Basement membrane biosynthesis. Secretion without deposition of underhydroxylated basement membrane collagen by parietal yolk sacs

Tyrosinemia I, a model for human diseases mediated by 2-oxoacid-utilizing dioxygenases: hepatotoxin suppression by NTBC does not normalize hepatic collagen metabolism

OBJECTIVES

Medical treatment of tyrosinemia I relies on the herbicide NTBC [Orfadin 2-(2-nitro-4-trifluoromethylbenzoyl)-cyclohexane-1,3-dione], an inhibitor of plant and mammalian 2-oxoacid-utilizing dioxygenases with a collective catalytic cycle ('HAG' mechanism). We hypothesize that NTBC-treated tyrosinemia I is a human model for the pathogenic role of two major enzymes in this class, 4-hydroxyphenylpyruvate dioxygenase (4-HPPD; EC 1.13.11.27) and prolyl 4-hydroxylase (P4-H; E.C. 1.14.11.2), essential for tyrosine and collagen metabolism, respectively.

METHODS

In a patient with established tyrosinemia I, we monitored the in vivo activities of 4-HPPD and P4-H via five biomarkers before and during NTBC medication. Hypothesis testing at the molecular level was performed by computational modeling of NTBC binding to the crystal structure-derived active site of 4-HPPD, and then relating these findings to our experimental results and to known P4-H data.

RESULTS

NTBC rapidly normalized the biomarkers for 4-HPPD activity. However, those for P4-H activity remained uniformly elevated after one hundred days on NTBC, the PIIINP biomarker even increasing above its grossly abnormal, initial level. This selective enzyme inhibition despite a collective catalytic cycle is attributed to the conformation of NTBC, which only fits the active site of 4-HPPD, as confirmed by its crystal structure.

CONCLUSIONS

Normalization of hepatic collagen formation, highly desirable in all fibrotic liver diseases, is not achieved by NTBC in tyrosinemia I. By establishing the molecular cause for this failure, our results also establish a rational approach to identify inhibitors that achieve that goal, either by joint 4-HPPD / P-4H inhibition, or by inhibition of only P-4H.

Expression of type IV collagen-degrading activity during early embryonal development in the sea urchin and the arresting effects of collagen synthesis inhibitors on embryogenesis

Type IV collagen-degrading activity was expressed in homogenates of Lytechinus pictus embryos during embryogenesis. Activity was concentrated 1,600-fold by ammonium sulfate fractionation, ion exchange, and gel chromatography and could not be activated further upon trypsin or organomercurial treatment. This enzyme activity could also degrade gelatin but had no affinity for type I, III, and V collagens. Activity was inhibited by addition of excess type IV collagen or gelatin, but was unaffected by addition of excess amounts of non-collagenous proteins of the extracellular matrix. Chelators such as 1,10-phenanthroline or Na2EDTA reduced activity to control levels. Inhibitors of plasmin and of serine and thiol proteases were without effect. Type IV collagen-degrading activity first became apparent at the stage of early mesenchyme blastula. It then increased by a small increment and remained stable up to the stage of late mesenchyme blastula, coinciding with first detection of collagen synthesis and the appearance of the archenteron. Thereafter, a sharp increase in activity was observed, concurrently with remodelling of the archenteron. Maximum activity was attained at prism stage and was retained throughout to pluteus-larva stage. The specific inhibitors of collagen biosynthesis 8,9-dihydroxy-7-methyl-benzo[b]quinolizinium bromide and tricyclodecane-9-yl xanthate arrested sea urchin embryo development at early blastula, prevented the invagination of the archenteron, and reverted the expression of type IV collagen-degrading activity to non-detectable levels. Removal of the inhibitors allowed embryos to gastrulate and express type IV collagen-degrading activity.

Comparative evaluation of mitogenicity and basement-membrane-degrading activity of Pseudomonas aeruginosa slime glycolipoprotein and alginate

Alginate from a heavily mucoid Pseudomonas aeruginosa strain and slime glycolipoprotein obtained from the revertant nonmucoid variant of the mucoid strain were tested for mitogenic activity on human peripheral lymphocytes and for degradation of 3H-labeled basement membranes of the anterior lens capsule of bovine eyes. Slime glycolipoprotein exerted mitogenic activity in concentrations from 50 to 200 micrograms/ml, whereas alginate was not mitogenic as shown by [3H]thymidine uptake. Alginate did not show any basement membrane degradation, whereas slime glycolipoprotein exhibited basement-membrane-degrading activity from 35 to 450 micrograms/ml in a dose-related manner. This activity was inhibited by metal chelators but not thiol protease inhibitors. The results suggest that alginate lacks the mitogenic and biodegrading activities of slime glycolipoprotein; these activities nevertheless need further investigation.

Degradation of intact basement membranes by human and murine tumor enzymes

Homogenates from malignant tumors, obtained from surgery specimens or from transplants of Walker 256 carcinosarcoma in rats, contained an enzyme activity capable of degrading intact 3H-acetylated basement membranes from bovine lens. The enzyme activity from murine tumor was purified about 7500-fold by (NH4)2SO4 fractionation, ion exchange and gel chromatography. The apparent molecular weight of the purified enzyme was approximately 50,000. The rate of degradation of 3H-labelled basement membrane by the murine tumor enzyme was reduced by addition of excess type IV collagen, but not of excess type I, type III or type V collagen. These results suggested specificity of this enzyme for type IV collagen. Inhibitors of serine proteinases, thiol proteinases and soybean trypsin inhibitor were without effect on the enzyme activity. Chelators such as 1,10-phenanthroline or EDTA reduced the activity to control levels, indicating that the enzyme activity was due to a metalloproteinase. Chromatographic and electrophoretic separation of the enzymatic products from 3H-labelled basement membrane and type IV collagen indicated that the enzyme activity was due to a type IV collagenase. The use of basement membrane in the native physiological state as a substrate for the study of basement membrane-degrading activity by homogenates of solid malignant tumors offers an in vitro model for the investigation of the metastatic potential of these tumors.

