Effects of the vitamin D3 analog 1 alpha, 25-dihydroxyvitamin D3-3 beta-bromoacetate on rat osteosarcoma cells: comparison with 1 alpha, 25-dihydroxyvitamin D3

Enhanced Antiproliferative Effect of Combined Treatment with Calcitriol and All-Trans Retinoic Acid in Relation to Vitamin D Receptor and Retinoic Acid Receptor α Expression in Osteosarcoma Cell Lines

The main objective of this study was to analyze changes in the antiproliferative effect of vitamin D3, in the form of calcitriol and calcidiol, via its combined application with all-trans retinoic acid (ATRA) in osteosarcoma cell lines. The response to treatment with calcitriol and calcidiol alone was specific for each cell line. Nevertheless, we observed an enhanced effect of combined treatment with ATRA and calcitriol in the majority of the cell lines. Although the levels of respective nuclear receptors did not correlate with the sensitivity of cells to these drugs, vitamin D receptor (VDR) upregulation induced by ATRA was found in cell lines that were the most sensitive to the combined treatment. In addition, all these cell lines showed high endogenous levels of retinoic acid receptor α (RARα). Our study confirmed that the combination of calcitriol and ATRA can achieve enhanced antiproliferative effects in human osteosarcoma cell lines in vitro. Moreover, we provide the first evidence that ATRA is able to upregulate VDR expression in human osteosarcoma cells. According to our results, the endogenous levels of RARα and VDR could be used as a predictor of possible synergy between ATRA and calcitriol in osteosarcoma cells.

Snail-1 regulates VDR signaling and inhibits 1,25(OH)-D₃ action in osteosarcoma

Previous research has shown that vitamin D could suppress proliferation, migration and invasion of cancers, but the effects of vitamin D may be related to the expression of Snail-1, which could inhibit the expression of the vitamin-D gene receptor (VDR). Snail-1 is overexpressed in osteosarcoma, this study was conducted to determine whether inhibiting Snail-1 could increase the role of vitamin D as an anti- osteosarcoma agent. We used stable transfection of the SaOS₂ cell line as in vitro model to study the effect of 1,25(OH)-D₃, which is the most active metabolite of vitamin D. The in vitro antiproliferative, pro-apoptotic and inhibiting of invasion effects were examined. The effects of 1,25(OH)-D₃ on the expression of β-catenin signaling pathways were also studied. Then in vivo antiproliferative effect of 1,25(OH)-D₃ was also detected in nude mice injected with either mock-infected or Snail-1 SaOS₂ cells. We found that inhibition of Snail-1 signaling by transfection could increase the expression of VDR, enhance the anti-proliferative activity of 1,25(OH)-D₃ in osteosarcoma cells, and induce apoptosis and lower invasion in vitro. The effect of 1,25(OH)-D₃ was also associated with decreased expression of β-catenin signaling, which is related to VDR signaling. In vivo, the effect of antiproliferative was higher in mice injected with either Snail-1-infected cells than with mock-infected cells. Our findings suggest that canonical Snail-1/VDR/β-catenin signaling reflects an important underlying mechanism of osteosarcoma progression. Therefore, strategies to suppress Snail-mediated signaling may lead to the better action of 1,25(OH)-D₃ as an anti osteosarcoma treatment.

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.

Regulation of cerebellar neuronal migration and neurite outgrowth by thyroxine and 3,3',5'-triiodothyronine

The timing of granule cell migration in the developing cerebellum is regulated by thyroid hormone. Granule cell migration depends on the recognition of extracellular neuronal guidance molecule(s), such as laminin, and this, in turn, requires cell surface adhesion molecules (integrins) that are anchored on the cell membrane by the actin cytoskeleton. While many of the actions of thyroid hormone, specifically 3,5,3'-triiodothyronine (T3), are mediated by regulated gene expression, both thyroxine (T4) and 3,3',5'-triiodothyronine (rT3) also exert direct, positive control of the quantity of polymerized actin in cultured astrocytes without affecting gene expression. T4-dependent actin polymerization has been shown to (i) participate in the immobilization of laminin to the cell surface, (ii) help deposit laminin in the molecular layer of the developing cerebellum, and (iii) anchor integrin(s) that recognize laminin present in the extracellular matrix. In this study, we show that both T4 and rT3, but not T3, directly regulate the F-actin content of elongating neurites of cerebellar neurons. T4 and rT3 also promoted extensive granule cell migration from cerebellar explants, as well as, dense cell clustering and extensive neuronal process formation when granule cells were grown on a laminin-coated surface. Both granule cell migration and neuronal process outgrowth were markedly attenuated by the addition of integrin-blocking antibodies or binding peptides, by the absence of thyroid hormone or the presence of T3. These data suggest that the T4-dependent actin polymerization in developing neurons is necessary for these migrating cells to recognize the laminin guidance molecule, thereby providing a novel molecular mechanism for the profound influence of thyroid hormone on brain development that is independent of regulated gene expression.

Degradation of RXRs influences sensitivity of rat osteosarcoma cells to the antiproliferative effects of calcitriol

Several cell lines, including ROS17/2.8 rat osteosarcoma (ROS) cells, contain functional VDRs and RXRs but are resistant to the antiproliferative effects of calcitriol and retinoids. We explored the role of receptor degradation in this hormone resistance. Results of transactivation assays indicated that ROS cells contain insufficient amounts of RXR to activate a DR-1 reporter, and Western blot analyses of cell extracts showed that the degradation of RXR is accelerated and produces an aberrant 45-kDa RXR. We stably expressed functional fluorescent chimeras of VDR and RXR [green fluorescent protein (GFP)-VDR; yellow fluorescent protein (YFP)-RXR] to evaluate degradation mechanisms and the impact of excess receptor expression on antiproliferative effects. Microscopy showed a diminished expression of YFP-RXR in ROS cells compared with the expression in CV-1 cells. Treatment with inhibitors of proteasomal degradation (lactacystin and MG132) selectively enhanced GFP-VDR and YFP-RXR expression and also increased the endogenous levels of VDR and RXR. Expression of GFP-VDR had no effect on the sensitivity of ROS cells to calcitriol. Increases of RXR levels by YFP-RXR expression, drug treatments, or the combination of the two, however, restored the growth-inhibitory effects of calcitriol and 9-cis-RA and restored p21 induction by calcitriol. These studies revealed that an accelerated and aberrant RXR degradation could cause resistance to the antiproliferative effects of calcitriol and retinoids in ROS cells.

Nongenomic effects of 1alpha,25-dihydroxyvitamin D(3)

The hormonally active form of vitamin D, 1alpha,25-dihydroxyvitamin D(3), is the key molecule of the vitamin D endocrine system, which produces biological effects in about 30 target cell systems. Growing experimental evidence supports the hypothesis that these biological effects can be generated both by a signal transduction mechanism involving a nuclear receptor (nVDR) that modulates gene transcription, and via a nongenomic receptor located in the plasma membrane (mVDR), which modulates a complex signaling system involving the rapid opening of Ca(2+) channels. Some data reviewed herein also indicate that crosstalk between genomic and nongenomic pathways operates in several cell types, and suggest that the physiological role of the rapid, nongenomic actions might involve the regulation of hormone-mediated gene activation.