A radioautographic study of the hepatic uptake of circulating carcinoembryonic antigen by the mouse

Coagulation factor X interaction with macrophages through its N-glycans protects it from a rapid clearance

Factor X (FX), a plasma glycoprotein playing a central role in coagulation has a long circulatory half-life compared to closely related coagulation factors. The activation peptide of FX has been shown to influence its clearance with two N-glycans as key determinants of FX’s relatively long survival. To decipher FX clearance mechanism, organ biodistribution and cellular interactions of human plasma FX (pd-FX), recombinant FX (rFX), N-deglycosylated FX (N-degly-FX) and recombinant FX mutated at both N-glycosylation sites (rFX^{N181A–N191A}) were evaluated. Biodistribution analysis of ¹²⁵I-labelled FX proteins after administration to mice revealed liver as major target organ for all FX variants. Liver tissue sections analysis showed an interaction of pd-FX and N-degly-FX to different cell types. These findings were confirmed in cell binding studies revealing that FX and FX without N-glycans interact with macrophages and hepatocytes, respectively. N-degly-FX appeared to be degraded in hepatocytes while interestingly pd-FX was not by macrophages. Furthermore, the chemical inactivation of macrophages by gadolinium chloride resulted in a significant decrease of circulating pd-FX into mice and not of N-degly-FX. Altogether our data lead to the conclusion that FX interaction with macrophages through its N-glycans protects it from a rapid clearance explaining its relatively long circulatory half-life.

A carcinoembryonic antigen (CEA) binding protein from ascites influences CEA uptake by macrophages

A variant of CEA which is less readily endocytosed by macrophages has been isolated from malignant ascites. In vivo, CEA is cleared more slowly by the liver (t1/2 = 15.1 minutes) than CEAs isolated from hepatic metastases (t1/2 = 3.1 minutes). In vitro, rat and human Kupffer cells and rat alveolar macrophages endocytose this CEA less effectively. This slow clearing form of CEA is associated with a smaller (45kD) acidic glycoprotein (CORA) with which it forms a stable complex. CORA can be visualized on reducing gels but not on non reducing gels or by HPLC run under non reducing conditions. This suggests a non-covalent complex between the two glycoproteins. Analysis of protein conformation by circular dichroism revealed changes in the ascites CEA consistent with binding of CORA to the molecule. Western blot showed that CORA crossreacts with antisera to alpha 1-acid glycoprotein and double immunodiffusion demonstrated cross-reactivity but not identify. Sequencing of CNBr peptides showed sequence homology with alpha 1-acid glycoprotein but areas of unique sequence were also found. It is suggested that binding of CORA to CEA blocks the macrophage receptor binding of CEA.

Carcinoembryonic antigen: function in metastasis by human colorectal carcinoma

Carcinoembryonic antigen (CEA) is a glycoprotein that has been useful as a tumor marker to predict recurrence in gastrointestinal malignancies, but whose biological function has not been elucidated. With the recent evidence that CEA is a member of the immunoglobulin supergene family, CEA may be involved in intercellular recognition and binding. This review examines the role that CEA plays in the development of metastases by colorectal carcinoma.

Hepatic clearance and metabolism in the rat of a human breast cancer associated glycoprotein (GCDFP-15)

Gross Cystic Disease Fluid Protein (GCDFP-15) is a 60,000 dalton glycoprotein isolated from human breast cyst fluid, composed of four 15,000 dalton monomers. Carbohydrate analysis indicates that each monomer has a single carbohydrate chain of the complex type. GCDFP-15 intravenously injected into rats showed a rapid circulatory clearance, the rate of clearance being faster in female animals [t1/2 = 12.8 (+/- 2.0) min. females, and 16.7 (+/- 2.6) min. males]. The major organs of clearance were the liver (70%) and kidneys (15%). Immunoperoxidase staining showed localization in Kupffer cells and the proximal convoluted tubules of the kidney. Removal of sialic acid from GCDFP-15 resulted in a more rapid clearance (t1/2 = 2.2 min) by the liver (85%). This clearance was inhibited by coinjection of asialo alpha 1 acid glycoprotein. About 3% of GCDFP-15 was excreted in bile with a transit time through the liver of 38 min. Examination of the uptake of GCDFP-15 by isolated rat Kupffer cells showed that yeast mannan, fucosylated BSA, and carcinoembryonic antigen (CEA) failed to inhibit uptake, though the binding of GCDFP-15 was clearly saturable. This suggests that a novel receptor system on the rat Kupffer cell may be responsible for GCDFP-15 clearance.

