In this prospective study, men with high VDR protein tumor expression had more favorable clinical characteristics, including lower pathologic Gleason score and more locally confined tumors. Moreover, we found a strong inverse association between tumor VDR protein expression and lethal prostate cancer. After adjusting for PSA at diagnosis, Gleason score, and tumor stage, this inverse association was somewhat attenuated but remained strongly inverse and statistically significant. Our findings could either reflect that VDR is involved in a biologic pathway leading to lethal prostate cancer or that it is indirectly associated with lethal prostate cancer as a marker of other causal mechanisms associated with poorly differentiated or advanced-stage prostate cancer. In either case, VDR expression does seem to be a predictive marker of prostate cancer progression beyond standard clinical features. Furthermore, in vitro and in vivo studies that show VDR inhibits growth of prostate tumor cells and reduces prostate tumor size in xenografts support a biologic role of VDR expression in aggressive prostate cancer.28,29
The reduction in risk of lethal prostate cancer was restricted to the highest quartile of VDR expression. This result may suggest a threshold of VDR protein expression that tumors must achieve for VDR to influence tumor biology. Alternatively, because the CIs for quartiles 2 and 3 were relatively wide, this appearance of a threshold could be a result of chance variation.
We found that VDR expression varied across a molecular subclass of prostate cancer defined by TMPRSS2:ERG
, which fuses TMPRSS2
, a gene regulated by androgens, and ERG
, a known oncogenic transcription factor.23,30
Men whose tumors were TMPRSS2:ERG
positive through deletion in genomic DNA had significantly higher VDR protein expression in tumor tissue than men who were fusion negative or fusion positive through rearrangement. In prior studies, fusion through deletion has been associated with a more aggressive prostate cancer phenotype and would therefore be expected to be more prevalent in men with low VDR protein expression.24,30–32
The relationship we found between VDR expression and fusion status could be explained by the potential effect of VDR and 1,25(OH)2
D on TMPRSS2:ERG
expression as suggested by Washington and Weigel.33
D induces TMPRSS2:ERG
expression, prostate tumor growth is still inhibited in the fusion-positive cell line. Therefore, fusion-positive tumors may be more responsive to vitamin D signaling, and so with high VDR protein expression, growth is still inhibited.
We found no associations between circulating levels of vitamin D metabolites and VDR protein expression. However, we assessed vitamin D metabolites 6 years before diagnosis. Levels more proximate to diagnosis may be more relevant in terms of tumor VDR protein expression. Within HPFS, we previously found a good correlation of circulating vitamin D levels across a 3-year window, with an ICC of 0.70, suggesting that a single measure does provide some reliable estimate of exposure in the short term.12
Although VDR polymorphisms (BsmI and FokI) were not associated with VDR protein expression, these gene variants could influence VDR function and thus alter prostate cancer progression.34–38
Consistent with previous studies comparing VDR polymorphisms and aggressive disease, men with the less functional ff
allele had an increased risk of developing lethal prostate cancer, although our results were not statistically significant.17
We noted a significant interaction between the BsmI allele (BB v Bb/bb
) and VDR protein expression and lethal prostate cancer; among men with the BB
allele, those with low VDR expression had a 2.6-fold increased risk of lethal prostate cancer risk, but men with high VDR expression were at lower risk. This difference could reflect the importance of VDR protein expression or simply reflect chance.
The long clinical follow-up permitted use of lethal prostate cancer as the outcome, an important clinical phenotype of such a heterogeneous cancer. Furthermore, we used unbiased quantitative image analyses to score VDR protein expression, and Gleason scores were standardized by study pathologists. Heterogeneity of VDR staining across the tumor specimens may be an issue when using TMAs. However, we sampled at least three TMA cores per prostate cancer specimen and showed a relatively high intraclass correlation of VDR staining.
This study was nested among men with available prostate tumor tissue, primarily men undergoing prostatectomy as curative treatment. We retrieved 73% of eligible tumor specimens, and the clinical characteristics of the prostatectomy patients with and without available tumor tissue are quite similar.
The VDR staining in the prostate tumors was apparent in the cytoplasm and membrane but not in the nucleus. Although nuclear staining of VDR is likely relevant, it is plausible that the VDR antigens may be processed quickly in the nucleus after functioning. Thus, cytoplasmic staining may be a surrogate of VDR actions that are ultimately mediated in the nucleus. At the same time, a significant portion of VDR resides in the cytoplasm and cytoplasmic membrane, where it exerts nongenomic actions outside of the nucleus.39,40
The nongenomic actions of VDR include regulation of calcium and chloride channel activity, protein kinase C activation, and phospholipase C activity occurring through cytoplasmic signaling pathways such as protein kinase and mitogen-activated protein kinase.41–46
Cytoplasmic VDR may cooperate with important driver mutations, such as a positive association with PIK3CA mutations in colorectal cancer.22
Cytoplasmic VDR localization in cancer has been well described in various cancers.22,47–52
In conclusion, these results support an important role of the vitamin D pathway on risk of progression to lethal prostate cancer. If these findings are confirmed, VDR protein expression in tumor could help improve prognostic prediction and guide treatment decisions.