It is now well established that, in addition to its central role in the maintenance of extracellular calcium levels and bone mineralization, 1,25-dihydroxyvitamin D 3 (1,25(OH) 2 D 3 ), the active form of vitamin D, also acts as a modulator of cell growth and differentiation in a number of cell types, including breast cancer cells. The anti-proliferative effects of 1,25(OH) 2 D 3 have been linked to suppression of growth stimulatory signals and potentiation of growth inhibitory signals, which lead to changes in cell cycle regulators such as p21WAF-1/CIP1 and p27 kip1 , cyclins and retinoblastoma protein as well as induction of apoptosis. Such studies have led to interest in the potential use of 1,25(OH) 2 D 3 in the treatment or prevention of certain cancers. Since this approach is limited by the tendency of 1,25(OH) 2 D 3 to cause hypercalcaemia, synthetic vitamin D analogues have been developed which display separation of the growth regulating effects from calcium mobilizing actions. This review examines mechanisms by which 1,25(OH) 2 D 3 and its active analogues exert both anti-proliferative and pro-apoptotic effects and describes some of the synthetic analogues that have been shown to be of particular interest in relation to breast cancer.

Today, it is well established that besides playing a crucial role in the establishment and maintenance of the calcium homeostasis in the body, the active form of vitamin D, 1,25(OH)2D3, also acts an effective regulator of cell growth and differentiation in a number of different cell types, including cancer cells. This has led to an increased interest in using 1,25(OH)2D3 in the treatment or prevention of cancer patients and to a substantial number of studies investigating the effect of 1,25(OH)2D3 on cancer cells. The results are encouraging, but clearly demonstrate that the therapeutic window of 1,25(OH)2D3 is extremely narrow due to the calcemic adverse effects of this compound. Much effort has consequently been directed into identifying vitamin D analogs with potent cell regulatory effects but with weaker effects on the calcium metabolism than those of 1,25(OH)2D3. In an attempt to clarify the mechanisms implicated in the cell regulatory effects of 1,25(OH)2D3 and eventually facilitate the process of developing new specific vitamin D analogs, numerous investigations have been carried out with 1,25(OH)2D3 and its analogs. The present review will focus on the results obtained in these studies and describe some of the synthetic analogs, which have shown to be of particular interest in relation to cancer.

It is well established that the metabolically active form of vitamin D, 1alpha,25-dihydroxyvitamin D3 (1alpha,25(OH)2D3) plays a key role in the establishment and maintenance of the calcium metabolism in the body. In addition to this classic effect of 1alpha,25(OH)2D3, substantial evidence has emerged demonstrating that 1alpha,25(OH)2D3 is able to regulate cell growth and differentiation in a number of different cell types, including cancer cells. However, the clinical usefulness of 1alpha,25(OH)2D3 is limited by its tendency to cause hypercalcaemia. Much effort has therefore been directed to identifying new vitamin D analogues with potent cell regulatory effects, but with weaker effects on the calcium metabolism than those of 1alpha,25(OH)2D3. One of these new synthetic analogues is Seocalcitol (EB 1089). Despite being 50-200 times more potent than 1alpha,25(OH)2D3 with respect to regulation of cell growth and differentiation in vitro as well as in vivo, EB 1089 displays a reduced calcaemic activity in vivo compared to that of 1alpha, 25(OH)2D3. These characteristics make EB 1089 a potentially useful compound for the treatment of cancer. Recent clinical evaluation of EB 1089 has focused mainly on establishing a maximum tolerated dose in cancer patients. Early results confirm that the low calcaemic activity observed in animals can be reproduced in the clinic. Furthermore, EB 1089 has been shown to induce regression of tumours, especially in hepatocellular carcinoma where complete remission has been obtained. In conclusion, the development of EB 1089 as an anti-cancer drug holds promise. However, its final evaluation must await the completion of ongoing controlled clinical trials.

The biologically active form of vitamin D3, the nuclear hormone 1 alpha,25-dihydroxyvitamin D3 (VD), is an important regulator of cellular growth, differentiation, and death. The hormone mediates its action through the activation of the transcription factor VDR, which is a member of the superfamily of nuclear receptors. In most cases the ligand-activated VDR is found in complex with the retinoid X receptor (RXR) and stimulates gene transcription mainly from VD response elements (VDREs) that are formed by two hexameric core binding motifs and are arranged either as a direct repeat spaced by three nucleotides (DR3) or as an inverted palindrome spaced by nine nucleotides (1P9). The two VD analogues CB1093 and EB1089 are both very potent inhibitors of the proliferation of MCF-7 cultured breast cancer cells displaying approximately 100-fold lower IC50 values (0.1 nM) than the natural hormone. In addition, CB1093 is even more potent in vivo than EB1089 in producing regression of experimental mammary tumors. Moreover, both VD analogues induce apoptosis in MCF-7 cells, but CB1093 is effective at concentrations approximately 10-fold lower than EB1089. In accordance, the reduction of Bcl-2 protein expression showed CB1093 to be more potent than EB1089. This suggests that the antiproliferative effect of CB1093 may be related mainly to its apoptosis inducing effect, whereas EB1089 may preferentially have effects on growth arrest. EB1089 is known to result in a selectivity for the activation of IP9-type VDREs, whereas CB1093 shows a preference for the activation of DR3-type VDREs. This promoter selectivity suggests that the effects of VD and its analogues on growth arrest and the induction of apoptosis may be mediated by different primary VD responding genes. In conclusion, CB1093 was found to be a potent inhibitor of rat mammary tumor growth in vivo. CB1093 also displayed a high potency in vitro in the induction of apoptosis, a process that may be linked to a promoter selectivity for DR3-type VDREs.

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