Stabilization of warfarin-binding pocket of VKORC1 and VKORL1 by a peripheral region determines their different sensitivity to warfarin inhibition

The mechanism of blood coagulation induced by sodium dehydroacetate via the regulation of the mTOR/ERK pathway in rats

Sodium dehydroacetate (DHA-S), a potent antifungal and antibacterial agent, is widely used in food, feed and cosmetics. However, recent studies have shown that DHA-S could pose a risk for human and animal health. We had previously reported that DHA-S could cause coagulation disorders in rats and chicken. In the present study, we further confirmed that DHA-S induced blood coagulation via VKORC1 and VKORC1L1 in rats, and elucidated the role played by mTOR/ERK signaling. The in vivo studies demonstrated that PT, APTT, and DHA-S content and relative protein expressions in tissues rebounded after drug withdrawal. In BRL-3A cells, 1.0 mM DHA-S increased the expression levels of mTOR, p-mTOR and p-ERK and decreased the levels of VKORC1, VKORC1L1 and Vitamin K. Rapamycin significantly decreased the expression levels of p-mTOR and p-ERK, while FR180204 (p-ERK Inhibition) lead to a decrease in p-ERK level. Rapamycin and FR180202 attenuated the inhibitory effect of DHA-S on VKORC1, VKORC1L1 and vitamin K levels. In addition, DHA-S increased the expression levels of mTOR, p-mTOR and p-ERK in male and female rat livers and prolonged PT and APTT. In summary, this study indicated that DHA-S induced blood coagulation via the modulation of the mTOR/ERK pathway in rats.

Missense VKOR mutants exhibit severe warfarin resistance but lack VKCFD via shifting to an aberrantly reduced state

Missense vitamin K epoxide reductase (VKOR) mutations in patients cause resistance to warfarin treatment but not abnormal bleeding due to defective VKOR activity. The underlying mechanism of these phenotypes remains unknown. Here we show that the redox state of these mutants is essential to their activity and warfarin resistance. Using a mass spectrometry-based footprinting method, we found that severe warfarin-resistant mutations change the VKOR active site to an aberrantly reduced state in cells. Molecular dynamics simulation based on our recent crystal structures of VKOR reveals that these mutations induce an artificial opening of the protein conformation that increases access of small molecules, enabling them to reduce the active site and generating constitutive activity uninhibited by warfarin. Increased activity also compensates for the weakened substrate binding caused by these mutations, thereby maintaining normal VKOR function. The uninhibited nature of severe resistance mutations suggests that patients showing signs of such mutations should be treated by alternative anticoagulation strategies.

New insights into vitamin K biology with relevance to cancer

Phylloquinone (vitamin K1) and menaquinones (vitamin K2 family) are essential for post-translational γ-carboxylation of a small number of proteins, including clotting factors. These modified proteins have now been implicated in diverse physiological and pathological processes including cancer. Vitamin K intake has been inversely associated with cancer incidence and mortality in observational studies. Newly discovered functions of vitamin K in cancer cells include activation of the steroid and xenobiotic receptor (SXR) and regulation of oxidative stress, apoptosis, and autophagy. We provide an update of vitamin K biology, non-canonical mechanisms of vitamin K actions, the potential functions of vitamin K-dependent proteins in cancer, and observational trials on vitamin K intake and cancer.

Structural and cellular basis of vitamin K antagonism

Vitamin K antagonists (VKAs), such as warfarin, are oral anticoagulants widely used to treat and prevent thromboembolic diseases. Therapeutic use of these drugs requires frequent monitoring and dose adjustments, whereas overdose often causes severe bleeding. Addressing these drawbacks requires mechanistic understandings at cellular and structural levels. As the target of VKAs, vitamin K epoxide reductase (VKOR) generates the active, hydroquinone form of vitamin K, which in turn drives the γ-carboxylation of several coagulation factors required for their activity. Crystal structures revealed that VKAs inhibit VKOR via mimicking its catalytic process. At the active site, two strong hydrogen bonds that facilitate the catalysis also afford the binding specificity for VKAs. Binding of VKAs induces a global change from open to closed conformation. Similar conformational change is induced by substrate binding to promote an electron transfer process that reduces the VKOR active site. In the cellular environment, reducing partner proteins or small reducing molecules may afford electrons to maintain the VKOR activity. The catalysis and VKA inhibition require VKOR in different cellular redox states, explaining the complex kinetics behavior of VKAs. Recent studies also revealed the mechanisms underlying warfarin resistance, warfarin dose variation, and antidoting by vitamin K. These mechanistic understandings may lead to improved anticoagulation strategies targeting the vitamin K cycle.

Development of a sandwich enzyme-linked immunosorbent assay (ELISA) to quantify γ-glutamyl-carboxylated clotting factor IX and assess redox susceptibility of anticoagulant chemicals

Anticoagulant chemicals (ACCs) such as warfarin are widely used in medical applications as well as for their rodenticide properties. Their efficacy is greatly influenced by polymorphisms in the gene encoding vitamin K epoxide reductase (VKOR). Evaluation of the activity of ACCs toward VKOR variants is essential to determine their proper use. Presently, this is achieved by co-expressing VKOR of Rattus Norvegicus and human clotting factor IX in cultured cells and measuring inhibition of vitamin K-dependent gamma-glutamyl carboxylation of factor IX (glaFIX) activity. However, glaFIX has only been quantified using indirect methods like blood coagulation assays. We have developed a sandwich enzyme-linked immunosorbent assay using a glaFIX-specific antibody to quantify glaFIX and used this to analyze inhibition of VKOR activity by warfarin.

