Hormonal Stimulation of Eucaryotic Cell ADP-ribosylation: EFFECT OF THYROTROPIN ON THYROID CELLS

The effect of thyrotropin (TSH) on the ADP-ribosylation of endogenous thyroid cell acceptor proteins was examined. Cells were “permeabilized” at 4°C in hypotonic medium and then exposed to [³²P]- or [³H-adenine]NAD⁺. The net incorporation of labeled ADP-ribose was measured by trichloroacetic acid precipitation. TSH (100 mU/ml) enhanced ADP-ribosylation with a maximum effect after 30-60 min in the majority of experiments. TSH stimulation was observed even when the incubation contained 1,000-fold more exogenous NAD⁺ than the amount of NAD⁺ contributed by the permeabilized cells, indicating an effect on enzymatic activity rather than an alteration in NAD⁺ pool size or specific activity. No incorporation of radioactivity from labeled NAD⁺ was observed in cells not rendered permeable to NAD⁺ by hypotonic shock. TSH did not increase the rate of disappearance of trichloroacetic-precipitable radioactivity and did not contain intrinsic NAD⁺ glycohydrolase activity. Alkali and snake venom phosphodiesterase, but not ribonuclease or deoxyribonuclease digestion of trichloroacetic acid precipitable thyroid cell radioactivity, revealed primarily 5′-AMP, consistent with an effect of TSH on mono-ADP ribosylation. Nicotinamide and thymidine (50 mM) inhibited both basal and TSH-stimulated ADP-ribosylation of thyroid cell protein. Dibutyryl cyclic (c)AMP (0.1 mM) inhibited endogenous ADP-ribosylation by ∼35% but had no effect at lower concentrations. 0.5 mM isobutylmethylxanthine inhibited this reaction by ∼60%.

We suggest that TSH enhances thyroid cell ADP-ribosylation by a mechanism independent of cAMP as a second messenger, and that ADP-ribosylation plays a role in the expression of TSH.

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