Rat parathyroid hormone-like peptide: comparison with the human homologue and expression in malignant and normal tissue.

A rat Leydig cell tumor cDNA library was screened with a 32P-labeled genomic restriction fragment encoding human PTH-like peptide (hPLP), and three cDNA clones were isolated. The largest cDNA insert contained 1146 nucleotides. The cloned cDNA encodes a 177-amino acid protein consisting of a predicted 36-amino acid leader sequence and a 141-amino acid mature peptide in which 9 of 13 amino-terminal residues are identical to rat PTH (rPTH). Comparison of rPLP with hPLP reveals marked conservation of both the nucleotide and amino acid sequences through the prepro, amino-terminal, and midregion portions of the molecules. There is also striking conservation of the 3' noncoding regions of rPLP and hPLP mRNAs, both of which contain AU-rich repeated sequences that may affect mRNA stability. A single species of mRNA of approximately 1.4-kilobases was identified in the rat Leydig cell tumor and in normal rat stomach. Southern blot analysis is consistent with the presence of a single copy of the rPLP gene per haploid genome, and there is no major rearrangement or amplification of the rPLP gene in DNA isolated from the tumor per se. The results demonstrate the presence of a single gene transcript in a rat model of malignancy associated with hypercalcemia which encodes a peptide homologous to hPLP, document the marked interspecies sequence conservation that exists in major functional domains of the mRNAs and peptides, and show the expression of mRNA encoding rPLP in normal stomach as well as in neoplastic rat tissue.