In the 1920s, sunbathing and consumption of fish oil were prescribed to children who were at risk of developing rickets. Several observations highlight the wide range of effects of vitamin D on the human body. The influence of sunlight, latitude, and a vitamin D-rich diet on the development of many civilization diseases, including breast cancer, has been studied ^[1][23]. Numerous epidemiological studies have shown a direct relationship between geographic location and decreasing UV-B exposure as the distance from the equator increases. Low vitamin D levels due to low exposure to UV radiation, as well as consumption of a vitamin D-deficient diet, are associated with an increased risk of colon, breast, ovary, and prostate cancers ^{[2][3][4][5][6]}[17,18,19,20,24].

A study analyzed the serum level of 25-hydroxyvitamin D (25(OH)D; the main metabolite used to assess the vitamin D status in the body) in pre- and postmenopausal women, as well as genetic predisposition, such as VDR gene polymorphism and expression of vitamin D-metabolizing enzymes at various stages of cancer development ^[7][25].

The study revealed that higher serum 25(OH)D is strongly associated with better prognosis. Moreover, it was supposed that lower vitamin D serum levels in the summertime may speak for low vitamin D levels during the whole year. This in turn is of great importance according to the crucial role of vitamin D in various cellular processes, apart from maintaining calcium homeostasis, such as the regulatory processes of proliferation and differentiation of many normal and neoplastic cells, influence on the cell cycle, and stimulation of cell maturation and apoptosis through a mechanism dependent, among others, on VDR ^[8][9][10][26,27,28]. Vitamin D controls angiogenesis, influences signaling pathways, that are involved in migration and metastasis of cancer cells to distant tissues and organs, for example, by regulating the expression of adhesion molecules on the surface of these cells ^{[11][12][13][14][15][16][17]}[7,10,29,30,31,32,33]. Therefore, vitamin D is considered an important regulator of life processes in the human body.

Some clinical studies have analyzed the role of vitamin D in the pathogenesis of breast cancer, but not all of them support the protective role of vitamin D against breast cancer development. Women diagnosed with breast cancer revealed a significantly lower 25(OH)D level than healthy women. Patients with advanced or metastatic breast cancer showed significantly lower 25(OH)D status than patients with early-stage disease ^{[1][8][9][18][19]}[21,22,23,26,27].

On the other hand, a recent meta-analysis indicated that high serum level of 25(OH)D has a significant protective effect only in premenopausal women ^[15][31] or at the time of diagnosis. The European population-based cohort studies conducted among older adults confirmed that the risk of breast cancer increased with the concentration of 25(OH)D ^[16][32]. Recently, Kanstrup et al. showed poorer breast cancer survival among women with high levels of 25(OH)D (above 110 nmol/L) ^[17][33].

Studies on VDR expression revealed that low VDR expression in tumor was correlated with aggressive characteristics of breast cancer ^[10][28]. What’s more, a favorable tumor characteristic included smaller size, lower grade, estrogen receptor positivity and progesterone receptor positivity, and lower expression of Ki67 (overall better prognosis). Also, the nuclear and cytoplasmic VDR expression were associated with a low risk of breast cancer mortality, with hazard ratios 0.56 (95% CI 0.34–0.91) and 0.59 (0.30–1.16), respectively ^[13][29]. Moreover, the VDR expression in circulating tumor cells (CTCs) can be suggested as a potential prognostic biomarker of breast cancer ^[14][30].