Serum beta-N-acetyl hexosaminidase (beta-NAH) as a discriminant between malignant and benign extrahepatic biliary obstruction: comparison with carcinoembryonic antigen (CEA)

Fifty-one patients (16 with malignant extrahepatic biliary obstruction, ten with benign extrahepatic biliary obstruction, eight with alcoholic liver disease, five with viral hepatitis and 12 with liver metastases) and 19 adult healthy controls were studied with determinations of beta-N-acetyl hexosaminidase (a lysosomal enzyme which is cleared from the circulation by the Kupffer cells), carcinoembryonic antigen (CEA), serum bilirubin, alkaline-phosphatase and aspartate aminotransferase (AST). Both CEA and beta-NAH were elevated in each disease group. Elevated beta-NAH levels distinguished between benign and malignant extrahepatic biliary obstruction better than CEA levels. Beta-NAH levels for the malignant and the benign groups were 47.6 +/- 14.7 U/l and 23.0 +/- 4.7 U/l (mean +/- S.D.) respectively. The groups differed significantly (P less than 0.001). Plasma CEA levels for both groups were 18.7 +/- 38.9 and 7.2 +/- 3.3 ng/ml (mean +/- S.D.) respectively. Beta-NAH levels for the 19 normal controls were 15.8 +/- 3.5 U/l (mean +/- S.D.). Beta-NAH also was significantly elevated in patients with hepatic metastases (36.9 +/- 20.1 U/l). In 25 cancer patients with metastases other than in the liver beta-NAH levels (18.3 +/- 5.2) were not significantly elevated over the control group. It has potential value as a marker for non-CEA-producing liver metastases.

Role of carcinoembryonic antigen assay in the management of cancer

CEA is a molecule produced by a large number of malignant and benign tissues. Measuring levels of CEA circulating in the blood by radioimmunoassay can be used in the management of cancer patients. Because of high false positive and false negative percentages in normal populations, it has not been useful in screening for malignancy. However, in several types of cancer patients the test has been shown to be of considerable clinical value. Elevated CEA levels indicate a poor prognosis in patients with primary colorectal cancer, primary pancreatic cancer, primary breast cancer, and primary lung cancer. Serial CEA titers obtained following cancer treatments can be used to monitor the therapy. CEA can assess the adequacy of surgical removal of a primary colon or rectal cancer, monitor responses to chemotherapy, and assess response to radiation therapy. The greatest clinical impact of CEA has been in the detection of recurrent colon or rectal cancer following surgical resection of the primary malignancy. Early detection of recurrence, when combined with reoperative second-look surgery, may result in 30% long-term survivors.

Role of the liver in clearance and excretion of circulating carcinoembryonic antigen (CEA)

CEA is a glycoprotein with a molecular weight of 200,000 containing 55%-65% carbohydrate. The removal of only two sialic acid residues result in rapid uptake from the circulation by the liver and catabolism in the lysosomes. There is a receptor on the plasma membrane of the hepatocyte (hepatic binding protein) which recognizes galactosyl residues. About 70% of 125I-labeled intact CEA is cleared by the liver in 1 hr. The exposure of terminal galactose residues by removing sialic acids determines the rate of clearance. CEA is probably initially taken up by Kupffer cells and transferred to hepatocytes. About 10% of CEA added to an isolated perfused liver appears in bile. Biliary duct obstruction and cholestasis may elevate plasma CEA levels due to detergent effects on the liver cell receptors.

A new Kupffer cell receptor mediating plasma clearance of carcinoembryonic antigen by the rat

Native human carcinoembryonic antigen is rapidly removed from the circulation by the rat liver Kupffer cell after intravenous injection. The molecule is subsequently transferred to the hepatocyte in an immunologically identifiable form. Carcinoembryonic antigen has a circulatory half-life of 3.7 (+/- 0.8) min, and cellular entry is by receptor-mediated endocytosis. Non-specific fluid pinocytosis and phagocytosis can be excluded as possible mechanisms by the kinetics of clearance and failure of colloidal carbon to inhibit uptake. Substances with known affinity for the hepatic receptors for mannose, N-acetylglucosamine, fucose and galactose all fail to inhibit carcinoembryonic antigen clearance. After two cycles of the Smith degradation, carcinoembryonic antigen is still able to inhibit clearance of the native molecule. Receptor specificity is apparently not dependent on those non-reducing terminal sugars of the native molecule. Performic acid-oxidized carcinoembryonic antigen also inhibits clearance of carcinoembryonic antigen in vivo. Receptor binding is not dependent on tertiary protein conformation. Non-specific cross-reacting antigen, a glycoprotein structurally similar to carcinoembryonic antigen, is cleared by the same mechanism.

