Abstract
PAT proteins (perilipin, adipophilin, and TIP47) are hypothesized to be critical regulators of lipid accumulation in eukaryotic cells. We investigated the developmental relationships between the expression of these proteins and cytoplasmic lipid droplet (CLD) accumulation in differentiating secretory epithelial cells in mouse mammary glands. Adipophilin (ADPH) specifically localized to CLD in differentiating and lactating mammary glands and was found exclusively in the secreted lipid droplet fraction of mouse milk. ADPH transcripts were selectively detected in secretory epithelial cells, and steady-state levels of both ADPH mRNA and protein increased during secretory differentiation in patterns consistent with functional linkage to CLD accumulation. TIP47 also was detected in secretory epithelial cells; however, it had a diffuse punctate appearance, and its mRNA and protein expression patterns did not correlate with CLD accumulation. Perilipin-positive adipose cells and steady-state levels of perilipin mRNA and protein decreased during mammary gland differentiation, suggesting a progressive loss of adipose lipid storage during this process. Collectively, these data demonstrate that increased ADPH expression is a specialized property of differentiated secretory epithelial cells and provide developmental evidence specifically linking increased ADPH expression to increased CLD accumulation. In addition, evidence is presented that the epithelial and adipose compartments of the mammary gland undergo concerted, developmentally regulated shifts in lipid metabolism that increase the availability of fatty acids necessary for lipid synthesis by milk-secreting cells.
Highlights
PAT proteins are hypothesized to be critical regulators of lipid accumulation in eukaryotic cells
To determine which PAT proteins are associated with cytoplasmic lipid droplet (CLD) in milkproducing mammary epithelial cells of the mouse, we compared the localizations of ADPH, TIP47, and perilipin in mammary glands of fully lactating mice
Our study provides evidence that ADPH mediates CLD accumulation in differentiating secretory epithelial cells in the mouse mammary gland
Summary
PAT proteins (perilipin, adipophilin, and TIP47) are hypothesized to be critical regulators of lipid accumulation in eukaryotic cells. We investigated the developmental relationships between the expression of these proteins and cytoplasmic lipid droplet (CLD) accumulation in differentiating secretory epithelial cells in mouse mammary glands. Perilipin-positive adipose cells and steady-state levels of perilipin mRNA and protein decreased during mammary gland differentiation, suggesting a progressive loss of adipose lipid storage during this process These data demonstrate that increased ADPH expression is a specialized property of differentiated secretory epithelial cells and provide developmental evidence linking increased ADPH expression to increased CLD accumulation. Previous studies in our laboratory demonstrating that ADPH forms a stable complex with xanthine oxidoreductase and butyrophilin [35], two proteins critically required for milk lipid secretion [36, 37], implicate ADPH in both the formation and secretion of CLDs. evidence linking ADPH expression to CLD formation in the mammary gland is lacking, and possible contributions of other PAT proteins to these processes have not been investigated
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