Expression of a human growth hormone (hGH) receptor isoform is predicted by tissue-specific alternative splicing of exon 3 of the hGH receptor gene transcript

Four members of the GH gene family, human (h) GH-V, human chorionic somatomammotropin-A (hCS-A), hCS-B, and hCS-L, are expressed in the human placenta. In attempting to define the role of these hormones in placental development, we have structurally characterized the human placental GH receptor (GHR) mRNA. Human GHR cDNAs were cloned from a human placental cDNA library. Clone pGHR-P1 encompasses the entire hGHR-coding region and is identical to the previously reported liver hGHR cDNA, except for a precise deletion of the sequences corresponding to exon 3. This mRNA with an exon 3 deletion is the sole form of the hGHR mRNAs in the placental villi. A reverse transcription/ polymerase chain reaction assay was used to further characterize GHR mRNA expression. Human GHR mRNA was detected in all placental tissues as well as in a wide spectrum of other tissues and cell lines. The distribution of the exon 3-retaining and exon 3-excluding isoforms of the hGHR mRNA shows distinct tissue specificity. Some tissues and cell lines contain only one of the two forms, and some contain a mixed population. Since exon 3 encodes a segment in the extracellular domain of the receptor, its alternative inclusion or exclusion may mediate critical alterations in hormone binding and physiological function.