Apart from the studies concerning the relationships between the expression and polymorphism of vitamin D-associated molecules (including VDR, CYP24A1, and CYP27B1) and breast cancer, those focusing on CaSR, which regulates the release of parathyroid hormone (PTH) and calcitonin in response to changes in blood calcium levels, seem to be of great importance ^[20][21][22][79,80,81]. Together with PTH, calcitonin, and calcitriol, CaSR contributes to maintaining the calcium homeostasis in the body. This receptor is found not only in cells involved in the regulation of calcium levels, such as thyroid C cells, kidney cells, colon epithelial cells, osteoclasts, and osteoblasts, but also in cells of the brain, pancreas, and stomach ^[23][82]. Furthermore, the presence of CaSR has been observed in neoplastic cells of the breast, prostate, and colon as well as in parathyroid neoplasms; however, the level of CaSR expression in neoplastic tissues significantly differs from that in normal tissues ^[24][25][26][83,84,85]. In normal mammary tissue, CaSR is found in epithelial cells, and its expression changes as a result of changes occurring in the breast. The level of CaSR remains lower before and during pregnancy, but during the lactation period it significantly increases due to changes in the concentration of calcium ions ^[26][27][85,86]. CaSR expression at the mRNA and protein level has been studied in breast cancer cells. In normal breast tissue, the secretion of PTH is inhibited by an increase in the calcium level. In prostate and breast tumors, an increase in calcium enhances the secretion of PTH-related protein (PTHrP) by influencing G protein activation, as demonstrated in the MCF-7 model, which often results in hypercalcemia ^[28][87]. The release of a high concentration of calcium from bone during breast cancer metastasis is believed to activate multiple CaSR signaling pathways that regulate the growth, proliferation, and migration of cancer cells to bone ^[27][29][30][86,88,89]. It was found that increased CaSR expression and increased PTHrP level in breast cancer cells correlate with higher predisposition of these cells to metastasize and colonize bones compared to other tissues and organs, such as the brain. Mamillapalli et al. have shown that the use of anti-PTHrP antibodies in mice bearing the MDA-MB-231 human mammary carcinoma xenograft reduced the number of bone metastases ^[28][87]. In response to an increase in calcium concentration, CaSR stimulates the activation of choline kinase and the production of phosphocholine, thereby accelerating the proliferation of cancer cells and enhancing their invasiveness and resistance ^[31][90]. Moreover, observations indicate that CaSR can regulate the proliferation of tumor cells by activating EGFR ^[32][33][91,92]. In MCF-7 human breast cancer cells, calcitriol was found to activate apoptosis via a nongenomic pathway involving CaSR ^[34][35][36][93,94,95].

The presence of CaSR in breast cancer cells and in bones, as well as in metastases, opens up new possibilities of applying the knowledge about this receptor to regulate the disorders caused by changes in bone structure and the course of cancer, as well as to reduce the risk of developing malignant tumors.

Gnagnarella et al. indicated in their review that data collected from various published studies do not allow for drawing unequivocal conclusions about the influence of vitamin D supplementation on mortality in cancer patients, and more randomized clinical trials are therefore needed ^[37][96]. A study by Lin et al. on premenopausal women showed that simultaneous intake of increased doses of calcium and vitamin D in the diet or in the form of dietary supplements reduces the risk of developing breast cancer ^[38][42]. This finding was also confirmed in other studies, which demonstrated a significant correlation between a diet rich in calcium and vitamin D and the incidence of ER+ breast cancer ^[38][39][40][42,43,47]. However, other randomized controlled trials did not show any substantial relationship between the intake of calcium and vitamin D and the risk of developing breast cancer ^[41][42][43][39,44,97].

Antineoplastic treatment of hormone-dependent breast cancer is based on the use of selective estrogen receptor modulators (SERMs), such as tamoxifen (TAM). However, if used for a long term, TAM may exert an agonistic effect on estrogens, stimulating endometrial tissue to express high levels of ER, thus leading to the development of endometrial diseases including cancer ^[44][45][46][98,99,100]. In addition, the use of TAM can result in acquired resistance in many patients, which may depend on the presence of ERβ, as well as increased expression of EGFR, HER2, or insulin-like growth factor 1 receptor ^[47][101]. Due to the numerous side effects and resistance related to the use of SERMs, the treatment strategy was changed from restricting access to ERs to reducing or blocking estrogen synthesis. Aromatase inhibitors (AIs) were developed to prevent the conversion of androgens to estrogens, by binding to the active site of the enzyme. Currently, apart from SERMs, third-generation AIs, such as anastrozole or letrozole, are used in the treatment of breast cancer. These inhibitors outperform compounds of previous generations in the specificity of binding to the enzyme. They also have a higher efficacy and produce less intrusive side effects. Long-term use of anastrozole, as well as other AIs, leads to almost complete inhibition of estrogen synthesis, thus inhibiting the development of breast cancer. However, AIs also exert undesirable effects on the body. They are not tissue-specific and inhibit the activity of aromatase in each cell where it occurs. Complete elimination of estrogens results in abnormal functioning of the bone tissue, bone decalcification, reduction in bone density, greater bone fragility, and appearance of pain and aggravation of osteoporosis in patients suffering from this condition ^[48][49][102,103]. Since estrogens are also vital for the proper functioning of skeletal and smooth muscles, people treated with AI experience muscle pain due to estrogen deficiency. To overcome the side effects of AI on the skeleton, as well as to reduce the formation of bone metastases, bisphosphonates are used ^[50][51][52][104,105,106]. These compounds prevent bone resorption and metastasis by inhibiting the adhesion and proliferation of cancer cells in the bone, as well as hypercalcemia associated with cancer development. However, the use of bisphosphonates also causes side effects, including nephrotoxicity and hypophosphatemia ^[52][106]. Moreover, studies have demonstrated vitamin D deficiency among breast cancer patients with metastatic bone disease who were treated with bisphosphonates and recommend vitamin D supplementation for these patients ^{[53][54][55][56]}[107,108,109,110]. It was also proven that in patients treated with AIs for ER+ breast cancer the proper management of osteoporosis with the use of bisphosphonates and vitamin D allows reducing the risk of relapse and death by 50% ^[57][111]. In addition, some studies have indicated that anticancer therapy can be supported with the intake of high doses of vitamin D₃ and calcium, omitting bisphosphonates, due to their side effects ^[58][59][112,113]. The use of high concentrations of vitamin D₃ and calcium seems to be justified due to the fact that the development of breast cancer is accompanied by a decrease in the vitamin D₃ concentration in the serum ^[55][60][109,114] and its synthesis in the cancer and surrounding tissues ^[3][18]. Recently published studies showed that de novo postdiagnosis supplementation of vitamin D (n-5417) caused a reduction in breast cancer-specific mortality ^[61][62][115,116]. Not only patients with ER+ breast cancer and treated with endocrine therapies may benefit from vitamin D. A retrospective review analyzing the effect of trastuzumab treatment in nonmetastatic HER2+ breast cancer patients revealed that high vitamin D intake during therapy improved disease-free survival but not overall survival ^[63][117]. The literature also includes interventional studies. The results of a randomized, placebo-controlled trial carried out for 5 years on a large number of participants highlighted that supplementation with 2000 IU of vitamin D had no effect on the incidence of invasive breast cancer ^[64][118]. But on the other side, a 20 years of follow-up study in cancer-free postmenopausal women with daily supplementation of calcium (1000 mg) and vitamin D (starting from 400 IU/day) was associated with lower risk of Ductal Carcinoma in Situ (DCIS), which may raise the possibility that consequent supplement intake may provide long-term benefits in the prevention of DCIS ^[65][119]. Patients with nonmetastatic HER+ breast cancer supplemented with vitamin D during neoadiuvant chemotherapy revealed improved disease-free survival ^[63][117]. Another cohort study revealed that everyday supplementation with vitamin D and calcium by menopausal hormone therapy users is associated with decreased postmenopausal breast cancer risk ^[66][120].

