Title: Vitamin K: key vitamin in controlling vascular calcification in chronic kidney disease
Abstract: Vascular calcification has emerged as an independent risk factor for cardiovascular morbidity and mortality, especially in chronic kidney disease. Deficiencies in calcium-regulatory proteins directly relate to development of calcifications. McCabe and colleagues report that vitamin K is a key regulator of vascular calcification, via carboxylation of vitamin K-dependent proteins such as matrix Gla protein. Knowledge about vitamin K status may propel therapeutic strategies to prevent and treat vascular calcification with high vitamin K supplementation. Vascular calcification has emerged as an independent risk factor for cardiovascular morbidity and mortality, especially in chronic kidney disease. Deficiencies in calcium-regulatory proteins directly relate to development of calcifications. McCabe and colleagues report that vitamin K is a key regulator of vascular calcification, via carboxylation of vitamin K-dependent proteins such as matrix Gla protein. Knowledge about vitamin K status may propel therapeutic strategies to prevent and treat vascular calcification with high vitamin K supplementation. Vascular mineralization is a process in which mineral is pathologically deposited in blood vessels, mainly in large elastic and muscular arteries such as the aorta, coronaries, and carotid and peripheral arteries. In the general population, the amount of vascular calcification, as measured and quantified by multislice computed tomography, is an important predictor of all-cause mortality, vascular complications, and myocardial infarction. The prevalence of both cardiovascular mortality and vascular calcification is much higher in patients with chronic kidney disease (CKD) than in the general population.1Drüeke T.B. Massy Z.A. Atherosclerosis in CKD: differences from the general population.Nat Rev Nephrol. 2010; 6: 723-735Crossref PubMed Scopus (141) Google Scholar The first cardiovascular changes observed in patients with CKD are arteriosclerosis, characterized by arterial stiffening and calcification, altered left ventricular diastolic function, and left ventricular hypertrophy. Atherosclerotic plaque formation often occurs later or in parallel with initial cardiovascular changes aggravated by CKD.1Drüeke T.B. Massy Z.A. Atherosclerosis in CKD: differences from the general population.Nat Rev Nephrol. 2010; 6: 723-735Crossref PubMed Scopus (141) Google Scholar For decades vascular calcification was regarded as a passive process, an inevitable consequence of aging and disease. Understanding the molecular mechanisms that lead to accelerated vascular calcification in patients with CKD is of great importance in limiting vascular calcification and subsequently mortality. It is well known that the extracellular fluid is a metastable soup with regard to calcium and phosphate concentrations and that active inhibitors of calcification must be present, both circulating and locally, to prevent the spontaneous formation of apatite: a situation that certainly applies to the CKD population. Some two-thirds of patients with early stages of CKD have mild coronary calcification, whereas one-third have severe coronary calcification. The active inhibition process involves vascular smooth muscle cells and a number of proteins, including some that are vitamin K-dependent. The report by McCabe and colleagues2McCabe K.M. Booth S.L. Fu X. et al.Dietary vitamin K and therapeutic warfarin alter the susceptibility to vascular calcification in experimental chronic kidney disease.Kidney Int. 2013; 83: 835-844Abstract Full Text Full Text PDF PubMed Scopus (112) Google Scholar (this issue) shows that either the administration of therapeutic doses of vitamin K antagonists (VKAs) or the use of low vitamin K1 intake in rats with CKD markedly increases the degree of vascular calcification. Treatment of animals with CKD with high doses of vitamin K increases vitamin K tissue concentrations, attenuates development of calcification—despite hyperphosphatemia—and restores tissue calcium content comparable to that of