Circulating CEA levels in patients with fulminant hepatitis

CEA levels were determined in blood specimens from 14 persons suffering from fulminant hepatitis. Values ranged from 1.9 to 21 ng/ml with a mean of 7.0 ng/ml. All except one patient had a CEA level greater than 2.5 ng/ml, but only three exceeded 10 ng/ml. There was no correlation between single CEA levels and prognosis, routine liver chemistries, or histopathologic characterization of hepatic tissue obtained at postmortem examination. These data show that patients with fulminant hepatitis commonly had increased circulating levels of CEA, but these elevations were far less than may be found in patients with hepatic metastases. In severe hepatic dysfunction reduced hepatic clearance of CEA may be responsible for the increased levels.

The biliary excretion of trypsin in rats

The serum half-life of bovine [3H]acetyltrypsin was estimated to be 9 min following intravenous administration in rats. This was maintained when six successive doses of 200 micrograms each were given at 1-h intervals. The enzyme was removed from the circulation after complexing with alpha 2-macroglobulin (alpha 2-M). The amount of 3H label appearing in bile increased with each successive dose and this was associated with breakdown products ( less than 10,000 daltons) of the alpha 2-M/[3H]acetyltrypsin. Intact alpha 2-M/[3H]acetyltrypsin was recovered from bile but represented only 0.06% of the administered dose of active enzyme.

Studies in vivo of the tissue uptake, cellular distribution and catabolic turnover of exogenous glucocerebrosidase in rat

The kinetics of plasma clearance of highly purified human placental glucocerebrosidase in rats were biphasic with 75% of the infused dose showing a rapid clearance (t1/2 = 11 min) and the remaining 25% a considerably lower rate (t1/2 = 60 min). The majority of the enzyme (60%) was taken up by the liver. Although saturation kinetics for the clearance or uptake were not observed, the very high hepatic endocytic index (217 microliter/min) of glucocerebrosidase uptake indicated that liver uptake was mediated by an adsorptive endocytic process. Analysis of the cellular distribution of recovered glucocerebrosidase revealed predominantly parenchymal cell uptake with 38% of the exogenous enzyme in hepatocytes and only 2% in sinusoidal cells. High-mannose glycoproteins blocked hepatocyte and sinusoidal cell uptake of glucocerebrosidase equally. Kinetic experiments failed to demonstrate a transfer or shuttle of exogenous glucocerebrosidase from sinusoidal cells to hepatocytes. The possibility was raised that uptake of enzyme by the liver may be mediated by a common receptor that functions in both hepatocytes and sinusoidal cells. The catabolic turnover of exogenous glucocerebrosidase in rat liver was biphasic and the rate of decline was similar in hepatocytes and sinusoidal cells.

Studies on the mechanisms of biliary excretion of circulating glycoproteins. The carcinoembryonic antigen

The transport of carcinoembryonic antigen from the circulation of the bile has been studied in the rat. Biliary excretion was directly proportional to the intravenously administered dose. Approximately 1-1.5% of the injected is excreted in the bile. Asialo-(carcinoembryonic antigen), asialo-fetuin and asialo-(alpha 1-acid glycoprotein) behaved similarly. The transit time through the liver for both native- and asialo-(carcinoembryonic antigen) was calculated as 47 min. Sialic acid content was not affected during biliary excretion. Glycoproteins that entered the liver cell lysosomes were not excreted in ther bile. A mechanism by which circulating proteins may by-pass the hepatocytes before being excreted in bile is suggested. Alternative mechanisms of protein entry into bile are discussed.

Carcinoembryonic antigen (CEA) levels in benign gastrointestinal disease states

Elevated circulating CEA levels occur in patients with benign gastrointestinal and hepatic disorders. These are usually less than 10 ng/ml. Of clinical importance is the influence of liver disease on the interpretation of CEA. At least 50% of patients with severe benign hepatic disease have elevated CEA levels, most often active alcoholic cirrhosis, and also chronic active and viral hepatitis, and cryptogenic and biliary cirrhosis. Patients with benign extrahepatic biliary obstruction may have increased plasma CEA, the highest in patients with co-existent cholangitis and especially liver abscess. The liver appears to be essential for the metabolism and/or excretion of CEA. Hence, liver work-up is needed to assess any patient with an elevated CEA. A damaged liver may further augment elevated CEA levels due to cancer. The increased circulating CEA observed in some patients with active ulcerative colitis tends to correlate with severity and extent of disease and usually returns to normal with remission. CEA levels also may be mildly elevated in patients with pancreatitis and in adults with colonic polyps. Smoking may contribute to the increased CEA levels seen in patients with alcoholic liver disease and pancreatitis. Therefore, in interpreting mildy elevated circulating CEA levels in patients with GI tract diseases, one must consider benign as well as malignant etiologies.