The classic function of vitamin D is maintenance of calcium homeostasis through tight regulation of circulating 1,25D via synthesis and degradation in the kidney. In addition, vitamin D signaling stimulates cellular differentiation and regulates cell proliferation and apoptosis in extra-renal tissues. In recent years, expression of CYP27B1, the enzyme which catalyzes synthesis of the active metabolite 1,25D, has been demonstrated in cells derived from colon, prostate and mammary tissue, prompting the hypothesis that local synthesis of 1,25D may act in an autocrine or paracrine fashion to regulate growth and differentiation in these tissues. This raises the possibility that loss of vitamin D signaling may contribute to the progression of cancer. Therefore, these studies were designed to elucidate function and regulation of the vitamin D pathway in both normal human mammary epithelial cells (HMEC) and MCF-7 breast cancer cells. We have found that both mammary cell lines express vitamin D pathway components and that vitamin D signaling is intact in these cell lines. Furthermore, we demonstrated internalization of vitamin D binding protein (DBP), CYP27B1 activity and 1,25D synthesis in mammary cells. These studies indicate that the normal mammary gland has the capacity to internalize and metabolize 25D-DBP from circulation. In contrast, DBP blunted the efficacy of 25D in MCF-7 cells, suggesting altered vitamin D signaling in breast cancer cells. To investigate abrogation in the vitamin D pathway during cancer progression, we utilized cells bearing a series of genetic manipulations that mimic mutations common in cancer. Although the vitamin D signaling pathway was intact in these cells, lower expression of VDR and decreased sensitivity to 1,25D mediated growth regulation occurred early in the tumorigenic process. Collectively, our work has demonstrated that local synthesis of 1,25D is of biological significance in mammary cells and that abrogation in the vitamin D pathway may contribute to loss of negative growth regulation by 1,25D.
|Author||Carly M Kemmis|
|Contributor||JoEllen Welsh, Committee Member|
|Degree Level||Doctoral Dissertation|
|Degree Discipline||Biological Sciences|
|Departments and Units|