Insulin and thyroid hormones stimulate matrix metabolism in primary cultures of articular chondrocytes from Young rabbits independently and in combination

These studies examined the effects of heat-inactivated horse serum, insulin, triiodothyronine (T₃), and thyroxine (T₄), individually and in combination, on collagen and proteoglycan synthesis by primary cell cultures of articular chondrocytes from immature male rabbits. Insulin concentrations of 25 to 100 ng/ml (4.4 to 17.4 × 10^-9M) increasingly stimulated collagen and proteoglycan synthesis in the absence of serum. The effects of 25 ng/ml (4.4 × 10^-9M) insulin or 15% heat-inactivated horse serum on collagen synthesis were similar. Triiodothyronine (10^-10 to 10^-6M) and T₄ (10^-8 to 10^-4M) also stimulated collagen synthesis in the absence of serum, with peak effects at 10^-8 and 10^-6M respectively. Biphasic stimulation of proteoglycan synthesis was obtained with 10^-11 to 10^-7M T₃ (maximum at 10^-8M) and 10^-8 to 10^-5M T₄ (maximum at 10^-7M). In these experiments, triiodothyronine was 10 to 100 times more potent than T₄ in stimulating cartilage matrix production. The cells retained their chondrocytic phenotype under hormonal stimulation, secreting almost exclusively Type II collagen and large, chondroitin sulfate-rich proteoglycans. The addition of insulin to maximally-stimulating concentrations of either T₃ or T₄ in serum-free medium further stimulated matrix synthesis, suggesting that these hormones modulate chondrocyte metabolism via multiple biosynthetic/receptor pathways.