The catalytic mechanism of vitamin K epoxide reduction in a cellular environment

Vitamin K epoxide reductases (VKORs) constitute a major family of integral membrane thiol oxidoreductases. In humans, VKOR sustains blood coagulation and bone mineralization through the vitamin K cycle. Previous chemical models assumed that the catalysis of human VKOR (hVKOR) starts from a fully reduced active site. This state, however, constitutes only a minor cellular fraction (5.6%). Thus, the mechanism whereby hVKOR catalysis is carried out in the cellular environment remains largely unknown. Here we use quantitative mass spectrometry (MS) and electrophoretic mobility analyses to show that KO likely forms a covalent complex with a cysteine mutant mimicking hVKOR in a partially oxidized state. Trapping of this potential reaction intermediate suggests that the partially oxidized state is catalytically active in cells. To investigate this activity, we analyze the correlation between the cellular activity and the cellular cysteine status of hVKOR. We find that the partially oxidized hVKOR has considerably lower activity than hVKOR with a fully reduced active site. Although there are more partially oxidized hVKOR than fully reduced hVKOR in cells, these two reactive states contribute about equally to the overall hVKOR activity, and hVKOR catalysis can initiate from either of these states. Overall, the combination of MS quantification and biochemical analyses reveals the catalytic mechanism of this integral membrane enzyme in a cellular environment. Furthermore, these results implicate how hVKOR is inhibited by warfarin, one of the most commonly prescribed drugs.

Structural basis of antagonizing the vitamin K catalytic cycle for anticoagulation

Vitamin K antagonists are widely used anticoagulants that target vitamin K epoxide reductases (VKOR), a family of integral membrane enzymes. To elucidate their catalytic cycle and inhibitory mechanism, we report 11 x-ray crystal structures of human VKOR and pufferfish VKOR-like, with substrates and antagonists in different redox states. Substrates entering the active site in a partially oxidized state form cysteine adducts that induce an open-to-closed conformational change, triggering reduction. Binding and catalysis are facilitated by hydrogen-bonding interactions in a hydrophobic pocket. The antagonists bind specifically to the same hydrogen-bonding residues and induce a similar closed conformation. Thus, vitamin K antagonists act through mimicking the key interactions and conformational changes required for the VKOR catalytic cycle.

Multiplexed measurement of variant abundance and activity reveals VKOR topology, active site and human variant impact

Vitamin K epoxide reductase (VKOR) drives the vitamin K cycle, activating vitamin K-dependent blood clotting factors. VKOR is also the target of the widely used anticoagulant drug, warfarin. Despite VKOR's pivotal role in coagulation, its structure and active site remain poorly understood. In addition, VKOR variants can cause vitamin K-dependent clotting factor deficiency or alter warfarin response. Here, we used multiplexed, sequencing-based assays to measure the effects of 2,695 VKOR missense variants on abundance and 697 variants on activity in cultured human cells. The large-scale functional data, along with an evolutionary coupling analysis, supports a four transmembrane domain topology, with variants in transmembrane domains exhibiting strongly deleterious effects on abundance and activity. Functionally constrained regions of the protein define the active site, and we find that, of four conserved cysteines putatively critical for function, only three are absolutely required. Finally, 25% of human VKOR missense variants show reduced abundance or activity, possibly conferring warfarin sensitivity or causing disease.

VKORC1L1, An Enzyme Mediating the Effect of Vitamin K in Liver and Extrahepatic Tissues

Vitamin K is an essential nutrient involved in the regulation of blood clotting and tissue mineralization. Vitamin K oxidoreductase (VKORC1) converts vitamin K epoxide into reduced vitamin K, which acts as the co-factor for the γ-carboxylation of several proteins, including coagulation factors produced by the liver. VKORC1 is also the pharmacological target of warfarin, a widely used anticoagulant. Vertebrates possess a VKORC1 paralog, VKORC1-like 1 (VKORC1L1), but until very recently, the importance of VKORC1L1 for protein γ-carboxylation and hemostasis in vivo was not clear. Here, we first review the current knowledge on the structure, function and expression pattern of VKORC1L1, including recent data establishing that, in the absence of VKORC1, VKORC1L1 can support vitamin K-dependent carboxylation in the liver during the pre- and perinatal periods in vivo. We then provide original data showing that the partial redundancy between VKORC1 and VKORC1L1 also exists in bone around birth. Recent studies indicate that, in vitro and in cell culture models, VKORC1L1 is less sensitive to warfarin than VKORC1. Genetic evidence is presented here, which supports the notion that VKORC1L1 is not the warfarin-resistant vitamin K quinone reductase present in the liver. In summary, although the exact physiological function of VKORC1L1 remains elusive, the latest findings clearly established that this enzyme is a vitamin K oxidoreductase, which can support γ-carboxylation in vivo.