Rate of basement membrane biosynthesis as an index to angiogenesis

A method was developed for assessing collagenous protein biosynthesis from [U-14C]proline in relation to angiogenesis in the chick chorioallantoic membrane (CAM). The rate of collagenous protein biosynthesis both in vitro and in vivo was maximum between days 8 and 11 of chick embryo development. This was the stage of maximum angiogenesis as shown by morphological evaluation of the vascular density. At day 10 the rate of collagenous protein biosynthesis was 11-fold higher than that of day 15, when angiogenesis had reached a plateau. The collagenous protein formed by CAM co-elutes on SDS-agarose chromatography with the collagenous component of [3H]-acetylated-basement membrane (BM) from bovine lens capsule. 8,9-dihydroxy-7-methyl-benzo[b]quinolizinium bromide (GPA1734), which was shown previously to be a specific inhibitor of BM collagen biosynthesis, caused about 80% reduction in collagenous protein synthesis by CAM. These results indicate that most of the collagenous protein synthesized by CAM was BM collagen and this can be used as a biochemical index of angiogenesis.

Assembly of chick and bovine lens-capsule collagen

Chick-embryo and adult bovine lens-capsular epithelia in organ culture synthesized 4-hydroxy[3H]proline-containing polypeptides when incubated in the presence of [3H]proline. These collagenous polypeptides of apparent Mr 180 000, 175 000 and 160 000 became incorporated with time into aggregates of higher molecular size. The formation of such aggregates was inhibited when the tissues were labelled in the presence of beta-aminopropionitrile, thereby implicating lysine-derived cross-links in aggregate formation. When the tissues were incubated in the presence of tunicamycin, the collagenous polypeptides synthesized exhibited increased electrophoretic mobilities on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. The addition to lens-capsule incubation medium of alpha alpha'-bipyridine led to the synthesis of underhydroxylated type IV collagen, also of increased electrophoretic mobility. Extended pulse-chase experiments indicated that such underhydroxylated collagen did not participate in aggregate formation, but was at least as stable as fully hydroxylated non-cross-linked collagen synthesized in the presence of beta-aminopropionitrile. Native type IV collagen, recovered from the culture medium when capsules were incubated with [3H]proline for 24h, was purified by ion-exchange chromatography. Separations conducted on CM-cellulose under denaturing and nondenaturing conditions suggested that the alpha 1(IV) and alpha 2(IV) chains occur in the same heterologous triple helix. Densitometric analyses of appropriate fluorograms indicated that these two polypeptides occur in a 2:1 ratio, suggesting that lens-capsule collagen is synthesized as a triple-helical molecule of composition [alpha 1(IV)]2 alpha 2(IV).

Effect of monensin on secretion of basement membrane collagen by cultured rabbit corneal endothelial cells in comparison with rabbit tendon fibroblasts

Monensin, a monovalent ionophore, affects differently the secretion of basement membrane collagen and interstitial procollagen. At 1 X 10(-8) M, monensin inhibited only slightly the secretion of procollagen synthesized by tendon cells while secretion of basement membrane collagen synthesized by endothelial cells was inhibited by 43% after 2 hours of labelling. At higher concentration (1 X 10(-6) M), the secretion of type I procollagen by tendon cells was inhibited after two h (36%) or 24 h (24%) of labelling. The pattern of basement membrane collagen secretion varied from inhibition to stimulation depending on the labeling time. No changes in the phenotypic expression of collagen were observed in either the monensin-treated tendon cultures or endothelial cells. Phase-contrast microscopy revealed differential responses of cells to monensin: tendon cells appeared spindle-shaped or rounded-up, in contrast to the enlarged shape displayed by endothelial cells.

Biosynthesis of two subunits of type IV procollagen and of other basement membrane proteins by a human tumor cell line

The major collagenous component secreted into the medium of cultured HT-1080 tumor cells was identified as type IV procollagen by specific antibodies and characteristic ratios of incorporated labeled 3-hydroxyproline and 4-hydroxyproline. The disulfide-bonded molecules consisted of two subunits, pro-alpha 1(IV) and pro-alpha 2(IV) chains with apparent molecular weights of 180 000 and 165 000. No conversion of the procollagen to collagen or to procollagen intermediates was detected in the cell cultures. The two subunits apparently represent different gene products, since enzymatic digestion of the separated chains produced quite different peptide maps. Pepsin degraded native type IV procollagen successively into several fragments, some still disulfide-linked, giving rise to a complex set of polypeptide chains (Mr = 30 000-140 000). This agrees with similar diverse patterns produced by pepsin from authentic type IV collagens. The ratio between the pro-alpha 1(IV) and pro-alpha 2(IV) chains varied in several experiments between 1.3 and 1.8, suggesting that the two chains belong to different triple-helical molecules. The cells also produced distinct amounts of fibronectin (subunit Mr = 230 000) and of the basement membrane glycoprotein laminin. The latter showed three subunits with Mr = 220 000, 210 000, and 400 000. A further disulfide-bonded, non-collagenous polypeptide (Mr = 160 000) was detected but not yet identified. Immunofluorescence demonstrated these proteins within the cells but not in a pericellular matrix. The production of basement membrane components by HT-1080 cells and lack of interstitial collagens disagree with the original classification of the cell line as a fibrosarcoma.