Hormonal control of protein synthesis in chick chondrocytes: a comparison of effects of insulin, somatomedin C and triiodothyronine.

Somatomedin C (SM-C), insulin, and triiodothyronine (T3) each result in a 2-fold stimulation of incorporation of [125]leucine into protein by cultured chick sternal chondrocytes. Maximal stimulation occurred at concentrations of 12.5 X 10(-9) M SM-C, 11 X 10(-9) M insulin, and 1.5 X 10(-9) M T3. Submaximal concentrations of SM-C and T3 were additive in their effect, and together stimulated [125]leucine incorporated to a level greater than that achieved by either alone. Submaximal concentrations of SM-C and insulin were also additive in their stimulatory effect, but only to the level achieved by either alone. It was possible to demonstrate specific binding of [14C]SM-C to chondrocytes, and bound SM-C could be displaced by either unlabelled SM-C or insulin at concentrations similar to concentrations that stimulated protein synthesis. Actinomycin D abolished stimulation by T3, but not by insulin or SM-C. Thus, it appears that SM-C and insulin increase protein synthesis by stimulating the translational process after binding to the same receptor. T3 appears to act through a different mechanism, which requires stimulation of transcription.