Studies of the effects of 1,25-dihydroxyvitamin D on skeletal and calcium homeostasis and on inhibition of tumor cell growth

Role of vitamin D and vitamin D receptor (VDR) in oral cancer

Oral cancer is known as one of the most common cancers, with a poor prognosis, related to delayed clinical diagnosis, either due to the lack of particular biomarkers related to the disease or costly therapeutic alternatives. Vitamin D executes its functions by interacting with the vitamin D receptor (VDR), both in healthy and diseased individuals, including oral cancer. This review discusses the role of vitamin D and VDR on tumorigenesis, emphasizing on oral cancer. Furthermore, regulation of VDR expression, mechanisms of anticancer effects of calcitriol, oral cancer chemoresistance and its relation with VDR and polymorphisms of VDR gene will be discussed. The manuscript is prepared mainly using the information collected from PubMed and MEDLINE.

Vitamin D deficiency promotes human breast cancer growth in a murine model of bone metastasis

Vitamin D exerts antiproliferative, prodifferentiation, and proapoptotic effects on nonclassic target tissues such as breast. Blood levels of 25-hydroxyvitamin D [25(OH)D], the most sensitive indicator of vitamin D status, are inversely correlated with breast cancer risk; however, a causal relationship between vitamin D deficiency and breast cancer growth in bone has not been assessed. We examined the effect of vitamin D deficiency on the intraskeletal growth of the human breast cancer cell line MDA-MB-231-TxSA in a murine model of malignant bone lesions. Subsets of mice were treated concurrently with osteoprotegerin (OPG) to abrogate bone resorption. Outcomes were assessed by repeated radiographic and end-point micro-computed tomography and histologic analyses. Mice weaned onto a vitamin D-free diet developed vitamin D deficiency within 4 weeks [mean +/- SE serum 25(OH)D: 11.5 +/- 0.5 nmol/L], which was sustained throughout the study and was associated with secondary hyperparathyroidism and accelerated bone turnover. Osteolytic lesions appeared earlier and were significantly larger in vitamin D-deficient than in vitamin D-sufficient mice after 2 weeks (radiographic osteolysis: +121.5%; histologic tumor area: +314%; P < 0.05). Although OPG treatment reduced the size of radiographic osteolyses and tumor area in both groups, tumors remained larger in OPG-treated vitamin D-deficient compared with OPG-treated vitamin D-sufficient mice (0.53 +/- 0.05 mm(2) versus 0.19 +/- 0.05 mm2; P < 0.05). We conclude that vitamin D deficiency promotes the growth of human breast cancer cells in the bones of nude mice. These effects are partly mediated through secondary changes in the bone microenvironment, along with direct effects of vitamin D on tumor growth.

Vitamin D and differentiation in cancer

This paper reviews the current understanding of the vitamin D-induced differentiation of neoplastic cells, which results in the generation of cells that acquire near-normal, mature phenotype. Examples of the criteria by which differentiation is recognized in each cell type are provided, and only those effects of 1alpha,25-dihydroxyvitamin D(3) (1,25D) on cell proliferation and survival that are associated with the differentiation process are emphasized. The existing knowledge, often fragmentary, of the signaling pathways that lead to vitamin D-induced differentiation of colon, breast, prostate, squamous cell carcinoma, osteosarcoma, and myeloid leukemia cancer cells is outlined. The important distinctions between the different mechanisms of 1,25D-induced differentiation that are cell-type and cell-context specific are pointed out where known. There is a considerable body of evidence that the principal human cancer cells can be suitable candidates for chemoprevention or differentiation therapy with vitamin D. However, further studies are needed to fully understand the underlying mechanisms in order to improve the therapeutic approaches.

Thyroid hormone-induced demineralisation of the vertebral skeleton of the eel, Anguilla anguilla

The role of thyroid hormones (TH) in bone remodelling is controversial. Indeed, in humans, while they are necessary for normal growth and development, their overproduction can induce important mineral bone loss and osteoporosis. Intense bone resorption is a natural phenomenon also observed in some teleosts, during reproductive migration and fasting. Our work aimed at investigating the effects of chronic treatments with TH (thyroxin, T4 or triiodothyronine, T3) on bone resorption in a migratory fish, the European eel (Anguilla anguilla), a representative species of an ancient group of teleosts (Elopomorphs). The incineration method showed that TH induced a significant mineral loss in eel vertebral skeleton. Histology and histophysical (qualitative and quantitative microradiographs) methods were then applied to vertebral sections to determine which types of resorption were induced by TH. Quantitative image analysis of microradiographs showed that TH significantly increased the porosity of the vertebrae, demonstrating the induction of a severe bone loss. Histology revealed the appearance of large osteoclastic lacunae, indicating a stimulation of osteoclastic resorption. Quantitative image analysis of ultrathin microradiographs showed a significant increase of the size of osteocytic lacunae, indicating a stimulation of periosteocytic osteolysis. Finally, quantitative microradiographs indicated a significant fall of mineralisation degree. TH treatments did not stimulate the production of the calcium-bonded lipo-phospho-protein vitellogenin, indicating that TH-induced bone demineralisation was not mediated by any indirect effect on vitellogenesis. Our study demonstrates that TH may participate in the mobilisation of bone mineral stores in the eel, by inducing different types of vertebral bone resorption, such as osteoclastic resorption and periosteocytic osteolysis. These data suggest that the stimulatory action of TH on bone resorption may be an ancient regulatory mechanism in vertebrates.

