Tissue-specific transcription initiation and effects of Growth Hormone (GH) deficiency on the regulation of mouse and rat GH-releasing hormone gene in hypothalamus and placenta

Hypothalamic GRH gene expression has been shown to be negatively regulated by GH in both rat and mouse. The recent reports of different 5′ untranslated sequences in mouse GRH cDNA from hypothalamus and placenta have raised the possibility of tissue-specific regulation of the GRH gene. To provide support for this possibility, we have studied rodent models with GH deficiency due to genetic defects in the pituitary. Complementary DNA probes for the hypothalamic and placental 5′ regions were used to determine the tissue specificity of each mRNA. Although the hypothalamic form of GRH mRNA was detected in placenta, it constituted less than 0.7% of total placental GRH mRNA. A placental 5′ probe (based on the previously reported sequence) hybridized only with a larger mRNA species and was not tissue specific, indicating that it was not related to GRH and was derived possibly from a cloning artifact. The correct 5′ sequence of mouse placental GRH cDNA was determined and shown to be distinct from both that previously reported and the hypothalamic sequence. Although the placental form of GRH mRNA was detected in hypothalamus using the polymerase chain reaction, its levels were undetectable by Northern blotting. The 5′ end of rat placental GRH cDNA was similarly sequenced and shown to exhibit no homology with the rat 5′ hypothalamic sequence, but a high degree of homology with the corresponding mouse placental sequence. In GH-deficient dwarf (dw/dw) rats, hypothalamic GRH mRNA levels were significantly increased above control levels in both females and males, and pregnancy did not alter the levels in either (dw) or control rats. However, GRH mRNA levels in placenta of (dw) and control animals were similar, indicating differential responses of GRH mRNA to GH deficiency in the two tissues. The differences in transcription initiation sites and in the regulation of expression in the two tissues suggest that the absence of effects of GH deficiency on placental GRH mRNA may be related to the use of different upstream sequences as regulatory regions in hypothalamus and placenta.