ID-RELATED GENES ENCODING HELIX-LOOP-HELIX PROTEINS ARE REQUIRED FOR G(1) PROGRESSION AND ARE REPRESSED IN SENESCENT HUMAN FIBROBLASTS

Three complete cDNA clones encoding Id-related helix-loop-helix (HLH) proteins lacking a basic region were isolated from a pcD2 cDNA express ion library prepared from TIG-3 human diploid fibroblasts (HDF). Of th ese cDNAs (Id-1H, Id-1H', and Id-2H), two (Id-IH and Id-1H') appeared to be derived by alternative RNA splicing. Id-1H and Id-2H seem to be human homologues of mouse Id-1 and Id-2, respectively, and have potent ial to encode 154 and 135 amino acid proteins. The Id-1H and Id-2H mRN As were barely detectable in quiescent early passage HDF; serum coordi nately induced both mRNAs, with two peaks of expression, in early and late in G1. Antisense oligomers complementary to Id-1H and Id-2H mRNA prevented early passage HDF from entering the S phase of the cell cycl e. The treatment of serum-stimulated early passage cells with the anti sense Id-1H oligomer completely abolished Id-1H. In senescent cells, s erum barely induced the Id-1H and Id-2H mRNAs, although the levels of c-myc expression induced were similar in early passage and senescent c ells. The expression levels of these Id genes vary among immortal huma n cell lines. Both genes were overexpressed in VA4 SV40-transformed lu ng fibroblasts and EJ-1 bladder carcinoma cells, while these genes wer e expressed at a very low level in SVts8 cells derived from SV40 tsA-t ransformed TIG-3 cells. SVts8 cells may acquire some function redundan t to Id proteins. HT1080 fibrosarcoma cells expressed the Id-1H gene b ut not the Id-2H gene, suggesting these Id genes may subserve redundan t functions.