Regulation of Glycosaminoglycan Synthesis by Thyroid Hormone in Vitro

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Abstract
Human skin fibroblasts synthesize and accumulate glycosaminoglycans (GAG). Recently, we reported that fibroblasts incubated in thyroid hormone-deficient media accumulate more GAG than do cultures incubated in the same media enriched with 0.1 μM triiodothyronine (T3) (1981. Endocrinology. 108: 2397). The current study characterizes that enhanced accumulation. Confluent cultures were maintained in thyroid hormone-deficient media without or with added T3, labeled with [3H]acetate and analyzed for total [3H]GAG and [3H]hyaluronic acid content. Addition of T3 to thyroid hormone-depleted media consistently inhibited the incorporation of [3H]acetate into GAG by 28-60% in fibroblast cultures from four different normal human donors. Maximal inhibitory effect was observed within 3 d after hormone addition at concentrations > 1 nM. 73% of the maximal inhibitory effect was observed in the presence of physiologic concentrations of T3 (0.16 nM total T3 or 1.4 pM free T3). The following observations indicated that T3 inhibition of [3H]GAG accumulation is most likely due to a decrease in GAG synthesis rather than to changes in the acetate pool or GAG degradation: (a) Addition of 0, 100, 500, and 2,500 μM unlabeled acetate progressively decreased [3H]acetate incorporation into GAG, up to 80%, without altering the further inhibitory effect of T3 (35-40%); (b). A similar effect of T3 on GAG (32% inhibition) was observed using [3H]glucosamine as substrate; (c) T3 decreased hyaluronate synthetase activity by 32%; and (d) There was no effect of T3 on GAG degradation in a pulse-chase experiment. The effect of T3 on [3H]GAG accumulation appears to be quite specific, since the hormone had no effect on the incorporation of [3H]leucine into trichloroacetic acid-precipitable material. Thus, thyroid hormone inhibits GAG accumulation in a dose-, time-dependent, and reversible manner. This inhibition is apparently due to specific effects on the rate of macromolecular synthesis.