non-CKD animals.2McCabe K.M. Booth S.L. Fu X. et al.Dietary vitamin K and therapeutic warfarin alter the susceptibility to vascular calcification in experimental chronic kidney disease.Kidney Int. 2013; 83: 835-844Abstract Full Text Full Text PDF PubMed Scopus (112) Google Scholar These findings confirm that vitamin K is key to the susceptibility to vascular calcification in an experimental model of CKD. If this finding holds true for patients with CKD, it would be of great importance with huge therapeutic implications for vitamin K treatment (Figure 1). McCabe and colleagues induced CKD in rats with high doses of adenine, resulting in interstitial nephritis, subsequently inducing fibrosis and atrophy of the proximal and distal renal tubules, leading to accumulation of urea and other uremic waste products in the blood. This treatment induces not only vascular calcification, but also severe bone loss.3De Schutter T.M. Neven E. Persy V.P. et al.Vascular calcification is associated with cortical bone loss in chronic renal failure rats with and without ovariectomy: the calcification paradox.Am J Nephrol. 2011; 34: 356-366Crossref PubMed Scopus (23) Google Scholar The latter might in turn favor the development of vascular calcification—known as the calcium paradox. Although the cumulative dosage of adenine was similar among all groups, minimizing any potential impact of bone loss on the effect of vitamin K on vascular calcification, the measurement of bone-specific markers such as bone alkaline phosphatase would need further investigation in future experiments. Accumulating evidence demonstrates that CKD patients suffer from subclinical vitamin K deficiency.4Holden R.M. Morton A.R. Garland J.S. et al.Vitamins K and D status in stages 3-5 chronic kidney disease.Clin J Am Soc Nephrol. 2010; 5: 590-597Crossref PubMed Scopus (152) Google Scholar, 5Cranenburg E.C.M. Schurgers L.J. Uiterwijk H.H. et al.Vitamin K intake and status are low in hemodialysis patients.Kidney Int. 2012; 82: 605-610Abstract Full Text Full Text PDF PubMed Scopus (145) Google Scholar, 6Westenfeld R. Krueger T. Schlieper G. et al.Effect of vitamin K(2) supplementation on functional vitamin K deficiency in hemodialysis patients: a randomized trial.Am J Kidney Dis. 2012; 59: 186-195Abstract Full Text Full Text PDF PubMed Scopus (225) Google Scholar This deficiency might be caused by exhaustion of vitamin K due to its high requirement by vitamin K-dependent proteins to inhibit calcification. However, the deficiency may also be related to dietary recommendations for these patients, including diets low in potassium (fewer leafy green vegetables; rich in K1) and low in phosphate (fewer dairy products; rich in K2). It is known that a phosphate-rich environment enhances the calcification process significantly. Lowering phosphate intake and/or absorption might thus be beneficial. Recently, Block et al. reported that phosphate binders significantly lowered serum and urine phosphorus among patients with CKD; however, unexpectedly, the binders also seemed to promote progression of vascular calcification.7Block G.A. Wheeler D.C. Persky M.S. et al.Effects of phosphate binders in moderate CKD.J Am Soc Nephrol. 2012; 23: 1407-1415Crossref PubMed Scopus (420) Google Scholar An explanation for this might be that the phosphate binder sevelamer also binds vitamin K, thereby further increasing the vitamin K deficiency. However, a word of caution is needed. The authors considered the effects on vascular calcification of three different phosphate binder classes together because of insufficient sample size. Several other studies, though not all of them, have previously shown that sevelamer reduced the progression of vascular calcification. In vitro and in vivo experiments show that VKAs induce vascular vitamin K deficiency, which relates to increased mineralization. Recently it was shown that ApoE mice—prone to develop atherosclerosis—treated with VKAs displayed increased atherosclerotic plaque calcification and an increase in plaque vulnerability.8Schurgers L.J. Joosen I.A. Laufer E.M. et al.Vitamin K-antagonists accelerate atherosclerotic