Vitamin D receptor-dependent inhibition of mammary tumor growth by EB1089 and ultraviolet radiation in vivo.

1,25-Dihydroxyvitamin D(3) (1,25D), the biologically active form of vitamin D(3), exerts antiproliferative and proapoptotic effects in multiple transformed cell types, and thus, the vitamin D signaling pathway represents a potential anticancer target. Although chronic treatment with 1,25D induces hypercalcemia, synthetic vitamin D analogs have been developed that inhibit tumor growth in vivo with minimal elevation of serum calcium. Furthermore, vitamin D is synthesized in skin exposed to UV light, and this route of vitamin D elevation is not associated with hypercalcemia. In this study, we examined whether enhancement of vitamin D status via exogenous (EB1089, a 1,25D analog) or endogenous (UV exposure) approaches could exert antitumor effects without hypercalcemia. We used mammary xenografts with differential vitamin D receptor (VDR) expression to examine whether the antitumor effects of either therapy are receptor mediated. We present evidence that both EB1089 and UV exposure inhibit tumor growth via induction of growth arrest and apoptosis. These antitumor effects were observed only in xenografts containing VDR-positive tumor cells; heterogeneous tumors containing VDR-negative tumor cells and VDR-positive stromal and endothelial cells were unresponsive to both therapies. No evidence for antiangiogenic effects of EB1089 were detected in this model system. Neither EB1089 nor UV was associated with overt toxicity, but keratinocyte proliferation was increased in UV-exposed skin. These data provide proof of principle that UV exposure modulates tumor growth via elevation of vitamin D signaling and that therapeutic approaches designed to target the vitamin D pathway will be effective only if tumor cells express functional VDR.

Dissociation of growth arrest and CYP24 induction by VDR ligands in mammary tumor cells.

Murine mammary tumor cells with differential vitamin D receptor (VDR) expression were used to study the mechanisms of growth inhibition by vitamin D steroids. In VDR-expressing WT145 cells, 1,25D and its synthetic analog EB1089 induce growth arrest and transcriptionally upregulate the well-characterized VDR target gene CYP24. 1,25D also induces apoptosis in WT145 cells through activation of initiator and executioner caspases and the calcium-dependent protease calpain. We also demonstrate that WT145 cells express CYP27B1, the enzyme that converts 25-hydroxyvitamin D(3) (25D) to 1,25D, and that 25D inhibits growth of these cells but does not trigger apoptosis or induce CYP24 expression. Comparative studies were conducted in KO240 cells, which were derived from VDR knockout mice and found to retain expression of CYP27B1. KO240 cells were not growth inhibited nor rendered apoptotic by any of the tested vitamin D compounds. These data conclusively demonstrate that VDR mediates the anti-proliferative and pro-apoptotic effects of vitamin D metabolites and analogs, but that the potency of a vitamin D compound to induce the VDR target gene CYP24 does not accurately predict its potency in mediating growth regulation.

Breast cancer cell regulation by high-dose Vitamin D compounds in the absence of nuclear vitamin D receptor.

1alpha,25-dihydroxyvitamin D(3) (1,25D(3)) inhibits growth and induces apoptosis in breast cancer cells in vivo and in vitro. To examine the role of the Vitamin D receptor (VDR) in mediating the actions of 1,25D(3) at nanomolar and micromolar concentrations, mammary epithelial tumor cell lines generated in wild type (WT) and VDR knockout (VDRKO) mice were utilized. WT cells express VDR and are growth inhibited by 1,25D(3) and synthetic analogs EB1089 and CB1093 at 1nM concentrations, while VDRKO cells do not express VDR and are insensitive to Vitamin D compounds at concentrations up to 100nM. In the current studies, we have confirmed and extended these previous observations. At nanomolar concentrations of 1,25D(3) and all analogs tested, including EB1089, CB1093, MC1288, and KH1230, WT cells are growth inhibited and exhibit apoptotic morphology, while VDRKO cells show no growth inhibition or apoptosis. At concentrations of 1-10microM, however, 1,25D(3) and synthetic analogs induce growth inhibition and apoptotic morphology in both WT and VDRKO cell lines. These data indicate that nanomolar concentrations of 1,25D(3) and analogs mediate growth regulatory effects via mechanisms requiring the nuclear VDR, but that micromolar concentrations of Vitamin D compounds can exert non VDR-mediated effects.