Clinical findings concerning the progression of tumor growth and metastasis via vitamin D deficiency are supported by studies performed using transgenic mouse models of spontaneous mammary gland cancer ^[67][129] or xenografts of human breast cancer cell lines ^[68][130] and allografted mouse tumors ^{[69][70][71][72]}[131,132,133,134]. In addition, a study on a breast cancer bone metastasis model showed that vitamin D deficiency resulting from 1α-hydroxylase (Cyp27b1) knockout increased the growth of TM40D mammary gland tumor in bone and accelerated tumor-induced bone destruction ^[70][132]. Moreover, in a mouse breast cancer model, targeted Cyp27b1 gene ablation in the mammary epithelium of polyoma middle T antigen-mouse mammary tumor virus (PyMT-MMTV) led to the initiation and acceleration of spontaneous mammary tumorigenesis ^[73][135]. The anticancer and antimetastatic activity of calcitriol and its analogs have been observed in various breast cancer models ^[74][75][76][136,137,138]. Furthermore, VDR knockdown was found to significantly accelerate the metastasis of MDA-MB-231 human breast cancer cells to bone ^[77][139], as well as increase primary tumor growth and metastasis of 168FARN mouse mammary gland tumors to the liver ^[69][131]. Additionally, the loss of VDR signaling in MMTV-Ron VDR⁻/⁻ mice caused an increase in spontaneous breast tumorigenesis and enhanced metastasis to the lungs and liver ^[78][140]. On the other hand, Trivedi et al. indicated that ablation of VDR (in the absence of ligand) reduced MCF-7 tumor growth in mammary fat pad and in bone ^[79][141]. Furthermore, the authors observed protumoral ^[80][142] or prometastatic activity of calcitriol in prostate ^[81][143] and breast cancer models ^[82][83][144,145].

Experimental studies also show that combined use of vitamin D or its analogs with chemotherapy agents has potential benefits. As reported by Krishnan et al. in a MCF-7 xenograft study the aromatase mRNA was reduced in the tumor and surrounding mammary adipose tissue after calcitriol treatment, but there was no change in aromatase mRNA noticed in the ovary. This selective aromatase gene regulation by calcitriol results from/is dependent on the presence of different promoters in these tissues. Calcitriol acts through the vitamin D response element present in the aromatase promoter II and, on the other way, suppresses the aromatase gene transcription by reducing the level of prostaglandins involved in estrogen synthesis in the breast tissue ^[84][146].