Effect of butyrate on thyroid hormone-mediated gene expression in rat pituitary tumour cells

These studies correlate the effects of (sodium) butyrate on intranuclear thyroid hormone receptor levels, with influences on both endogenous and transfected rat growth hormone (rGH) gene expression and regulation by l-triiodothyronine (T₃). In rat anterior pituitary tumour (GH3) cells, 5.0 mM butyrate elicits a biphasic reduction in the number of nuclear T₃ receptors. About 75% are depleted rapidly (t _{1 2} = 7 h), and the remaining receptors are depleted more slowly (t _{1 2} = 59 h). GH3 cells were treated with increasing concentrations of butyrate (0-5.0 mM), plus or minus 10 nM T₃ for 48 h. Total cytoplasmic RNA, cellular protein and medium were analysed for rGH levels with radiolabelled rGH cDNA or antibodies. A greater than 50-fold increase in rGH mRNA level was seen after T₃ treatment in the absence or presence of 0.1 mM butyrate. However, 1.0 and 5.0 mM butyrate decreased the stimulation of rGH mRNA levels by T₃ to 10- and less than 2-fold, respectively. Control mRNA levels were decreased slightly by increasing butyrate concentrations; rGH mRNA level was 2- to 3-fold higher in the absence of 5 mM butyrate. The pattern of butyrate/T₃ response displayed by both cellular and secreted rGH was similar to that seen with mRNA levels. Thus, the predominant effect of butyrate on T₃-mediated regulation of growth hormone gene expression is at the level of transcription or mRNA accumulation. A hybrid gene containing 5'-flanking DNA from the rGH gene fused to the bacterial gene coding for chloramphenicol acetyl transferase (CAT), was used to transfect rat pituitary tumour cells with or without butyrate and T₃ treatments. Conditions were obtained in which the addition of 0.5-5 mM butyrate could increase levels of CAT activity (3- to 30-fold). However, no T₃ response was observed in the presence of 5 mM butyrate. These data are consistent with the existence of at least two subclasses of thyroid hormone receptor, and that the more abundant butyrate-depletable subclass in largely responsible for mediating the effects of thyroid hormone on expression directed by the growth hormone gene promotor in pituitary tumour cells and after gene transfer.