Prognostic role of vitamin D receptor in breast cancer: a systematic review and meta-analysis

The global incidence of breast cancer (BC) has been on the rise since the late 1970s, seriously threatening women’s health [1]. According to statistics [2], there were 2.088 million new cases of BC in the world in 2018, and the incidence of BC in developed countries was significantly higher than that in developing countries. BC has become one of the most common malignant tumors that causes female death [3]. In the past few decades, despite improvements in surgical techniques and changes in chemotherapy and radiotherapy methods, the mortality rate of BC has significantly decreased, but the prognosis of BC patients is still not satisfactory [4]. Previous meta-analyses have supported an inverse association between vitamin D status/intake and BC occurrence [5, 6], and an association of low levels of vitamin D with increased risk of recurrence and death in BC patients [7, 8].

Vitamin D is a steroid hormone in structure, and its metabolic active substance is 1,25 (OH)₂D₃, which plays an important role in calcium and phosphorus metabolism. Preclinical studies have found that 1,25 (OH)₂D₃ can inhibit the proliferation of BC cell lines and promote their differentiation and apoptosis [9, 10]. Moreover, findings from a prospective study including 10,578 premenopausal and 20,909 postmenopausal women suggested that higher intakes of calcium and vitamin D may reduce the risk of BC in premenopausal women [11]. Therefore, in our clinical work, we suggest that early BC patients and premenopausal women should appropriately increase their vitamin D intake. However, it seems inappropriate to recommend that each patient take the same dose of vitamin D, because vitamin D as a ligand for its biological function depends on binding to the receptor, while the vitamin D receptor (VDR) is expressed vary in different patients. Therefore, we established a scientific hypothesis that the vitamin D intakes of BC patients should refer to their VDR expression levels. To prove this hypothesis, we first need to verify the correlation between the VDR expression and the prognosis of BC patients.

VDR is a ligand-dependent transcriptional regulator protein and a member of the nuclear receptor superfamily [12, 13]. In the breast epithelium, vitamin D interacts with VDR in the same place or in adjacent cells to maintain differentiation and quiescence [14]. A case-control study by Hemida et al. showed that VDR expression was upregulated in BC tissues and correlated with estrogen receptor alpha (ER-α) expression [15]. Retrospective studies by Heublein [16] and Huss [17] et al. showed that low expression of VDR is an indicator for poor prognosis of BC. However, there are certain differences between the various studies, and the sample size of the studies is small, so the relevant results cannot directly and effectively guide clinical work. In view of this, a meta-analysis was carried out by collecting literatures on VDR expression in BC to clarify the relationship between VDR expression and the prognosis of BC patients.

It has been reported that vitamin D can be regarded as a protective factor for reducing the risk of various cancers including BC, and can inhibit the cell proliferation of normal and malignant breast cells [28], and induce cell differentiation and apoptosis [29]. Vitamin D is involved in the process of regulating cell growth, differentiation, and apoptosis by binding to VDR [30]. VDR is a nuclear receptor that regulates gene expression and is expressed in 80 to 90% of BC patients [31]. VDR can be expressed in breast epithelial cells, which suggests that vitamin D may directly affect the sensitivity of the glands. In vitro studies have shown that the VDR ligand, 1,25 (OH)₂D₃, is involved in maintaining the differentiation of breast cells. Knocking out the VDR gene increases the susceptibility of BC in mice. The expression of VDR is down-regulated in invasive BC [30], suggesting that the expression of VDR is negatively correlated with the progress of BC, and the expression of VDR has a certain protective effect on the breast. Therefore, in theory, the high expression of VDR in BC should be related to a good prognosis.

In this meta-analysis, we found that the relationship between VDR expression and prognosis in BC was mainly affected by the staining location. Results of the subgroup analysis showed that only the total VDR expression in nucleus and cytoplasm was related to BC patients’ survival. VDR mainly functions as a nuclear receptor [32, 33], but it is widely distributed on multiple subcellular structures, including the nucleus, nuclear membrane, cytoplasm, and cell membrane [17]. Our results indicate that VDR in the cytoplasm also exerts specific biological functions in the progression of BC cells. In view of this, we recommend that when performing immunohistochemical analysis of BC specimens in clinical work, the total expression of VDR in nucleus and cytoplasm should be detected instead of only the expression of VDR in nucleus.

