Discrepancy between in vitro and in vivo effect of Galphas gene mutation on the mRNA expression of TRH receptor

ABSTRACT

We investigated the effect of alpha-subunit of the stimulatory GTP-binding protein (Galphas) gene mutation on the expression of the thyrotropin-releasing hormone (TRH) receptor (TRH-R) gene in GH3 cells and in growth hormone (GH)-secreting adenomas of acromegalic patients. In the presence of cycloheximide, forskolin and isobutylmethylxanthine, cholera toxin, and GH-releasing hormone (GBRH) decreased rat TRH-R (rTRH-R) gene expression by about 39%, 43.7%, and 46.7%, respectively. Transient expression of a vector expressing mutant-type Galphas decreased the rTRH-R gene expression by about 50% at 24 h of transfection, whereas a wild-type Galphas expression vector did not. The transcript of human TRH-R (hTRH-R) gene was detected in 6 of 8 (75%) tumors. Three of them (50%) showed the paradoxical GH response to TRH and the other three patients did not show the response. The relative expression of hTRH-R mRNA in the tumors from patients with the paradoxical response of GH to TRH did not differ from that in the tumors from patients without the paradoxical response. Direct PCR sequencing of GALPHAs gene disclosed a mutant allele and a normal allele only at codon 201 in 4 of 8 tumors. The paradoxical response to TRH was observed in 2 of 4 patients without the mutation, and 2 of 4 patients with the mutation. The hTRH-R gene expression of pituitary adenomas did not differ between the tumors without the mutation and those with mutation. The present study suggests that the expression of TRH-R gene is not likely to be a main determinant for the paradoxical response of GH to TRH, and that Galphas mutation may suppress the gene expression of TRH-R in GH-secreting adenoma. However, a certain predisposing factor(s) may play an important role in determining the expression of TRH-R.