A new pathogenic hypothesis for cystic fibrosis: hyperactivity of glycosyl transferases at microsomic level

Clinical aspects of glycoprotein biosynthesis

Glycoproteins are widely distributed among species in soluble and membrane-bound forms, associated with many different functions. The heterogenous sugar moieties of glycoproteins are assembled in the endoplasmic reticulum and in the Golgi and are implicated in many roles that require further elucidation. Glycoprotein-bound oligosaccharides show significant changes in their structures and relative occurrences during growth, development, and differentiation. Diverse alterations of these carbohydrate chains occur in diseases such as cancer, metastasis, leukemia, inflammatory, and other diseases. Structural alterations may correlate with activities of glycosyltransferases that assemble glycans, but often the biochemical origin of these changes remains unclear. This suggests a multitude of biosynthetic control mechanisms that are functional in vivo but have not yet been unraveled by in vitro studies. The multitude of carbohydrate alterations observed in disease states may not be the primary cause but may reflect the growth and biochemical activity of the affected cell. However, knowledge of the control mechanisms in the biosynthesis of glycoprotein glycans may be helpful in understanding, diagnosing, and treating disease.

Cystic fibrosis: a casualty of "detoxification"?

It is proposed that excessive and/or aberrant function of cytochromes P450, due to a combination of genetic and environmental influences, is the basic defect in cystic fibrosis. Organs that are involved in oxidative detoxification reactions in foetal life are thus at risk: tissue damage is initiated by excessive production of oxygen free radicals which deplete cellular antioxidants and provoke the secretion of mucus.

Cobalamin R binder as a possible model molecule for glycoprotein study in cystic fibrosis

The isoprotein pattern of semi-purified R binder (an acidic glycoprotein which binds cobalamin) from saliva and sera of 8 cystic fibrosis patients was compared to that of R binder from samples of 5 healthy children. In cases of cystic fibrosis, the mean isoelectric point of salivary R binder was increased from 3.78 up to 4.34 and its microheterogeneity was reduced. These significant physicochemical modifications were not observed with R binder from cystic fibrosis sera and they did not correlate with the beta-galactosidase, alpha-mannosidase, alpha-L-fucosidase nor neuraminidase activity of saliva. We propose the R binder as a model molecule to study the glycoprotein metabolism in cystic fibrosis since it contains 30-40% carbohydrate, is easily complexed with cyano[57Co]cobalamin and is present in most tissues and fluids of the human organism.

Mannosylation of glycoproteins and dolichol derivatives in fibroblasts from patients with cystic fibrosis

Increased incorporation of mannose into endogenous glycoprotein fractions has been found in whole cell lysates and crude membrane preparations of cultured skin fibroblasts from patients with cystic fibrosis (1.3-2.3-times normal) when GDP[14C]mannose served as the mannosyl donor. In contrast, the incorporation of mannose from GDPmannose into lipid fractions containing dolichol phosphate and dolichol pyrophosphate oligosaccharides as well as the incorporation of mannose from dolichol phospho[3H]mannose into both glycoproteins and dolichol derivatives were not significantly different among cell preparations from patients with cystic fibrosis and normal controls. Mannosyltransferase activity toward exogenous glycoproteins as well as the activities of soluble and membranous alpha-mannosidase and beta-mannosidase appeared to be normal and could not account for the observed differences. The altered incorporation of mannose into endogenous glycoprotein may reflect changes in glycosylation processes other than mannosylation.

Glycosylation of VSV glycoprotein is similar in cystic fibrosis, heterozygous carrier, and normal human fibroblasts

The single envelope glycoprotein of vesicular stomatitis virus was used as a specific probe of glycosyltransferase activities in fibroblasts from two cystic fibrosis patients, an obligate heterozygous carrier and a normal individual. Gel filtration of pronase-digested glycopeptides from both purified virions and infected cell-associated VSV glycoprotein which had been labeled with[3H] glucosamine did not reveal any significant differences in the glycosylation patterns between the different cell cultures. All 4 cell lines were apparently able to synthesize the mannose- and glucosamine- containing core structure and branch chains terminating in sialic acid which are characteristic of asparagine-linked carbohydrate side chains in cellular glycoproteins. Analysis of tryptic glycopeptides by anion-exchange chromotography indicated that the same 2 major sites on the virus polypeptide were recognized and glycosylated in all 4 VSV-infected cell cultures. These studies suggest that the basic biochemical defect(s) in cystic fibrosis is not an absence or deficiency in enzymes responsible for the biosynthesis of complex carbohydrate side chains.

Altered isoenzyme patterns of liver alpha-L-fucosidase in cystic fibrosis

The isoenzyme pattern of alpha-L-fucosidase was studied by isoelectric focusing in livers from seven patients with cystic fibrosis and in normal and pathological (GM1-gangliosidosis, Type II and Sanfilippo disease) controls. The controls had very reproducible patterns consisting of seven isoenzymes of alpha-L-fucosidase with the most neutral from (I) representing a small proportion of the total activity. All seven of the cystic fibrosis livers had altered alpha-L-fucosidase isoenzyme patterns. The chemical relationship of the seven isoenzymes of normal liver alpha-L-fucosidase was investigated using neuramindase. The five most acidic forms of alpha-L-fucosidase appear to be related to the most neutral form by sialic acid residues. Since the isoenzymes of liver alpha-L-fucosidase appear to be related by sialic acid residues, it is possible that the altered alpha-L-fucosidase isoenzyme patterns found in cystic fibrosis livers may result from aberrant sialylation.

Carcinoembryonic antigen and cystic fibrosis protein in blood from cystic fibrosis homozygotes and heterozygote carriers

Carcinoembryonic antigen (CEA) activity was measured by radioimmunoassay in blood from cystic fibrosis (CF) homozygotes, heterozygote carriers of CF, normal healthy controls, and other patient controls with carcinomas involving gastrointestinal organs. All samples were also screened by electrofocusing for cystic fibrosis protein (CFP), a metabolic marker previously shown to be associated with the CF gene. Significantly increased levels of CEA activity were found in all CFP-positive groups; however, with one exception all patient controls with marked increases in CEA activity were CFP-negative. Immunodiffusion of perchloric acid extracts of CEA-like material from heterozygote carrier blood indicated that the CEA-like material, which was elevated in homozygotes and heterozygotes for CF, showed only partial identity with two separate CEA preparations obtained from colon carcinomas and was not identical to either A, B, or O(H) blood group substances. This glycoprotein material did, however, react with three different anti-CEA antisera. Our finding of an abnormally increased glycoprotein in cystic fibrosis, taken together with previous reports demonstrating abnormalities in the carbohydrate portion of glycoproteins found in various exocrine secretions in CF, further suggests that the primary defect in this disease is manifested partly as a defect in glycoprotein metabolism. This defect may result from an abnormality in one or more of the glycosyltransferases, possibly caused by a more primary defect in polyamine metabolism.