Our study has made it clear that both the serum vitamin D level and the expression of VDR are related to the prognosis of BC patients, which suggests that the serum vitamin D level of different BC patients should be adjusted according to the expression of VDR. BC patients with high total VDR expression in nucleus and cytoplasm may not need too much vitamin D intake. Of course, this hypothesis needs to be further verified by controlled clinical trials with larger sample sizes. In addition, our results are also conducive to more accurate assessment of the prognosis of BC patients, which is important for formulating appropriate treatment plans.

At present, Tumor Node Metastasis (TNM) staging is the most important indicator for assessing the prognosis of BC patients, but the accuracy of prediction is reduced due to individual differences. It is well known that patients with same TNM staging may have different prognosis. Therefore, an effective biological indicator is urgently needed to help assess the prognosis of BC patients [20]. Although serum vitamin D level and VDR expression are both related to the prognosis of BC patients [7, 11], VDR expression seems a more suitable prognostic indicator of BC because serum vitamin D level fluctuates greatly due to diet and sunlight exposure.

In addition to the prognostic value of VDR protein expression, the prognostic value of VDR mRNA expression in BC has also been reported [34, 35]. Murray et al. [34] evaluated a pooled database of 12 publicly available BC datasets (n = 2592 patients) containing gene expression data, then found that the mRNA expression of VDR was not related to the DFS of BC patients as a whole. In this meta-analysis, we found that the protein expression of VDR was also not related to the DFS of BC patients. This suggests that the mRNA expression and the protein expression of VDR may be consistent. However, there is still a lack of reports on the relationship between VDR mRNA expression and OS of BC patients. The correlation between VDR polymorphism and BC has also been researched in previous studies [36‐38]. Raimondi et al. reported that the polymorphism of the third gene Bsml and fifth gene Fokl of the VDR gene may be able to regulate the risk of BC [39]. A high-quality meta-analysis showed that the Fokl polymorphism of the VDR gene was associated with an increased risk of BC [37]. However, another meta-analysis by Lu et al. showed that VDR polymorphism (Fok1, Bsm1, Taq1, and Apa1) were not associated with the risk of BC in general population [36]. The roles of VDR mRNA expression and polymorphism in BC need to be further explored by more prospective clinical studies.

Our meta-analysis is the first to study the relationship between VDR protein expression level and BC prognosis. Although only 8 studies were included, the present meta-analysis based on the data of 2503 patients can still provide some help and reference for assessing the prognostic role of VDR expression in BC. Of course, the small number of included studies may affect the reliability of the results of the subgroup analysis. In addition, this meta-analysis also has some other shortcomings. For example, part of the HRs is obtained from univariate analyses, which will overestimate the effect size because the influence of confounding factors is not excluded. The clinical information provided by the included studies is inadequate. Some studies did not provide the pathological and molecular types of patients, which prevented us from performing high-quality subgroup analyses based on these clinical features, leading to the omission of some valuable positive results. Furthermore, some HRs were estimated based on survival curves, which caused statistical errors.

Previous meta-analyses have shown an inverse association between serum vitamin D level and BC prognosis. Similarly, VDR expression is also related to the prognosis of BC patients. Our review and meta-analysis demonstrated that BC patients with high total VDR expression in nucleus and cytoplasm had better OS. This is an important finding in terms of future randomized controlled trials which might reveal a directive function of VDR expression for the adjustment of vitamin D intake in BC patients. VDR expression is not affected by diet and sunlight exposure, so it is more suitable for assessing the prognosis of BC than the serum vitamin D level. A more accurate assessment of the prognosis by combining TNM staging and VDR expression is conducive to formulating a more appropriate treatment plan for BC patients. VDR expression is expected to become a routine immunohistochemical examination item in the pathological diagnosis of BC.