Growth hormone increases Stat5 and Stat1 expression in lactating goat mammary gland: a specific effect compared to milking frequency.

In ruminants, both milking frequency and exogenous GH treatment affect milk production. In a previous report, we showed that the modulation of milk yield due to variations in milking frequency and GH treatment was associated with variations in mammary cell numbers. The aim of this study was to clarify the different mechanisms governing the effects of GH treatment and milking frequency on signal transducer and activator of transcription (Stat) expression and activation, and on the expression of genes involved in mammary cell differentiation. Six Saanen goats in late lactation were milked once daily from one half-udder and thrice daily from the other half-udder for 23 days. At the same time, the goats were divided into two groups: GH-treated versus control group. After slaughter of the goats, soluble mammary proteins and RNA were extracted from half-udder samples. Levels of Stat5, Stat3 and Stat1 proteins and the Stat activation by phosphorylation were analysed by Western blot. The amounts of Stat5 protein and mRNA were significantly elevated by GH treatment in all half-udders (milked once or thrice daily). Positive Stat5 immunoreactivity was principally localised in the nuclei of epithelial cells, with heterogeneous intensity between cells. No significant changes in Stat5 protein phosphorylation levels were observed. Furthermore, GH significantly increased Stat1 protein levels, without modifying the level of Stat1 tyrosine phosphorylation, and tended to reduce the abundance of Stat3 protein. In contrast, milking frequency failed to modify Stat gene expression, protein level and phosphorylation. Using Northern blot, we showed that levels of kappa casein and prolactin receptor mRNA were not affected by the treatments. These observations suggest that GH probably acts specifically on mammary cells by regulating the expression of Stat1, 3 and 5. In contrast, milking frequency does not act through this regulatory pathway.