Metabolic regulation in the lactating mammary gland: a lipid synthesizing machine

The mammary gland of the lactating mouse synthesizes and secretes milk lipid equivalent to its entire body weight in a single 20-day lactation cycle, making it one of the most active lipid synthetic organs known. We test the hypothesis that multiple control points and potential regulatory mechanisms regulate milk lipid synthesis at the level of gene expression. The mammary transcriptome of 130 genes involved in glucose metabolism was examined at late pregnancy and early lactation, utilizing data obtained from microarray analysis of mammary glands from quadruplicate FVB mice at pregnancy day 17 and lactation day 2. To correlate changes with physiological parameters, the metabolome obtained from magnetic resonance spectroscopy of flash-frozen glands at day 17 of pregnancy was compared with that at day 2 of lactation. A significant increase in carbohydrates (glucose, lactose, sialic acid) and amino acids (alanine, aspartate, arginine, glutamate) with a moderate increase in important osmolytes (myoinositol, betaine, choline derivatives) were observed in the lactating gland. In addition, diets containing 8% or 40% lipid were fed from lactation days 5-10 and mammary glands and livers of triplicate FVB mice prepared for microarray analysis. The results show that substantial regulation of lipid synthesis occurs at the level of mRNA expression and that some of the regulation points differ substantially from the liver. They also implicate the transcription factor SREBP-1c in regulation of part of the pathway.