Vitamin D and Breast Cancer: Insights in Possible Treatment Options of Breast Cancer

Epidemiologic studies have indicated that vitamin D might play a protective role against breast cancer. Incidence of breast cancer and mortality rate considerably vary worldwide and reveal a geographic pattern. The lowest rates of breast cancer generally occur in countries close the equator. With increasing latitude reported breast cancer incidence and mortality rates also increase. A negative correlation between available sunlight and breast cancer death rates has been shown. Because sunlight exposure is a measure of vitamin D produced in the skin, it has been hypothesized that vitamin D formed in the skin may reduce the risk of breast cancer [1-3]. Breast cancer is the most frequent cause of cancer death in women in the western world. Many studies have tried to identify the casual factors responsible for the uncontrolled growth of the tumor cells. A variety of biochemical and genetic changes have been identified in breast carcinomas and have been found to be related to breast cancer growth.


Mini Review
Epidemiologic studies have indicated that vitamin D might play a protective role against breast cancer. Incidence of breast cancer and mortality rate considerably vary worldwide and reveal a geographic pattern. The lowest rates of breast cancer generally occur in countries close the equator. With increasing latitude reported breast cancer incidence and mortality rates also increase. A negative correlation between available sunlight and breast cancer death rates has been shown. Because sunlight exposure is a measure of vitamin D produced in the skin, it has been hypothesized that vitamin D formed in the skin may reduce the risk of breast cancer [1][2][3]. Breast cancer is the most frequent cause of cancer death in women in the western world. Many studies have tried to identify the casual factors responsible for the uncontrolled growth of the tumor cells. A variety of biochemical and genetic changes have been identified in breast carcinomas and have been found to be related to breast cancer growth.
However, especially because of the heterogeneity of the disease on the clinical, biologic and genetic levels, the exact mechanism of breast cancer development and progression is still unclear. During the last 15 years, it has become evident that 1, 25-dihydroxyvitamin D 3 (1,25(OH) 2 D 3 ), the biologically most active form of vitamin D 3 , exerts effects on a variety of tissues which are apparently unrelated to calcium homeostasis. 1,25(OH) 2 D 3 has been shown to induce cellular differentiation and inhibit proliferation of hematopoietic cells and cancer cells. In addition, studies with animal cancer models have shown that 1,25(OH) 2 D 3 application can prolong the survival of leukemic mice and suppress the growth of tumors of different origins including breast [4,5]. These newly discovered properties suggest a possible role of the hormone in the treatment of cancer. However, a major drawback for a clinical application is that high doses are needed.

