The relation of RNA synthesis to chondroitin sulphate biosynthesis in cultured bovine cartilage

Modulation of aggrecan and link-protein synthesis in articular cartilage

The addition of serum or insulin-like growth factor-I (IGF-I) to the medium of explant cultures of bovine articular cartilage is known to stimulate the synthesis of aggrecan in a dose-dependent manner. The half-life of the pool of proteoglycan core protein was measured in adult articular cartilage cultured for 6 days in the presence and absence of 20 ng of IGF-I/ml and shown to be 24 min under both sets of conditions. The half-life of the mRNA pool coding for aggrecan was also determined and shown to be approx. 4 h in cartilage maintained in culture with or without IGF-I. The pool size of mRNA coding for aggrecan core protein increased 5-6-fold in cartilage explants maintained in culture in medium containing 20% (v/v) fetal-calf serum; however, in tissue maintained with medium containing IGF-I there was no increase in the cellular levels of this mRNA. This suggests that aggrecan synthesis is stimulated by IGF-I at the level of translation of mRNA coding for the core protein of this proteoglycan and that other growth factors are present in serum that stimulate aggrecan synthesis at the level of transcription of the core-protein gene. Inclusion of serum or IGF-I in the medium of cartilage explant cultures induced increases in the amounts of mRNA coding for type II collagen and link protein, whereas only serum enhanced the amount of mRNA for the core protein of decorin.

Synthesis of hyaluronate in cultured bovine articular cartilage

The synthesis and distribution of hyaluronate and proteoglycan were studied in bovine articular cartilage in short-term explant culture with [3H]acetate and H2(35)SO4 as precursors. The incorporation of [3H]acetate into hyaluronate and sulphated glycosaminoglycans was linear with time, except that hyaluronate synthesis showed a marked lag at the beginning of the incubation. [3H]Hyaluronate represented 4-7% of the total [3H]glycosaminoglycans synthesized over a 6 h period. However, the distributions of [3H]hyaluronate and 3H-labelled sulphated glycosaminoglycans were different: about 50% of the newly synthesized [3H]hyaluronate appeared in the medium, compared with less than 5% of the 3H-labelled sulphated proteoglycans. A pulse-chase experiment revealed that the release of newly synthesized [3H]hyaluronate from cartilage was rapid. No difference was observed in the distribution of [3H]hyaluronate between medium and tissue by cartilage from either the superficial layer or the deep layer of articular cartilage. When articular cartilage was incubated with 0.4 mM-cycloheximide, proteoglycan synthesis was markedly inhibited, whereas the synthesis of hyaluronate was only partially inhibited and resulted in more of the newly synthesized hyaluronate being released into the medium. Analysis of the hydrodynamic size of [3H]hyaluronate isolated from cartilage on Sephacryl-1000 revealed one population that was eluted as a broad peak (Kav. less than 0.7), compared with two populations (Kav. greater than 0.5 and less than 0.5) appearing in the medium of cultures. These data suggest that hyaluronate is synthesized in excess of proteoglycan synthesis and that the hyaluronate that is not complexed with proteoglycans is rapidly lost from the tissue.

Experimental syphilitic orchitis. Relationship between Treponema pallidum infection and testis synthesis of proteoglycans

The relationship between Treponema pallidum infection and the synthesis of proteoglycans by organ cultures of rabbit testes was investigated. Two proteoglycan fractions, large (CL-6B Kav 0.05) and small (Kav 0.25), that were not synthesized at detectable levels by cultures from rabbits infected with T. pallidum for 0 or 5 days, were produced by cultures from 10-, 15- and 20-day infected rabbits. The small proteoglycan appeared to be synthesized first because greater amounts of this fraction were detected in extracts of cultures from 10-day infected animals. The large proteoglycan fraction may have been induced directly by T. pallidum because increased synthesis correlated with maximal treponemal numbers, 15 days after infection. In contrast, the induction of the smaller proteoglycan did not appear to be directly related to numbers of elutable organisms. The proteoglycans synthesized between 10 and 20 days after infection were analyzed for glycosaminoglycan (GAG) size and composition. The size of GAGs beta-eliminated from the proteoglycans generally increased over the 10-20-day infection period. Whereas the composition of the small proteoglycan fraction was largely unchanged during the 10-20-day period (45% chondroitin sulphate (CS), 55% dermatan sulphate (DS)), the amount of DS in the high molecular weight proteoglycan fraction decreased from 50-20% during this period (CS 50-80%). Autoradiography studies revealed that increased proteoglycan synthesis in T. pallidum-infected testes was localized to cells lining the seminiferous tubules and to fibroblasts infiltrating peritubular spaces.