Vitamin D deficiency associated with number of neurofibromas in neurofibromatosis 1

Neurofibromatosis 1 (NF1) is a tumour suppressor gene syndrome characterized by multiple cutaneous and plexiform neurofibromas. Focal osseous abnormalities, short stature, and decreased bone mineral density are also frequent in people with NF1. We measured serum 25-hydroxyvitamin D concentrations in 55 patients with NF1 and 58 healthy controls, and correlated the findings in the patients with NF1 with their estimated number of dermal neurofibromas. Geometric mean (SD) serum 25-hydroxyvitamin D concentration was 14.0 (1.6) ng/mL among the patients with NF1 compared with 31.4 (1.7) ng/mL among healthy controls (p<<0.0001). The serum vitamin D concentration and number of dermal neurofibromas reported by patients with NF1 were inversely correlated (Spearman's rho = -0.572, p<0.00001). The occurrence of low serum vitamin D concentrations in people with NF1, especially those with many dermal neurofibromas, may provide new pathogenic insights and have important therapeutic implications.

Vitamin D and skin cancer: a problem in gene regulation

The skin is the major source of Vitamin D(3) (cholecalciferol), and ultraviolet light (UV) is critical for its formation. Keratinocytes, the major cell in the epidermis, can further convert Vitamin D(3) to its hormonal form, 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] (calcitriol). 1,25(OH)(2)D(3) in turn stimulates the differentiation of keratinocytes, raising the hope that 1,25(OH)(2)D(3) may prevent the development of malignancies in these cells. Skin cancers (squamous cell carcinoma (SCC), basal cell carcinoma (BCC), and melanomas) are the most common cancers afflicting humans. UV exposure is linked to the incidence of these cancers-UV is thus good and bad for epidermal health. Our focus is on the mechanisms by which 1,25(OH)(2)D(3) regulates the differentiation of keratinocytes, and how this regulation breaks down in transformed cells. Skin cancers produce 1,25(OH)(2)D(3), contain ample amounts of the Vitamin D receptor (VDR), and respond to 1,25(OH)(2)D(3) with respect to induction of the 24-hydroxylase, but fail to differentiate in response to 1,25(OH)(2)D(3). Why not? The explanation may lie in the overexpression of the DRIP complex, which by interfering with the normal transition from DRIP to SRC as coactivators of the VDR during differentiation, block the induction of genes required for 1,25(OH)(2)D(3)-induced differentiation.

Vitamin D and skin cancer

Skin cancer is the most common cancer afflicting humans. These cancers include melanomas and 2 types of malignant keratinocytes: basal-cell carcinomas (BCC) and squamous-cell carcinomas (SCC). UV light exposure is linked to the incidence of these cancers. On the other hand, the skin is the major source of vitamin D-3 (cholecalciferol) and UV light is critical for its formation. Keratinocytes can convert vitamin D-3 to its hormonal form, 1,25 dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] (calcitriol). 1,25(OH)(2)D(3) in turn stimulates the differentiation of keratinocytes, raising the hope that 1,25(OH)(2)D(3) may prevent the development of malignancies in these cells. We identified a number of mechanisms by which 1,25(OH)(2)D(3) regulates the differentiation of keratinocytes and explored where this regulation breaks down in SCCs. 1,25(OH)(2)D(3) regulates gene expression by activating the vitamin D receptor (VDR). When activated, the VDR binds to one of two coactivator complexes: DRIP or p160/SRC. Binding to DRIP occurs in the undifferentiated keratinocyte, but, as the cell differentiates, DRIP(205) levels fall and p160/SRC binding takes over as SRC3 expression increases. SCCs fail to respond to the prodifferentiating actions of 1,25(OH)(2)D(3). These cells have normal levels of VDR and normal binding of VDR to vitamin D response elements. However, they overexpress DRIP(205) such that the p160/SRC complex is blocked from binding to VDR. We hypothesize that failure of 1,25(OH)(2)D(3) to induce differentiation in SCCs lies at least in part with its failure to induce the replacement of the DRIP complex with the SRC complex in the promoters of genes required for differentiation.

PTH-1R responses to PTHrP and regulation by vitamin D in keratinocytes and adjacent fibroblasts

Vitamin D and PTHrP are essential for the differentiation of keratinocytes and epidermal development. The action of PTHrP on skin is mediated via its PTH-1R receptors present in both epidermal and dermal cells. This suggests that PTHrP may have a paracrine/autocrine role, and its receptors may act in association or in negative cooperativity. We compared the intracellular signaling pathways in response to PTHrP (1-34) and to various PTHrP peptides, the N-terminal (1-34), Mid region (67-89), and C-terminal (107-139) fragments, and the possible modulation of PTHrP and its receptor mRNA expressions by vitamin D. Adjacent dermal fibroblasts as freshly isolated keratinocytes expressed both PTHrP and PTH-1R mRNAs, and responded to the various PTHrP fragments. bPTH and PTHrP(1-34) increased both cellular cAMP and [Ca(2+)]i in keratinocytes and fibroblasts. In contrast, PTHrP (107-139) increased [Ca(2+)]i but not cAMP in the two cell types. PTHrP (67-89) had no effect in keratinocytes, and only increased [Ca(2+)]i in fibroblasts. Vitamin D deficiency in weaned rats increased the expression of PTHrP mRNA in keratinocytes, and decreased it in fibroblasts and kidneys. Vitamin D deficiency increased PTH-1R mRNA expression in keratinocytes and kidneys, but not in fibroblasts. Although keratinocytes and skin fibroblasts are target cells for PTHrP and express PTH-1R, the two adjacent cell types differ as regards their intracellular signaling in response to PTHrP peptides. Moreover vitamin D regulates PTHrP and PTH-1R in a cell-specific manner.