Molecular cloning of complementary DNA for a novel human hepatoma-derived growth factor. Its homology with high mobility group-1 protein

A novel hepatoma-derived growth factor was purified from the conditioned medium of human hepatoma-derived cell line, HuH-7, by the assay of [3H]thymidine incorporation into Swiss 3T3 cells. Molecular cloning of a complementary DNA from the cDNA library of HuH-7 cells was done on the basis of the N-terminal amino acid sequence. This protein was acid- and heat-labile heparin-binding protein and inactivated by reducing condition. The cDNA is 2.4 kilobase pairs long, and the deduced amino acid sequence from cDNA contained 240 amino acids without a signal peptide-like N-terminal hydrophobic sequence. Heparin column-eluted fraction of the conditioned medium of Cos-7 cells transfected by the cDNA stimulated the DNA synthesis. Northern blot analysis revealed its ubiquitous expression in several tumor-derived cell lines, as well as in normal tissues. The primary sequence shares homology with the high mobility group (HMG)-1 protein (23.4% amino acid identity and 35.6% similarity). However, its hydrophobic profile was distinct from that of HMG-1 except for the C-terminal acidic region, and it contained no apparent HMG box motif. Immunofluorescence study showed that it is localized in the cytoplasma of hepatoma cells. These findings suggest that this factor is a novel heparin-binding protein, with mitogenic activity for fibroblasts.