The effect of retinoic acid on proteoglycan biosynthesis in bovine articular cartilage cultures

The addition of retinoic acid to adult bovine articular cartilage cultures produces a concentration-dependent decrease in both proteoglycan synthesis and the proteoglycan content of the tissue. Total protein synthesis was not affected by the presence of retinoic acid, indicating that the inhibition of proteoglycan synthesis was not due to cytotoxicity. The proteoglycans synthesized in the presence of retinoic acid were similar in hydrodynamic size, ability to form aggregates with hyaluronate, and glycosaminoglycan composition to those of control cultures. However, the presence of larger glycosaminoglycan chains suggests that the core protein was substituted with fewer but longer glycosaminoglycan chains. In cultures maintained with retinoic acid, a decreased ratio of the large proteoglycan was synthesized relative to the small proteoglycan compared to that measured in control cultures. In cultures maintained with retinoic acid for 1 day and then switched to medium with 20% (v/v) fetal calf serum, the rate of proteoglycan synthesis and hexuronate contents increased within 5 days to levels near those of control cultures. Within 2 days of switching to medium with 20% (v/v) fetal calf serum, the relative proportions of the proteoglycan species were similar to those produced in cultures maintained in medium with 20% (v/v) fetal calf serum throughout. The rate of proteoglycan synthesis by bovine articular cartilage cultures exhibited an exponential decay following exposure to retinoic acid, with estimated half-lives of 11.5 and 5.3 h for tissue previously maintained in medium alone or containing 20% (v/v) fetal calf serum, respectively. The addition of 1 mM benzyl beta-D-xyloside only partially reversed the retinoic acid-mediated inhibition of proteoglycan synthesis. This indicates that the inhibition of proteoglycan synthesis by retinoic acid was due to both a decreased availability of xylosylated core protein and a decreased capacity of the chondrocytes to synthesize chondroitin sulfate chains.

Stimulation of proteoglycan biosynthesis by serum and insulin-like growth factor-I in cultured bovine articular cartilage

The addition of foetal calf serum to explant cultures of adult bovine articular cartilage is known to stimulate proteoglycan synthesis in a dose-dependent manner. We have now shown the activity in serum responsible for this effect to be heat- and acid-stable, to be associated with a high-Mr complex in normal serum but converted to a low-Mr form under acid conditions. The activity has an apparent Mr approximately 10,000 and isoelectric points similar to those reported for insulin-like growth factors (IGFs). Addition of a monoclonal antibody against insulin-like growth factor-I (IGF-I) prevented foetal calf serum from stimulating proteoglycan synthesis. Physiological concentrations of recombinant IGF-I or pharmacological levels of insulin when added to cartilage cultures mimicked the proteoglycan-stimulatory activity of serum. IGF-I appeared to act by increasing the rate of proteoglycan synthesis and did not change the nature of the proteoglycan synthesized nor the rate of proteoglycan catabolism by the tissue, suggesting that IGF-I may be important in the regulation of proteoglycan metabolism in adult articular cartilage. Furthermore, IGF-I can replace foetal calf serum in the culture medium, thereby allowing the use of a fully-defined medium which will maintain the synthesis and tissue levels of proteoglycan in adult articular cartilage explants for up to 5 days.

Control of proteoglycan biosynthesis. Further studies on the effect of serum on cultured bovine articular cartilage

Proteoglycan synthesis in explant cultures of adult bovine articular cartilage is stimulated in a dose-dependent manner when the tissue is cultured in the presence of foetal-calf serum. The stimulation of proteoglycan synthesis is paralleled by a similar increase in DNA synthesis; however, when DNA synthesis is inhibited by hydroxyurea the stimulation of proteoglycan synthesis by serum remains essentially the same. The apparent half-life of the pool of proteoglycan core protein precursor was measured in freshly isolated tissue as well as in tissue cultured for 7 days in the presence and in the absence of foetal-calf serum; under all conditions the half-life was the same, suggesting that this value is independent of the net rate of proteoglycan synthesis. In the presence of actinomycin D, an inhibitor of RNA synthesis, there was a difference in the apparent half-life of the available pool of mRNA coding for proteoglycan core protein: 8.5 h for tissue maintained in the presence of serum and 3.8 h for tissue cultured in the absence of serum. It is suggested that proteoglycan synthesis is stimulated by serum factors at the level of DNA-dependent RNA synthesis. Concomitant with an increase in the rate of proteoglycan synthesis induced by the presence of serum in the culture medium, an increase in the concentrations of several glycosyltransferases involved in chondroitin sulphate synthesis was also observed.