NPT HAPT Fig. 1. Electrophoretic analysis on 10% polyacrylamide gels <strong>of</strong> purified NPT <strong>and</strong> HAPT. The samples (25
NPT HAPT 10 5 2.5 1.25 CHARACTERIZATION OF A HEPATOMA ABNORMAL PROTHROMB1N 1.25 Fig. 3. Binding <strong>of</strong> 125I-concanavalin A to NPT <strong>and</strong> HAPT. Serial concentra tions <strong>of</strong> proteins were applied to nitrocellulose paper <strong>and</strong> incubated with radiolabeled concanavalin A. A, radiograph <strong>of</strong> the dot blot: B, densitometric scans <strong>of</strong> HAPT (gray) <strong>and</strong> NPT (black). different assay methods, other investigators have also reported increased blood levels <strong>of</strong> APT in 63-74% <strong>of</strong> patients with hepatocellular carcinoma (11-13). We have named the APT found in the blood <strong>of</strong> these patients the hepatoma-associated abnormal prothrombin antigen. APT is found in the blood <strong>of</strong> patients with vitamin K defi ciency, patients with acquired disorders <strong>of</strong> hepatic function, <strong>and</strong> patients treated with coumarin anticoagulants (8, 9). In patients with vitamin K deficiency <strong>and</strong> patients taking cou marin, circulating APT rapidly disappears from the blood with parenteral vitamin K. In patients with hepatocellular carci noma, HAPT does not disappear with the administration <strong>of</strong> parenteral vitamin K (10, 13). Therefore, these patients do not have vitamin K deficiency. In fact, measurements <strong>of</strong> blood vitamin K levels in two hepatoma patients found elevated levels <strong>of</strong> the vitamin.3 HAPT, furthermore, is eliminated or reduced with resection <strong>of</strong> the hepatoma (10, 13). Therefore, it could be concluded that HAPT is synthesized by the malignant hepatocyte <strong>and</strong> is characteristic <strong>of</strong> an acquired tumor defect in vitamin K-dependent carboxylation. HAPT has been detected in blood by immunochemical meth ods. However, antigenic identity between HAPT <strong>and</strong> APT found in the blood <strong>of</strong> patients taking vitamin K antagonists does not exclude the possibility <strong>of</strong> significant structural differ ences between these proteins. An example is Factor IX Cam bridge (23). Factor IX Cambridge has antigenic characteristics similar to those <strong>of</strong> abnormal (des-7-carboxy) Factor IX. How ever, Factor IX Cambridge has a point mutation at the -1 residue <strong>of</strong> the propeptide resulting in a protein with an un- 3 H. Liebman. unpublished observations. 6496 processed 18-residue amino-terminal propeptide. Therefore, the protein has a higher molecular weight than either native or abnormal Factor IX. In this report I have purified a HAPT from the ascites <strong>of</strong> a patient with hepatocellular carcinoma <strong>and</strong> have compared this protein to NPT <strong>and</strong> APT. Analysis <strong>of</strong> this protein shows structural identity to APT <strong>and</strong> further supports the hypothesis that HAPT results from a defect in the vitamin K-dependent posttranslational carboxylation <strong>of</strong> the prothrombin precursor by the malignant hepatocyte. Purified HAPT has the same molecular weight, amino acid analysis (exclusive <strong>of</strong> 7-carboxyglutamic acid content), <strong>and</strong> amino-terminal structure as NPT <strong>and</strong> APT. Analysis <strong>of</strong> the 7carboxyglutamic acid content <strong>of</strong> HAPT shows the protein to be partially carboxylated with an average <strong>of</strong> 5 Gla residues/ molecule. Electrophoretic analysis, in the presence <strong>of</strong> Ca(II), suggest that HAPT is more heterogeneous with regard to its Gla content than either NPT or APT. However, this apparent difference in Gla content between HAPT <strong>and</strong> APT is probably secondary to the methods used to isolate these proteins. The purification <strong>of</strong> APT (9) includes a barium citrate absorption step resulting in the removal <strong>of</strong> partially carboxylated pro thrombin species. The purification <strong>of</strong> HAPT does not include the barium absorption <strong>and</strong> utilizes an anti-prothrombin:Ca(II) column to remove contaminating native prothrombin. The immunoaffmity removal <strong>of</strong> NPT would leave partially carboxyl ated variants with less than 8 Gla residues in the HAPT preparation. Other investigators have prepared APT using dif ferent methods <strong>and</strong> this has resulted in an APT preparation which is more heterogeneous with regard to its Gla content (25, 26). Hepatocellular carcinomas are associated with production <strong>of</strong> aberrant <strong>and</strong> ectopie proteins including «-fetoprotein (27), ab normal vitamin B12 binders (24), erythroprotein (28), <strong>and</strong> abnormal fibrinogens (29). The abnormal vitamin B12-binding proteins <strong>and</strong> fibrinogens are believed to result from aberrant or excessive glycosylation <strong>of</strong> these proteins. I studied the glycosylation <strong>of</strong> HAPT using a concanavalin A-binding assay to deter mine if this protein is also abnormally glycosylated. There were minor differences between HAPT <strong>and</strong> NPT in the binding <strong>of</strong> 1251-labeled concanavalin. These studies suggest that HAPT is /V-glycosylated to an equal or slightly lesser degree than NPT. Also previous studies on the role <strong>of</strong> sugar residues on the function <strong>of</strong> the vitamin K-dependent proteins find no evidence that glycosylation is essential for biologic activity. A rat model for the production <strong>of</strong> abnormal prothrombin by hepatocellular carcinoma was recently reported by Shah et al. (30). Their data are consistent with the "hypothesis that the tumors that increase circulating abnormal prothrombin are those that are capable <strong>of</strong> expressing the prothrombin gene, but that have lost the ability to express significant levels <strong>of</strong> the vitamin K-dependent carboxylase enzyme." The structural char acterization <strong>of</strong> the HAPT is consistent with this model since the only defect noted was a deficiency in 7-carboxyglutamic acid content. The expression <strong>of</strong> H APT by the malignant hepatocyte results from different biochemical derangements than those responsi ble for secretion <strong>of</strong> «-fetoprotein. APT is the product <strong>of</strong> a defective posttranslational processing step in the malignant hepatocyte. n-Fetoprotein production results from the reexpression <strong>of</strong> a suppressed fetal gene. The poor correlation between these two tumor markers supports the independence <strong>of</strong> these two acquired cellular defects (10-13). When measurements <strong>of</strong> blood HAPT are used in combination with the assav for the Downloaded from cancerres.aacrjournals.org on July 30, 2013. © 1989 American Association for <strong>Cancer</strong> <strong>Research</strong>.
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