We conducted this study to examine the relationship between vitamin D receptor (VDR) expression and mutations in PIK3CA
in colorectal cancer. Accumulating evidence indicate a substantial role of vitamin D in prevention of various forms of human cancer (1
). A potential link between VDR and the PI3K/AKT or RAS/MAPK pathway has been suggested (17
). Thus, examining VDR in colorectal cancer may shed lights on biological mechanisms of vitamin D action and its failure. We have found that VDR overexpression in colorectal cancer is significantly associated with both PIK3CA
mutations, independent of other clinical and molecular features. Our data support the hypothesis that the VDR pathway interact with the PI3K/AKT and RAS/MAPK pathways in colonic neoplastic cells.
Our resource of a large number of colorectal cancers derived from the two prospective cohort studies has enabled us to precisely estimate the frequency of colorectal cancers with a specific molecular feature (such as VDR expression, KRAS mutation, PIK3CA mutation, etc.). The large number of cases has also provided a sufficient power in our multivariate logistic regression analysis and survival analysis.
Studying risk modifying factors or molecular changes is important in cancer research (39
). Previous studies have consistently reported the preventive effect of vitamin D on colorectal cancers (1
), and that higher plasma levels of vitamin D confer a greater reduction in the risk of colorectal cancer (2
). Studies have reported that 1,25(OH)2
D potentiates the effects of many cytotoxic and anti-proliferative drugs (1
), and that higher plasma levels of 25(OH)D is associated with a significant reduction in colon cancer mortality (2
). Thus, accumulating evidence indicates important roles of vitamin D in preventing the development and progression of colorectal cancer. Interestingly, VDR and 1-α-hydroxylase (encoded by CYP27B1
), which converts 25(OH)D into 1,25(OH)2
D, are frequently overexpressed in colon cancer cells (1
). The anti-proliferative action of 1,25(OH)2
D appears to depend on VDR expression level and differentiation status of tumor cells (45
). Therefore, it is possible that the effect of serum vitamin D level, which is protective against cancer incidence and mortality, may differ according to VDR expression status in colorectal cancer. Additional studies are necessary to address this intriguing hypothesis.
A potential link between VDR and the PI3K/AKT or RAS/MAPK pathway has been suggested (16
). In these pathways, PIK3CA
mutation plays an important role in the progression of colorectal cancer. Mutant PIK3CA
stimulates the downstream AKT pathway, and promotes cell growth in various cancers, including colorectal cancer. The PI3K/AKT pathway has been known to mediate signals from growth factors, which is influenced by the state of energy balance. In addition, PI3K/AKT signaling is influenced by KRAS
). In fact, PIK3CA
mutation is positively associated with KRAS
mutation in colorectal cancer (25
). Our data support a potential link between PIK3CA
and VDR and suggest that VDR expression may affect the regulation of PI3K/AKT pathway in colorectal cancer. In myeloid leukemia cells, VDR activated by 1,25(OH)2
D can inhibit tumor cell proliferation by inducing differentiation, which depends on the formation of activated VDR and PI3K complexes (1
). A combination of 1,25(OH)2
D with AKT pathway inhibitors is strongly anti-proliferative and should be considered for differentiation therapy of myeloid leukemia (17
). These previous observations (16
) and our data suggest that VDR-expressing cells may need the activation of PI3K/AKT pathway in order to acquire malignant characteristics, and that therapy or chemoprevention targeting VDR and downstream pathways may be influenced by PIK3CA
mutation in colon cancer cells.
The vitamin D pathway may also interact with RAS signaling. A recent case-control study has reported that the VDR
poly A, rs10735810 (so-called “FokI SNP”) and rs11568820 (so-called “CDX2 SNP”) polymorphisms are associated with KRAS
mutation in colon cancer (54
). VDR protein expression has been shown to be down-regulated in KRAS
-mutated colon cancer cells (47
). In contrast, VDR expression has been associated with the activation of the RAS/MAPK pathway in leukemia cells (18
), which is in agreement with our current data. In addition, RAS-transformed human keratinocytes are shown to be resistant to the growth-inhibitory effects of 1,25(OH)2
). Further analysis is needed to clarify how vitamin D and its downstream pathway influence colorectal cancer development in relation to KRAS
We did not observe a significant relation between VDR expression and microsatellite instability (MSI)-high or the CpG island methylator phenotype (CIMP) in colorectal cancer. A molecular classification of colorectal cancer based on MSI and CIMP is increasingly important (55
), because MSI and CIMP status represent genomic and epigenomic alterations, respectively, in tumor cells and largely determine clinical, pathologic and molecular characteristics (55
). Nonetheless, our data do not support a link between VDR and CIMP or MSI in colorectal cancer. A recent study has reported that the VDR
rs10735810 (so-called “FokI SNP”) polymorphism is significantly associated with CIMP-high and inversely with MSI-high (54
); however, either of these could be a chance association given multiple hypothesis testing.
In the current study, we have shown that VDR expression is inversely associated with high grade tumors in univariate analysis, but not in multivariate analysis. Thus, our data are not incompatible with the inverse association between VDR expression and high tumor grade in the previous study (12
), but do not support a mechanistic link between VDR loss and high tumor grade. With regard to disease stage, the previous study (12
) has shown that VDR expression in colon cancer cells is commonly lost in a metastatic focus, but not in a primary lesion. Therefore, the absence of the relation between loss of VDR expression in primary tumor and advanced disease stage in the current study is not incompatible with the previous data (12
As one limitation in our cohort studies, data on cancer treatment were limited. Nonetheless, it is unlikely that chemotherapy use differed according to tumoral VDR status, since such data were not available to patients or treating physicians. In addition, beyond cause of mortality, data on cancer recurrences were not available in these cohorts. Nonetheless, given the median survival for metastatic colorectal cancer was approximately 10 to 12 months during much of the time period of this study, colorectal cancer-specific survival should be a reasonable surrogate for colorectal cancer-specific outcomes.
As another limitation of this study, there are currently no standardized methods to assess VDR overexpression in colorectal cancer. Thus, our current study is exploratory by nature, and our data and method to determine a cutpoint for VDR overexpression need to be validated and confirmed by independent datasets.
In summary, this large cohort of colorectal cancers has shown that VDR expression is significantly associated with PIK3CA and KRAS mutations, independent of clinical, pathological and molecular features. On the other hand, VDR expression is not significantly related with patient survival. Our data support the hypothesis that PIK3CA and/or KRAS mutations may influence biological effect of VDR and its downstream pathway. Thus, targeting VDR for chemoprevention or cancer therapy likely needs to consider the effect of KRAS or PIK3CA mutation. Likewise, therapy targeting EGFR or the downstream RAS or PI3K pathway may be influenced by VDR status. Considering that VDR regulates the transcription of various genes involved in cellular differentiation and inhibition of proliferation, our findings may have considerable clinical implications. Further studies are necessary to elucidate exact roles of vitamin D and VDR in prevention of colorectal neoplasias, as well as to examine a potential mechanistic link between VDR and the RAS/MAPK and PI3K/AKT pathways.