Mechanisms of milk lipid secretion

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DESCRIPTION (provided by applicant):. Lipid synthesis is induced during differentiation of the mammary gland into a secretory organ; and during lactation in humans and rodents the mammary gland is estimated to be among the most active lipogenic organs in the body. Milk lipid secretion is a tightly regulated process requiring synthesis, assembly and transport of triglyceride and cholesterol ester containing droplets (cytoplasmic lipid droplets -CLD) to the apical membrane of mammary epithelial cells where they are secreted by unique membrane budding mechanism. The long-term objectives of this proposal are to elucidate the molecular and cellular mechanisms underlying formation, transport, and secretion of CLD. We hypothesize that adipophilin (ADPH), a member of the perilipin (PAT) family of lipid droplet binding proteins, is required for CLD formation and secretion, thus integrating lipid synthesis and secretion during lactation. We propose to test this hypothesis by determining the functional consequences of selectively disrupting expression of ADPH and/or the closely related PAT protein, TIP47, in the mouse mammary gland using transgenic and adenoviral approaches. Transgenic mice expressing N- and C-terminally truncated forms of ADPH will be used to define effects of specific ADPH domains on milk lipid formation and secretion. Adenoviral vectors will be used to express variants of ADPH and/or TIP47, or mutations of specific functional domains of these proteins, in mammary glands of ADPH-null mice to identify molecular determinants that will rescue defects induced by absence of this protein. ADPH and TIP47 are both hypothesized to function in lipid accumulation in many mammalian cell types. Because lipid synthesis in the mammary gland is robust and developmentally regulated by well defined promoter systems, and mammary epithelial cells can be manipulated by transgenic and adenoviral techniques, the proposed studies offer an excellent opportunity to understand molecular interactions and structure-function relations of lipid storage in many cells and tissues. NICHD Health Relatedness: CLD are the source of milk lipids, which are required neonatal growth, and essential fatty acids and cholesterol needed for membrane synthesis, particularly in the CNS. However, there is increasing recognition of the general importance of CLDs in storage and intracellular trafficking of lipids in eukaryotic cells. These cellular structures are the primary storage depots for triglycerides in liver, adipose and muscle cells; and the source of triglycerides and cholesterol esters in serum lipoprotein particles secreted by the liver, and milk fat globules secreted by mammary epithelial cells. CLDs are important homeostatic elements in cellular and tissue lipid metabolism and may serve to limit the availability of potentially toxic free fatty acids. Furthermore, elevated CLD accumulation in non-adipose tissue is a prominent pathological feature of many human metabolic diseases, such as obesity, type-II diabetes and non-alcoholic hepatic steatosis, which are increasing health concerns for children. Characterization of the fundamental molecular and cellular mechanisms regulating normal formation and metabolism of CLD as proposed in this application is essential to understanding the molecular abnormalities contributing the pathophysiology of lipid metabolism disorders. PUBLIC HEALTH RELEVANCE: The long-term objectives of this proposal are to elucidate the molecular and cellular mechanisms underlying formation, transport, and secretion of CLD. We hypothesize that adipophilin (ADPH), a member of the perilipin (PAT) family of lipid droplet binding proteins, is required for CLD formation and secretion, thus integrating lipid synthesis and secretion during lactation. We propose to test this hypothesis by determining the functional consequences of selectively disrupting expression of ADPH and/or the closely related PAT protein, TIP47, in the mouse mammary gland using transgenic and adenoviral approaches. Transgenic mice expressing mutated forms of ADPH will be used to define effects of specific ADPH domains on milk lipid formation and secretion. Adenoviral vectors will be used to express variants of ADPH and/or TIP47, or mutations of specific functional domains of these proteins, in mammary glands of ADPH-null mice to identify molecular determinants that will rescue defects induced by their absence.