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Insulin and Insulin-Like Growth Factor-I Stimulate DNA Synthesis in Bovine Mammary Tissue in Vitro¹

The Pennsylvania State University^,4, University Park 16802

Abstract

Effects of insulin and insulin-like growth factor I (IGF-I) on [³H]thymidine incorporation, in vitro, by mammary tissue slices obtained from prepartum and lactating cows were investigated. Both insulin and IGF-I induced up to a 10-fold increase in [³H]thymidine incorporation in the mammary slices cultured in serum-free media. The effect of insulin-stimulated [³H]thymidine incorporation occurred at a threshold of >1.75 pmol/ml and appeared to reach maximum at >8.8 nmol/ml. The response to IGF-I occurred at >6.5 pmol/ml and reached the equivalent of maximal insulin-stimulated incorporation at 39 pmol/ml. No synergistic or additive effects were observed between these two factors. The in vitro response took 3 to 4 d to reach maximum and was inhibited by cytarabine. Mammary tissue obtained from lactating cows incorporated more [³H]thymidine per microgram DNA in response to insulin (175 pmol/ml) than mammary tissue from pregnant cows. Culture of mammary tissue slices with growth hormone, cortisol, prolactin, or triiodothyronine showed no stimulation of [³H]thymidine incorporation over control. Autoradiography of the cultured lactating tissue showed incorporation of [³H]thymidine by 51, 24 and 29% of the ductal epithelial, secretory alveolar epithelial and myoepithelial cells, respectively. All alveolar epithelial cells that incorporated [³H]thymidine contained secretory products. Among nonsecretory cells, 25 and 28% of the fibroblasts and white blood cells, respectively, were labeled. Insulin-like growth factor I, but not bovine somatotropin, stimulated [³H]thymidine uptake into DNA in lactating bovine mammary tissue. Thus, our data support the concept that bovine somatotropin acts through IGF-I to increase DNA synthesis in mammary cells.

¹ This research was supported by the Pennsylvania Agric. Exp. Station and USDA Competitive Grant 85-CRCR-1-1881. Authorization for publication as paper no. 7940 in the journal series of the Perm. Agric. Exp. Station.

² Dept. of Dairy and Anim. Sci. and for reprint requests, 302 Henning Bldg., telephone:(814)863-0712.

³ Dept. of Food Sci.

⁴ We thank K. P. Deemer, R. M. Stover and P. G. Campbell for their assistance.