Date of Award
MS in Agriculture - Animal Science
The ability of exogenous growth hormone (GH) to increase milk yield through insulin-like growth factor-1 (IGF-I) in dairy cows is well characterized. However, recent studies utilizing mammary epithelial cell lines indicate a direct effect of GH on mammary epithelial cells (MEC). To test if these observations are relevant in vivo and if this response differs between dairy breeds, three mammary models were utilized. Mammary explants from a lactating Jersey cow were cultured in classical lactogenic media (dexamethasone, insulin, and prolactin) with 0 or 10 ng/mL of recombinant bovine GH for 12h. Primary MEC from lactating Holstein and Jersey cows were cultured in classical lactation media with 0 or 10 ng/mL of GH for 2, 4, and 7 days. And lastly, MEC isolated from pooled Holstein or pooled Jersey milk were cultured in the same conditions as primary MEC. The response to GH was quantified by the relative abundance of mRNA for two milk protein genes (α-lactalbumin and αS1-casein), the GH receptor, IGF-I and insulin-like growth factor binding protein-3 (IGFBP-3) as determined by quantitative RT-PCR. The abundance of α-lactalbumin mRNA in explants was increased in response to GH. After 2 days, Jersey primary MEC showed an increase in GH receptor mRNA, in addition to a noteworthy trend of increasing abundance of IGFBP-3 regardless of GH treatment. After 4 days, Holstein primary cells cultured with GH had decreased IGFBP-3 mRNA. After 7 days, primary cells isolated from Holstein and Jersey mammary tissue showed a slight response to GH. Mammary cells from milk mirrored the responses to GH observed in primary cells: MEC isolated from Holsteins had decreased IGFBP-3 mRNA after 4 days of treatment with GH and MEC isolated from Jerseys showed the same trend of increasing IGFBP-3 abundance between 2 and 4 days, but with no difference between GH treatments. These results indicate that the effect of GH may differ between breeds and indicate GH has a direct effect on mammary epithelial cells, possibly including effects on the abundance of IGFBP-3 mRNA.