MECHANISM OF THE ABNORMAL VITAMIN-K-DEPENDENT GAMMA-CARBOXYLATION PROCESS IN HUMAN HEPATOCELLULAR CARCINOMAS

Background. An important marker for hepatocellular carcinoma is the pr esence of des-gamma-carboxy (abnormal) prothrombin. However, the molec ular basis for the reduced carboxylation of prothrombin is unknown. Me thods, Two groups of patients were defined according to the absence (G roup I, n = 7) or presence (Group II, n = 8) of des-gamma-carboxy prot hrombin. The enzymatic activity of gamma-carboxylase and the total mic rosomal prothrombin concentration were determined in all tumors. The k inetic parameters for the synthetic peptide Phe-Leu-Glu-Glu-Leu (FLEEL ) were measured in eight tumors. The gamma-carboxylase mRNA expression was evaluated by Northern blot analysis in 12 of 15 tumors. In additi on, the total vitamin K content (K-1, K-1 epoxide, and menaquinones 4- 10) in 10 tumors was investigated by high performance liquid chromatog raphy. Results. Concentrations of menaquinones 4-10 were normal in the nontumorous part of the liver but significantly decreased (P = 0.02) in all the tumors (Groups I and II). This decrease was more severe in Group II (P = 0.02). The tumors in Group I had normal or increased gam ma-carboxylase activity and increased mRNA expression (P < 0.02) as co mpared with their nontumorous counterparts. The tumors in Group II wer e heterogeneous. Five tumors displayed low gamma-carboxylase activity, associated with low mRNA expression in two, whereas two others had hi gh gamma-carboxylase activity and mRNA expression. The concentration o f FLEEL at half-maximal velocity was normal in all the tumors examined (Groups I and II), and a relation was found between the level of expr ession of gamma-carboxylase and the maximal velocity for FLEEL carboxy lation In the tumors in Group II(r = 0.98; P < 0.01). The microsomal c ontent of normal prothrombin was within normal limits in all tumors (G roups I and II). Conclusions. Tumor vitamin K content has a critical r ole in the synthesis of des-gamma-carboxy prothrombin. Furthermore, th e gamma-carboxylase defect, which is observed in some secreting tumors , is the result of the defective gene expression of a normal enzyme an d not the consequence of the presence of a competitive inhibitor. It i s possible that a 75% reduction in gamma-carboxylase gene expression c ould take a part in the secretion of des-gamma-carboxy prothrombin, bu t this mechanism is not predominant.