We have previously reported that 5, 5’-diphenylhydantoin (DPH) inhibits total cellular and specific nuclear T3 binding by cultured GC cells, a rat pituitary tumor cell line that produces GH. DPH decreased competitively the rate of T3 accumulation by GC cells and noncompetitively inhibited specific nuclear T3 binding as well. To determine the biological consequences of these DPH effects on cellular and nuclear T3 binding, we studied the effect of DPH on the growth rate and GH production of GC cells cultured in Dulbecco’s Modified Eagle–s Medium with 10% serum. Incubation with T3 stimulated the GC cell growth rate in a dose-dependent manner. The halfmaximal growth rate was achieved at a T3 concentration of 0.18 nM, and the maximal effect was observed at 0.4 nM T3. Addition of DPH to GC cells cultured with 0.15 nM T3 resulted in a dosedependent decrease in the GC cell growth rate. The half-maximal depression of the rate of GC cell growth occurred at 185 βM DPH, a concentration that results in an approximately 50% decrease in cellular and nuclear T3. The DPH-induced decrease in GC cell growth was abolished by the addition of increasing concentrations of T3 (maximal concentration, 1.0 nM). Similarly, DPH effected a dose-dependent decrease in GH production in cells cultured with physiological concentrations of T3 (0.15 nM). The decrease in GH production of cells incubated with 200 nM DPH was associated with a decrease of similar magnitude in GH mRNA. These findings suggested that the DPH effect on GH production was mediated at a pretranslational level. Addition of increasing concentrations of T3 up to 5 nM completely abolished the DPH-associated decrease in GH production. Finally, studies of the effects of DPH on cell growth and GH production in cultures maintained with T3-depleted conditions were carried out to detect putative agonist activity of DPH. In the present investigation, we were unable to detect agonist activity of DPH that was more than 10–15% of the effect of maximal doses of T3. The data suggest that DPH attenuates the action of T3 in GC cells, probably because of a decrease in the steady state concentrations of cellular and nuclear T3. These attributes of DPH suggest that the drug or related analogs may serve as prototypes of agents that may decrease thyroid hormone activity at target tissues.
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