Dip1 inhibits growth and gene transcription in MCF-7 breast cancer cells

In previous studies we identified a novel gene Dipl, also designated CCNDBP1, which encodes a 42kDa helix-loop-helix (HLH) nuclear protein. Although this protein was originally identified by its ability to bind to cyclin D1 its precise biochemical functions are not known. In the present study we carried out mechanistic studies on Dip1 focusing on the human breast cancer cell line MCF-7. We found that overexpression of Dip1 in MCF-7 cells inhibited colony formation and cell proliferation. Reporter assays in MCF-7 cells indicated that Dip1 strongly inhibited the transcriptional activities of the cyclin D1, c-fos, NF-kappaB, SRE and p21cP1 promoters. Furthermore studies with truncated and mutant forms of the cyclin D1 promoter suggest that Dip1 does not act on specific transcriptional elements. Assays with mutant and truncated forms of Dip1 indicated that both the LXXLL motif and the HLH domain play important, but not exclusive, roles in these inhibitory effects. Dip1 co-immunoprecipitated with the histone deacetylase (HDAC) proteins HDAC1 and HDAC3. Nevertheless Dip1 markedly inhibited the stimulation of cyclin D1 promoter activity obtained with trichostatin A [1], an inhibitor of HDAC. Taken together these findings suggest that Dip1 functions as a general repressor of transcription. Although the precise mechanism by which Dip1 inhibits gene transcription and the growth of MCF-7 cells remain to be determined, the present results suggest that Dip1 is a candidate tumor suppressor gene.