Progesterone Regulation of Milk Fat Globule Size Is VLDL Dependent
Nurit Argov-Argaman, Chen Raz, Zvi Roth
Abstract
Progesterone plays a pivotal role during mammogenesis and serves as an inhibitor of the secretory activation of mammary cells in the last days of gestation. However, its role during lactogenesis, in particular its involvement in lipid metabolism, milk fat content and composition, is unknown. Here we provide new evidence of progesterone's involvement in the regulation of milk fat globule (MFG) synthesis and secretion. Findings from both in-vivo and in-vitro studies indicated that the concentration and the direction (increase vs. decrease) of progesterone concentration to which the mammary epithelial cells (MEC) are exposed, affect MFG size. This was found to be VLDL-dependent: in the presence of VLDL, the proportion of MEC with small lipid droplets (1 µm) increased 4-fold; in the absence of VLDL, no differences were found. The findings add to our understanding of the mechanism underlying the regulation of MFG size and provide new evidence for progesterone's role in lipid metabolism in the mammary gland during lactogenesis. The fact that the size, synthesis and composition of MFG are affected by the cyclic pattern of progesterone concentration in the circulation might have physiologically relevant consequences, in particular on milk as a nutritional source.