Alternative transcription of the mouse Gh gene identifies an immune-associated transcript with species-specific...

Growth hormone (GH) in mice is primarily expressed in the anterior pituitary, although Gh expression has been reported in extrapituitary tissues, including immune organs. However, the structure of immune-associated Gh transcripts remains poorly characterized. To determine whether splenic Gh transcripts differ from pituitary Gh mRNA, 5'- and 3'-rapid amplification of cDNA ends (RACE) analyses were performed. While 3' RACE showed a shared polyadenylation site, 5' RACE identified a novel exon located approximately 2 kb upstream of the conventional exon 1, generating a transcript (spl-Gh mRNA) with a distinct first exon but shared downstream exons with pituitary Gh mRNA (pit-Gh mRNA). RT-PCR analysis revealed that spl-Gh mRNA is predominantly expressed in immune tissues such as spleen and bone marrow, and its distribution did not correlate with Pit-1 mRNA expression. Quantitative RT-PCR further demonstrated that spl-Gh mRNA was expressed at levels comparable to those of pit-Gh mRNA in the mouse spleen, indicating that spl-Gh is one of the major Gh transcript forms in this tissue. Sequence analysis indicated that spl-Gh mRNA is predicted to retain coding potential for a GH protein. Comparative genomic analyses further demonstrated that genomic features associated with the spl-Gh transcriptional unit are conserved only in a subset of closely related Mus species. In contrast, although a spl-Gh-related transcript was detected in rat spleen, no properly spliced mouse-like transcript was identified under the present experimental conditions. The detected transcript exhibited intron retention and an in-frame stop codon, suggesting that it is unlikely to produce a functional GH protein. These findings identify a distinct immune-associated Gh transcript generated through alternative transcription of the mouse Gh gene and suggest that immune-associated Gh transcriptional mechanisms have undergone species-specific divergence among rodents. Together, these findings reveal previously unrecognized complexity in Gh gene regulation and highlight species-specific differences in immune-associated Gh transcripts.