15, 26, and 29) expressed oIGF-I mRNA and recombinant IGF-I in the mammary gland, although stimulation with exogenous dexamethasone was required for expression. Lactating mice from four transgenic lines (2, 15, 26, and 29) expressed oIGF-I mRNA in mammary tissue, but only three of those lines (15, 26, and 29) produced detectable recombinant IGF-I in milk and mammary tissue. Recombinant IGF-I was not detected in plasma from any of the transgenic lines. The biological activity of recombinant IGF-I secreted into the milk of lactating transgenic mice was demonstrated in vitro by stimulation of ([³H]thymidine incorporation into DNA of immortalized bovine mammary epithelial cells. The MMTV-IGF-I transgenic mice will provide a model for evaluating the effects of increased mammary synthesis of IGF-I on mammary gland development and function.
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    The Role of Insulin-like Growth Factor-I and IGF-binding Proteins in Mammary Gland Development
    Weber, Miriam S. (Virginia Tech, 1998-11-19)
    Development of the mammary gland is likely mediated by locally produced growth factors acting in concert with circulating mitogens. To investigate the importance of mammary synthesis of insulin-like growth factor-I (IGF-I) and IGF-binding proteins (IGFBP), the initial objective was to evaluate the physiological effects of recombinant IGF-I synthesis in the mouse mammary gland. Expression of recombinant IGF-I was targeted by the mouse mammary tumor virus - long terminal repeat (MMTV-LTR) to the mammary glands of two lines (15 and 29) of transgenic mice. Mammary synthesis of recombinant IGF-I increased the frequency of appearance of mammary alveolar buds (71% vs. 21%) in transgenic compared with non-transgenic CD-1 mice. During lactation, mammary synthesis of recombinant IGF-I reduced the amount of endogenous native IGF-I secreted into milk of transgenic mice. Regardless of transgenesis, a shift in the milk IGFBP profile from predominantly IGFBP-3 to a lower molecular weight IGFBP occurred between d 8 and d 12 of lactation. The altered composition of milk from transgenic line 29 dams reduced by 27% the average daily gain of suckling litters, compared with CD-1 dams. Moreover, mammary glands of transgenic mice were less regressed after weaning than controls and were characterized by the presence of more organized secretory lobules. The second overall objective was to evaluate the regulation and physiological effects of mammary IGF-I and IGFBP synthesis in prepubertal heifers. Serum and extracts of mammary tissue at 5% concentration in media stimulated DNA synthesis 545% and 28%, respectively, in primary mammary epithelial organoids in collagen gel culture. Addition of IGFBP-3 strongly inhibited this growth response. High feeding level tended to increase IGFBP-3 levels in mammary tissue and reduced by 30% the growth response to mammary tissue extracts. Somatotropin increased the mitogenic response to mammary extracts at high feeding level and increased the tissue content of IGF-I by 46%. In summary, local synthesis of IGF-I and IGFBP is influenced by feeding level and exogenous somatotropin and contributes substantially to effects on mammary cell proliferation. Interactions of locally produced IGFBP-3 with IGF-I and other growth factors appear to be especially important when mammary growth is modulated by feeding level.