Increased adiposity is a feature of aging in both mice and humans, but the molecular mechanisms underlying age-related changes in adipose tissue stores remain unclear. In previous studies, we noted that 18-month-old normocalcemic vitamin D receptor (VDR) knockout (VDRKO) mice exhibited atrophy of the mammary adipose compartment relative to wild-type (WT) littermates, suggesting a role for VDR in adiposity. Here we monitored body fat depots, food intake, metabolic factors, and gene expression in WT and VDRKO mice on the C57BL6 and CD1 genetic backgrounds. Regardless of genetic background, both sc and visceral white adipose tissue depots were smaller in VDRKO mice than WT mice. The lean phenotype of VDRKO mice was associated with reduced serum leptin and compensatory increased food intake. Similar effects on adipose tissue, leptin and food intake were observed in mice lacking Cyp27b1, the 1alpha-hydroxylase enzyme that generates 1,25-dihydroxyvitamin D(3), the VDR ligand. Although VDR ablation did not reduce expression of peroxisome proliferator-activated receptor-gamma or fatty acid synthase, PCR array screening identified several differentially expressed genes in white adipose tissue from WT and VDRKO mice. Uncoupling protein-1, which mediates dissociation of cellular respiration from energy production, was greater than 25-fold elevated in VDRKO white adipose tissue. Consistent with elevation in uncoupling protein-1, VDRKO mice were resistant to high-fat diet-induced weight gain. Collectively, these studies identify a novel role for 1,25-dihydroxyvitamin D(3) and the VDR in the control of adipocyte metabolism and lipid storage in vivo.

The vitamin D(3) receptor (VDR) is a ligand-dependent transcription factor implicated in regulation of cell cycle, differentiation and apoptosis of both normal and transformed cells derived from mammary gland. In these studies we examined whether VDR status altered mammary gland morphology or transformation in the well-characterized MMTV-neu transgenic model of breast cancer. We demonstrate that VDR protein is highly expressed in neu-positive epithelial cells of preneoplastic lesions, established tumors and lung metastases from MMTV-neu mice. Furthermore, MMTV-neu mice lacking VDR exhibit abnormal mammary ductal morphology characterized by dilated, distended ducts containing dysplastic epithelial cells. From 12 months of age on, MMTV-neu mice lacking VDR also experience body weight loss, atrophy of the mammary fat pad, estrogen deficiency and reduced survival. The limited survival of MMTV-neu mice lacking VDR precluded an accurate assessment of the impact of complete VDR ablation on tumor development. MMTV-neu mice heterozygous for VDR, however, did not exhibit body weight loss, mammary gland atrophy or compromised survival. Compared with MMTV-neu mice with two copies of the VDR gene, haploinsufficiency of VDR shortened the latency and increased the incidence of mammary tumor formation. Tumor histology and expression/subcellular localization of the neu transgene were not altered by VDR haploinsufficiency despite a significant decrease in tumor VDR expression. Collectively, these studies suggest that VDR gene dosage impacts on age-related changes in ductal morphology and oncogene-induced tumorigenesis of the mammary gland in vivo.

Active cell death (ACD) in hormone-dependent tissues such as the prostate and mammary gland is readily induced by hormone ablation and by treatment with anti-androgens or anti-estrogens, calcium channel agonists and TGF beta. These agents induce a variety of genes within the hormone-dependent epithelial cells including TRPM-2, transglutaminase, poly(ADP-ribose) polymerase, Hsp27 and several other unidentified genes. Not all epithelial cells in the glands are equally sensitive to the induction of ACD. In the prostate, the secretory epithelial cells that are sensitive to hormone ablation are localized in the distal region of the prostatic ducts, and are in direct contact with the neighboring stroma. In contrast, the epithelial cells in the proximal regions of the ducts are more resistant to hormone ablation, probably because the permissive effects of the stroma are attenuated by the presence of the basal epithelial cells, which are intercalated between the epithelium and stroma. The underlying biology of ACD in prostate and mammary glands, and its relevance to hormone resistance, is discussed in this review.

The major circulating form of vitamin D is 25-hydroxycholecalciferol [25(OH)D3], which is delivered to target tissues in complex with the serum vitamin D binding protein (DBP). We recently observed that mammary cells can metabolize 25(OH)D3 to 1,25-dihydroxycholecalciferol [1,25(OH)(2)D3], the vitamin D receptor (VDR) ligand, and the objective of our study was to elucidate the mechanisms by which the 25(OH)D3-DBP complex is internalized by mammary cells prior to metabolism. Using fluorescent microscopy and temperature-shift techniques, we found that T-47D breast cancer cells rapidly internalize DBP via endocytosis, which is blunted by receptor-associated protein, a specific inhibitor of megalin-mediated endocytosis. Endocytosis of DBP was associated with activation of VDR by 25(OH)D3 but not 1,25(OH)(2)D3 (as measured by induction of the VDR target gene, CYP24). We also found that megalin and its endocytic partner, cubilin, are coexpressed in normal murine mammary tissue, in nontransformed human mammary epithelial cell lines, and in some established human breast cancer cell lines. To our knowledge, our studies are the first to demonstrate that mammary-derived cells express megalin and cubilin, which contribute to the endocytic uptake of 25(OH)D3-DBP and activation of the VDR pathway.