Transcriptionally active haptoglobin-related (Hpr) gene in hepatoma G2 and leukemia molt-4 cells.

The aim of the present study was to answer the question: Is the haptoglobin-related (Hpr) gene expressed in tumor cells? Our strategy of cloning the cDNA was to screen a human hepatoma G2 cDNA expression library in lambda gt11 using three different probes complementary to the coding strands of regions of the Hpr gene that contain codon changes permitting a discrimination from haptoglobin gene Hp1F. Among 8 x 10(5) recombinant phages screened, 2 hybridized to all three probes under stringent conditions. A 1.5 kb cDNA designated ST-1 was subcloned and sequenced. Almost total identity was found with the Hpr predicted exons 2-5, although exon 1 was missing. The ST-1 partial cDNA clone was used as a probe to screen a human leukemia molt-4 cDNA expression library in lambda gt11. Among 10(6) recombinant phages screened, 1 hybridized under stringent conditions. A 1.5 kb cDNA designated ST-2 was subcloned and sequenced. ST-1 and ST-2 cDNA were identical except for an insert of A at position 500 of ST-1 cDNA. Two different nucleotide changes were observed in the ST-1 and ST-2 sequences as compared with the expected Hpr cDNA sequence. An alternative processing of Hpr pre-mRNA was found in both cDNA clones that included 126 bp of the 3'-region of intron 1. This intronic sequence is thereby retained in the mature mRNA. cDNA analysis revealed an in-frame ATG in intron 1. Transcription/translation assay was used to demonstrate that the Hpr message could be translated from the internal methionine codon. We have thus shown for the first time that the Hpr gene is expressed in the human hepatoma G2 and leukemia molt-4 cell lines.