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status of functional VDR has been demonstrated in breast cancer cells [40,41]. The underlying mechanism of this VDR-independent resistance to growth inhibition is unknown. The first studies on the effect of 1,25(OH) 2 D 3 on breast cancer cells showed a biphasic growth response of the estrogen receptor-positive T47-D human breast tumor cell line. At low concentrations (10-11 M), a stimulation of cell growth was observed [10,42,43]. This growthinhibitory effect of 1,25(OH) 2 D 3 was confirmed in other breast tumor cell lines and shown to be independent of the estrogen receptor status [44,45]. In the growth inhibited breast cancer cells 1,25(OH) 2 D 3 and 1,25(OH) 2 D 3 analogs caused an increase of the number of cells in the GO/GI and occasionally in the G2 phase together with a decrease of the number of cells in the S phase [40,44,[46][47][48][49] indicating a cell cycle block in the GO/GI phase.
Apoptosis (programmed cell death), is an asynchronous cellular process with cytoplasmic and nuclear condensation, disruption of the cytoskeleton and condensation of intermediate filaments around the nucleus and is related to the cell cycle [50]. Induction of apoptosis can be a possible way in which 1,25(OH) 2 D 3 inhibits tumor cell growth. Recently it has been shown that 1,25(OH) 2 D 3 induces apoptosis in various tumor cells [40,49,51,52]. A central role for apoptosis in the action of 1,25(OH) 2 D 3 is uncertain because growth inhibition of several breast cancer cells appeared to be independent of apoptosis [40]. Thus, the growth stimulation of MCF-7 cells, that were growth inhibited by 1,25(OH) 2 D 3 , after removal of 1,25(OH) 2 D 3 indicates independence of growth inhibition from apoptosis [53]. Possibly in these latter cases induction of differentiation is more prominent. Treatment of breast cancer cells with 1,25(OH) 2 D 3 resulted in morphologic changes, which may resemble a more differentiated status of the cells [43,54,55].
Induction of differentiation was recently shown in several breast cancer cells [40]. The results of these studies suggest that induction of differentiation and growth inhibition are two independent processes. The various synthetic vitamin D 3 analogs have been shown to be more potent than 1,25(OH) 2 D 3 in the growth inhibition of several cancer cell types, whereas there in vivo calcemic activity was similar or even reduced compared with 1,25(OH) 2 D 3 . Mammary tumors can be induced in rats by oral administration of the carcinogens N-nitroso-N-methylurea (NMU) or 7,12-dimethylben(a)anthracene (DMBA). Application of 1,25(OH) 2 D 3 resulted in an inhibition of the growth of NMU- [31,56] and DMBA-induced rat mammary tumors [57,58], whereas Noguchi et al. [59] did not find an effect of 1,25(OH) 2 D 3 on the incidence and growth of DMBA-induced rat mammary tumors. To achieve tumor suppression, high doses of 1,25(OH) 2 D 3 about 0,5 ug/kg BW) were needed with the subsequent development of hypercalcemia and weight loss. Thus, synthetic vitamin D 3 analogs with low in vivo calcemic activity have been developed.
Only a few analogs have been evaluated in vivo for their potential use in the treatment of breast cancer. To date, two clinical studies on the effect of vitamin D 3 analogs on cancer growth in humans have been reported. Topical application of calcipotriol (MC903) in a small group of patients with locally advanced or cutaneous metastatic breast cancer showed a reduction in the size of treated lesions in 4 of 14 patients [60], whereas another study could not confirm this observation [61]. In a phase I trial the analog EB1089 is being examined in advanced breast cancer, but no detailed analyses have been published. The antiestrogen tamoxifen is the most widely used endocrine agent in the treatment of breast cancer [62]. A major problem of tamoxifen therapy is that in case of response, the tumor almost inevitably progresses to a tamoxifen-resistant state during prolonged therapy. Furthermore, long-term tamoxifen therapy has been linked to an increased risk of endometrial cancer. Therefore, despite the efficacy of tamoxifen for breast cancer, alternative additional endocrine therapies are needed.
Thus, several studies have focused on possible future combination treatment with 1,25(OH) 2 D 3 and 1,25(OH) 2 D 3 analogs in estrogen receptor-positive and -negative breast cancer. A synergistic antiproliferative effect of submaximum dosages of the vitamin D analog, 22-oxacalcitriol, and tamoxifen in breast cancer cells has been described in vitro and in vivo [63]. Thus, the combined treatment with 1,25(OH) 2 D 3 and tamoxifen resulted in stronger growth inhibition of MCF-7 cells than treatment with either compound alone [53]. With a number of vitamins D3 analogs a similar effect was observed [37]. In combination with tamoxifen the cells were more sensitive to the antiproliferative action of 1,25(OH) 2 D 3 and the analogs [37]. The ability of tamoxifen to reduce the total tumor burden of rats treated with the carcinogen NMU is significantly enhanced by a combination of the vitamin D 3 analog, Ro24-5531, with low doses of tamoxifen [64]. Thus, implications for the use of vitamin D analogs not only in treatment but also in the prevention of breast cancer have been indicated.
Vitamin A-derivatives like fenretinide are currently being tested in clinical trials as preventive agents against recurrence of breast cancer, and animal studies point to a potential use of these compounds as therapeutic agents for breast cancer [65]. A combination therapy of retinoic acid and 1,25(OH) 2 D 3 showed a synergistic growth inhibition of breast cancer cells [66]. Furthermore, combinations with vitamin D 3 compounds and cytotoxic drugs (TNF, GMCSF, Adriamycin, 5-FU, Carboplatin, Cisplatin) have been studied [45,57,[67][68][69]. The data on combinations of 1,25 (OH) 2 D 3 and 1,25(OH) 2 D 3 analogs with various other anticancer compounds are promising and justifies further analyses. For example, the development of effective combination therapies may result in better response rates and lower dosages combined with a reduced risk of negative side-effects. Invasion and metastasis of tumor cells are the primary causes for the fatal outcome of cancer diseases. A recent report by Mork Hansen et al. [70] indicated that 1,25(OH) 2 D 3 may be effective in reducing the invasiveness of breast cancer cells.
They have shown 1,25(OH) 2 D 3 inhibited the invasion and migration of a metastatic human breast cancer cell line (MDA-MB-231). A fact to be considered in relation to metastasis is that bone is the most frequent site of metastasis of advanced breast cancer [71]. There are some indications from clinical studies that bone metastases develop preferentially in areas with high bone

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turnover [72,73]. By contrast, agents that inhibit bone resorption have been reported to reduce the incidence of skeletal metastasis [74]. As 1,25(OH) 2 D 3 is an important stimulator of bone resorption and consequently of bone tumover, treatment with 1,25(OH) 2 D 3 or vitamin D3 analogs for breast cancer might increase the risk of skeletal metastases. However, Krempien [75] reported that following intraarterial injection with Walker 256 tumor cells, rats treated with 1,25(OH) 2 D 3 developed significantly more bone metastases than untreated controls. In relation to this aspect, it was shown that tamoxifen, which exerts positive estrogenic effects on bone [76], considerably suppressed the bone resorption induced by 1,25(OH) 2 D 3 ED1089, and KH1060 [77]. These data suggest that in vitamin D-tamoxifen combination therapy for breast cancer tamoxifen may offer protection against the bone resorption induced by vitamin D 3 compounds and thereby decreases the risk of bone metastases.
Vitamin D 3 analogs have potent antiproliferative effects on breast cancer cells in vitro and suppress breast cancer growth in vivo without marked calcemic effects. However, apart from the strong calcemic activity of 1,25(OH) 2 D 3 other negative sideeffects may arise, in particular immunosuppressive effects and an increased risk of bone metastases. The development of new vitamin D3 analogs continues. In the future vitamin D 3 analogs with even stronger antiproliferative effects and better selectivity may become available. Vitamin D treatment theoretically could be beneficial for a large group of patients, since the VDR is expressed in about 80% of human breast cancers. Another promising aspect of vitamin D treatment might be its combination with other established endocrine (tamoxifen) or cytotoxic agents. Finally, epidemiologic studies and in-vitro results have suggested a role for vitamin D in the prevention of breast cancer. In the next few years, clinical studies are needed to confirm that vitamin D3 analogs, either alone or in combination with other antitumor agents, can provide an effective treatment for breast cancer.