Distribution and source of lipoprotein lipase in mouse mammary gland.

During lactation lipoprotein lipase (LPL) is elevated in mammary tissue and depressed in adipose tissue to redirect lipids for incorporation into milk fat. The cellular origin of lipo- protein lipase in mammary tissue is thought to be the mammary epithelial cell which is the predoinant cell type noticeable in the lactating gland; however, mammary adipocytes are also present. If lipoprotein lipase is produced by adipocytes in other sites of the body, then the question remains as to why mammary adipo- cytes have not been shown to produce lipoprotein lipase. In this study we present several lines of evidence that indicate that the mammary adipocyte is a source of LPL in the lactating mam- mary gland of mice. This evidence includes the absence of extra- cellular and intracellular lipoprotein lipase activity in two types of primary mammary epithelial cell cultures and a similarity in the changes of lipoprotein lipase activity in genital adipose tissue from nonpregnant mice and lactating mammary tissue to the nutritional state of the animal. Other evidence presented here includes strong localization of lipoprotein lipase protein and messenger RNA by fluoresence immunohistochemistry and in situ hybridization, respectively, to interstitial cells located be- tween epithelial structures. We postulate that these interstitial cells are regressed, lipid-deleted mammary adipocytes. -Jensen, D. R., D. H. Bessesen, J. Etienne, R. H. Eckel, and M. C. Neville. Distribution and source of lipoprotein lipase in mouse mammary gland. J Lipid Res. 1991. 32: 733-742.