THE ROLE OF VITAMIN K-DEPENDENT PROTEINS IN THE PATHOGENESIS OF PSEUDOXANTHOMA ELASTICUM

Pseudoxanthoma elasticum (PXE) is a genetic disease characterized by progressive mineralization of the elastic component, which has been related to a reduced expression of the active form of matrix gla protein (MGP), a strong inhibitor of ectopic calcifications. To be fully effective, MGP has to be carboxylated (c-MGP) through a vitamin K-depend- ent pathway. The observation that in PXE, patients have lower levels of circulating vitamin K, arose the question whether elastic fiber mineralization is due to an insufficient amount of the vitamin reaching peripheral connective tissues, or to the inability of PXE mesenchymal cells to utilize the vitamin. Unexpectedly, vitamin K supplementation does not exert bene- ficial effects on soft connective tissue mineralization in the PXE animal model. For a better understanding of the role of vitamin K-dependent carboxylation of MGP in PXE patho- genesis, aim of the present study was to investigate the effects of vitamin K1 (phylloquinone) and vitamin K2 (menaquinone- 4, MK-4) supplementation on control and on PXE dermal fibroblasts cultured in standard conditions and in calcifying medium. Results demonstrate that vitamin K1 and K2 can be taken up and accumulated at similar levels by control and PXE fibroblasts, that the carboxylation process can be conse- quently similarly up-regulated, that both vitamins K1 and K2, independently from concentration, similarly down-regulate the expression of CALU in all cell strains, whereas changes are negligible in the case of PDI, indicating that the effect of vita- min K supplementation on the expression of ER-proteins involved in vitamin K cycle is not pathway-specific. Surprisingly, MGP cannot be adequately carboxylated, even at increased levels of vitamin K. It can be therefore excluded that PXE fibroblasts are not capable to utilize the vitamin, thus suggesting that altered MGP characteristics/properties could contribute to defective carboxylation. Moreover, the observa- tion that in an in vitro calcification model, both vitamin K1 and K2 are ineffective in inhibiting the mineralization process, also in control fibroblasts, i.e. in cells that do not exhibit reduced cMGP, may underline the importance and the com- plexity of the extracellular environment in mineral deposit for- mation and in regulating cell behavior