calcification and induce a vulnerable plaque phenotype.PLoS One. 2012; 7 ([online]): e43229Crossref PubMed Scopus (117) Google Scholar This also accounts for patients treated with VKA who are at increased risk of vascular mineralization as compared to non-VKA treated patients.9Chatrou M.L.L. Winckers K. Hackeng T.M. et al.Vascular calcification: the price to pay for anticoagulation therapy with vitamin K-antagonists.Blood Rev. 2012; 26: 155-166Abstract Full Text Full Text PDF PubMed Scopus (121) Google Scholar CKD patients who are prone to develop arterial calcification may not benefit with a decreased thrombosis risk in the short term and may be compromised by the detrimental effect of excessive arterial mineralization in the long term. Approximately 5% of CKD patients develop calciphylaxis, an often fatal complication of end-stage renal disease. Notably, about 50% of patients with CKD stage 5D (CKD5D) who develop calciphylaxis are on VKAs.9Chatrou M.L.L. Winckers K. Hackeng T.M. et al.Vascular calcification: the price to pay for anticoagulation therapy with vitamin K-antagonists.Blood Rev. 2012; 26: 155-166Abstract Full Text Full Text PDF PubMed Scopus (121) Google Scholar The question therefore arises of whether CKD patients might benefit from extra vitamin K intake. In the population-based Rotterdam study, 4800 elderly patients were followed to assess the occurrence of diseases of the elderly and to clarify their determinants.10Geleijnse J.M. Vermeer C. Grobbee D.E. et al.Dietary intake of menaquinone is associated with a reduced risk of coronary heart disease: the Rotterdam Study.J Nutr. 2004; 134: 3100-3105PubMed Scopus (394) Google Scholar In these elderly patients, none of whom had a history of myocardial infarction at baseline, dietary intake data including vitamin K were available. The authors assessed the risk of incident coronary heart disease, all-cause mortality, and aortic calcification based on tertiles of energy-adjusted vitamin K intake. They found that low vitamin K2 intake was associated with a higher incidence of severe aortic calcification and increased mortality. In line with this observation, a recent pilot study investigated whether daily vitamin K2 supplementation would improve the bioactivity of vitamin K-dependent proteins in CKD5D patients, as assessed by circulating dephosphorylated-undercarboxylated matrix Gla protein (dp-ucMGP), undercarboxylated osteocalcin (ucOC), and undercarboxylated prothrombin (ucFII; protein induced by vitamin K absence-II).6Westenfeld R. Krueger T. Schlieper G. et al.Effect of vitamin K(2) supplementation on functional vitamin K deficiency in hemodialysis patients: a randomized trial.Am J Kidney Dis. 2012; 59: 186-195Abstract Full Text Full Text PDF PubMed Scopus (225) Google Scholar There was an increase in undercarboxylated forms for all vitamin K-dependent proteins, indicating an overall vitamin K deficiency in these CKD patients. After only 6 weeks of daily vitamin K2 supplementation, a significant decrease in dp-ucMGP, ucOC, and ucFII levels was observed. This shows that vitamin K is able to reach the tissues, including the vessel wall, and to correct the biochemical and local tissue consequences of vitamin K deficiency. The question remains: what is the best marker to study vitamin K deficiency, and what should this marker reflect? Traditional screening tests for vitamin K deficiency are based on global coagulation assays including prothrombin time and activated partial thromboplastin time. These tests are simple to perform but are not specific to vitamin K deficiency. The major dietary and circulating form of vitamin K is K1, and measurement of this form in serum might be a useful indicator of vitamin K status. However, circulating vitamin K closely relates to the diet consumed the day before and thus is subject to variation. Another limitation is the failure to measure vitamin K2, which is often below the detection limit in serum although present in the diet. With the availability of assays enabling the determination of undercarboxylated Gla proteins, which are synthesized and released by the liver (ucFII, prothrombin), bone (ucOC), and vasculature (dp-ucMGP), true functional, tissue-specific biomarkers are at hand. Unlike the vitamin K-dependent coagulation proteins, fractions of both osteocalcin and MGP circulate in undercarboxylated form in healthy people not taking VKAs. UcOC probably is not an ideal marker of vitamin K status in CKD patients, because of the concomitant bone resorption in association with secondary hyperparathyroidism and the potential for retention of osteocalcin fragments in the setting of reduced kidney function.4Holden R.M. Morton A.R. Garland J.S. et al.Vitamins K and D status in stages 3-5 chronic kidney disease.Clin J Am Soc Nephrol. 2010; 5: 590-597Crossref PubMed Scopus (152) Google Scholar Dp-ucMGP, reflecting the vitamin K status of the vessel wall, might be a better biomarker, as supported by recent observations.5Cranenburg E.C.M. Schurgers L.J. Uiterwijk H.H. et al.Vitamin K intake and status are low in hemodialysis patients.Kidney Int. 2012; 82: 605-610Abstract Full Text Full Text PDF PubMed Scopus (145) Google Scholar,6Westenfeld R. Krueger T. Schlieper G. et al.Effect of vitamin K(2) supplementation on functional vitamin K deficiency in hemodialysis patients: a randomized trial.Am J Kidney Dis. 2012; 59: 186-195Abstract Full Text Full Text PDF PubMed Scopus (225) Google Scholar Vitamin K supplementation in CKD patients showed ucFII responding first in that it was significantly reduced at the lowest vitamin K supplementation dose.6Westenfeld R. Krueger T. Schlieper G. et al.Effect of vitamin K(2) supplementation on functional vitamin K deficiency in hemodialysis patients: a randomized trial.Am J Kidney Dis. 2012; 59: 186-195Abstract Full Text Full Text PDF PubMed Scopus (225) Google Scholar Thus ucFII is probably the most sensitive marker for vitamin K status in this patient population. More research is needed to establish more precisely the relative value of these markers in the clinic. McCabe et al. report that CKD animals had significantly higher menaquinone-4 levels in the kidney and lower K1 levels in the liver and heart.2McCabe K.M. Booth S.L. Fu X. et al.Dietary vitamin K and therapeutic warfarin alter the susceptibility to vascular calcification in experimental chronic kidney disease.Kidney Int. 2013; 83: 835-844Abstract Full Text Full Text PDF PubMed Scopus (112) Google Scholar The authors hypothesize that this might be caused by factors enhancing the efficiency of conversion of K1 to menaquinone-4 and/or impaired utilization of menaquinone-4 in CKD. Menaquinone-4 is a member of the vitamin K2 family, and one might hypothesize that K2 is of greater importance than K1 for vascular health, which is supported by data of the Rotterdam study.10Geleijnse J.M. Vermeer C. Grobbee D.E. et al.Dietary intake of menaquinone is associated with a reduced risk of coronary heart disease: the Rotterdam Study.J Nutr. 2004; 134: 3100-3105PubMed Scopus (394) Google Scholar Both K1 and K2 function as cofactors for the enzyme γ-glutamylcarboxylase, in both the liver and extrahepatic tissues. K1 and K2 appear to be different in physiological processes such as absorption and transport to tissues. For dietary vitamin K both the absorption and the bioactivity of K2 are better than those of K1, probably because of the much longer half-life of K2. Thus, in the dietary range, K2 seems to have a clear benefit compared with K1.10Geleijnse J.M. Vermeer C. Grobbee D.E. et al.Dietary intake of menaquinone is associated with a reduced risk of coronary heart disease: the Rotterdam Study.J Nutr. 2004; 134: 3100-3105PubMed Scopus (394) Google Scholar Supplemental K1 and K2, however, are equally good in terms of absorption, and as K1 can be converted into K2, both K vitamins might work equally well at higher doses, both in rodents and in humans. In conclusion, in a CKD rat model, vitamin K plays a key role in the inhibition of vascular calcification. Whether increased vitamin K intake also is protective against vascular calcification in patients with CKD and is able to decrease their high cardiovascular mortality remains to be tested in prospective clinical trials.