Physiological role and molecular mechanism of a novel growth hormone sGH system in the chicken

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The chicken growth hormone (GH) gene produces two distinct GH mRNAs by alternative usage of two transcription initiation sites : the pituitary GH (p-GH) mRNA encoding circulating GH, and the extra-pituitary GH mRNA encoding a GH cytosolic isoform (s-GH). The s-GH mRNA is transcribed from the third intron of the GH gene which consists of 5 exons and 4 introns (1). Here we demonstrate for the first time that the chicken GH gene is bi-directionally transcribed in a wide variety of tissues to produce GH sense (p-GH and/or s-GH) mRNAs and multiple alternatively spliced anti-sense transcripts. Seven GH anti-sense (GH-as) transcripts with distinct exon composition were isolated by RT-PCR with RNA from embryonic chicken eyes. The 5' and 3' RACE revealed that their transcription initiates in the 3' downstream from the poly (A) site of the GH sense mRNAs, and each transcript was polyadenylated by using either of three poly (A) signals located 5' upstream from the promoter for p-GH mRNA. Those GH-as transcripts were composed of at least 5 exons, most of which overlap exons of the GH sense mRNAs. Interestingly, no open reading frame expanding over plural exons was observed in all GH-as transcripts. The RT-PCR analysis revealed that the GH sense mRNAs and the GH-as transcripts are co-expressed widely in adults and 3-day-old chickens, and the relative abundance of GH sense mRNAs and GH-as transcripts varied with tissue types. Moreover, the B-cell cell line DT40 expressed both GH sense mRNAs and GH-as transcripts, indicating that the bi-directional transcription occurs in a single cell. Protein analyses are required ; however, these findings imply the possibility that p-GH and/or s-GH expression is regulated at post-transcriptional levels by interaction between GH sense and anti-sense transcripts in the chicken.