Our results indicate that plasma MMP-9 expression is inversely and independently associated with circulating 25(OH) D concentration among ESRD patients. We also found anti-inflammatory IL-10 concentration to be associated with 25(OH) D concentration. This is the first report to our knowledge to assess the relationship of 25(OH)D concentration with MMP-9 and IL-10 in an ESRD population, and may help to shed light on mechanisms by which vitamin D deficiency is associated with an increased risk of atherosclerosis and cardiovascular disease.
Our results are consistent with previous data among non-ESRD populations that report a strong association between 25(OH) D deficiency and elevated markers of vascular remodeling: Vitamin D supplementation of vitamin D deficient, healthy British Bangladeshi adults resulted in significant reductions in plasma MMP-9 and serum CRP concentrations . Vitamin D supplementation of congestive heart failure patients significantly increased serum concentrations of anti-inflammatory cytokine IL-10 and suppressed TNF-α synthesis over a nine month period . Further, previous reports show that anti-inflammatory IL-10 is increased by vitamin D supplementation in vitro and in mice . Vitamin D acts on dendritic cells, T-cells, and B-cells to enhance IL-10 expression [21–23].
While others have reported reductions in CRP concentrations following vitamin D supplementation [5, 6, 24], and negative correlations between serum 25(OH)D concentration and TNF-α concentrations , this was not observed in our study, possibly due to our sample size or that a greater differential in 25(OH)D concentration may be needed in order to observe an association with these biomarkers. Whilst we identified associations between both MMP-9 and IL-10 with 25(OH) D concentration, we also observed several important negative findings in this cross-sectional study following multivariate adjustment. First, while PTH concentrations were greater among patients with 25(OH)D < 15 ng/dL, they were not significantly different among the vitamin D cohorts, as we would have expected to observe, possibly due to our relatively small sample size. Second, there was no significant association observed between 25(OH) D and CRP, the most common biomarker of inflammation. This may be due to our relatively small sample size, however, other cross-sectional reports among non-ESRD populations have also failed to find an association between 25(OH)D concentration and CRP concentration following multivariate analysis, supporting our conclusion [4, 14, 18]. We did not observe elevations in MMP-9 and CRP concentrations as large as those reported previously in comparable patients , likely because MMP-9 gene expression is upregulated by several factors that were not included in our analysis. Finally, while MMP-2 concentrations tended to be higher with 25(OH)D concentrations <15 ng/dL, this did not reach significance, likely due to our sample size.
MMP's promote the remodeling of connective tissue and basement membranes via degradation of collagen, and act as important regulatory molecules in inflammation and vascular diseases. MMP-9 is released by neutrophils and is a key effector molecule of inflammatory cells, aiding migration of inflammatory leukocytes through tissue barriers, lysing protein substrates, modulating smooth muscle cell migration, and promoting angiogenesis [26, 27]. MMP's also regulate inflammation by directly and indirectly acting on pro-inflammatory cytokines, such as TNF-α and TGF-β, to control chemokine activity . 25(OH)D may act to reduce MMP-9 in several ways. An increase in IL-10 concentration, as we observed, may also act to suppress MMP-9 secretion . Pulmonary tuberculosis in vitro studies suggest that vitamin D blunts MMP-9 expression by inhibition of c-Jun-N-terminal kinase(JNK) and NFkappaB signaling cascades . MMP-9 concentrations are increased in the setting of several chronic inflammatory diseases, and MMP elevations are associated with abdominal aortic aneurysm rupture  and acute myocardial infarction . These findings suggest that the risk for acquiring or further progression of these disorders may be positively impacted by limiting MMP concentrations.
Another important finding of our study is that vascular access type modifies the relationship between plasma MMP-9 and circulating 25(OH) D concentrations. When we stratified AVF versus AVG, the association between MMP-9 and 25(OH) D concentrations no longer remained significant among AVF patients, whereas the relationship remained significant among patients using an AVG, suggesting that MMP-9 concentrations in AVF patients were too low for a relationship with 25(OH)D to be detected. This may occur because the prosthetic material within an AVG may induce increased systemic inflammation. This finding is supported by data examining the role of vascular access type on systemic inflammation among prevalent HD patients, which noted that CRP concentrations were significantly lower among autogenous AVF users compared with patients using a prosthetic AVG .
Limitations of this study should be considered. First, our relatively small sample size may affect our results, however the 25(OH) D concentrations observed among our subjects are comparable to those reported among larger ESRD patient cohorts, and the observed inverse association between 25(OH) D and MMP-9 has been reported in a larger, non-renal patient cohort . Second, the cross-sectional design of our study does not allow for causal inference. We cannot exclude the possibility that an association exists between 25(OH) D and the additional inflammatory biomarkers examined in our study, but due to our relatively small sample size, we may be underpowered to detect it. For example, the power to detect a statistical difference, if any, between 25(OH)D groups was less than 50% for TNF-α and MMP-2. Third, although we have attempted to reduce the effect of confounding by limiting our study population to include only patients free of infection or inflammation, residual confounding inherent to the cross-sectional design cannot be excluded.