Functional and physical interactions between BRCA1 and p53 in transcriptional regulation of the IGF-IR gene.

The insulin-like growth factor-I receptor (IGF-IR) mediates the biological actions of the IGFs, and is critical for normal mammary gland development as well as for malignant transformation. Transcription of the IGF-IR gene is under inhibitory control by a number of transcription factors with tumor suppressor activity, including BRCA1 and p53. To assess the potential functional interactions between BRCA1 and p53 in transcriptional control of the IGF-IR gene, co-transfections were performed on MCF-7 breast cancer cells using an IGF-IR promoter luciferase reporter construct together with expression vectors encoding BRCA1 and wild-type and mutant p53. Similar experiments were performed in the colorectal cancer cell line HCT116 (+/+), which expresses a wild-type p53 gene, and its HCT116 (-/-) derivative, which lacks p53. BRCA1 was able to suppress IGF-IR promoter activity both in the absence and presence of p53. However, BRCA1 had no effect in mutant p53-expressing cells. Co-immunoprecipitation experiments showed that BRCA1 and p53 physically interact. In summary, our data suggest that the transcriptional activity of BRCA1 depends on the cellular status of p53. Inability of mutant tumor suppressors to repress IGF-IR gene expression may result in increased IGF-IR levels and IGF binding, leading to a reduction in apoptosis and enhanced survival capacity of malignant cells.

Regulation of the insulin-like growth factor-I receptor gene by oncogenes and antioncogenes: implications in human cancer.

The insulin-like growth factor-I receptor (IGF-I-R) has a central role in normal cellular proliferation as well as in transformation processes. Transcription of the IGF-I receptor gene is controlled by a number of tumor suppressors, including WT1, p53, and BRCA1. It has been demonstrated that, in their wild-type form, these transcription factors can suppress the activity of the IGF-I-R promoter, with ensuing reduction in the levels of cell-surface IGF binding. On the other hand, a number of oncogenes, including mutant p53 and c-myb, and the fusion protein EWS-WT1 significantly stimulate promoter activity. Interactions between stimulatory and inhibitory transcription factors may determine the level of expression of the IGF-I-R gene and, consequently, the proliferative status of the cell.

BRCA1 suppresses insulin-like growth factor-I receptor promoter activity: potential interaction between BRCA1 and Sp1.

The insulin-like growth factor I receptor (IGF-I-R) has an important role in breast cancer etiology. The receptor is overexpressed by most breast cancers, where it functions as a potent antiapoptotic agent. BRCA1 is a tumor suppressor gene that is mutated in a large fraction of familial breast and ovarian cancers. Cotransfection of Saos-2, MCF7, and CHO cells with IGF-I-R promoter constructs driving luciferase reporter genes, and with a BRCA1 expression vector, suppressed promoter activity in a dose-dependent manner. Functional interactions between BRCA1 and Sp1 in the regulation of the IGF-I-R gene were studied in Schneider cells, a Drosophila cell line which lacks endogenous Sp1. In these cells BRCA1 suppressed 45% of the Sp1-induced trans-activation of the IGF-I-R promoter. These results suggest that BRCA1 is capable of suppressing the IGF-I-R promoter in a number of cell lines, thus resulting in low levels of receptor mRNA and protein. Mutant versions of BRCA1 lacking trans-activational activity can potentially derepress the IGF-I-R promoter. Activation of the overexpressed receptor by locally produced or circulating IGFs may be a crucial step in breast and ovarian cancer progression.

Cleavage of the glycosylphosphatidylinositol anchor affects the reactivity of thy-1 with antibodies.

Thy-1 protein, a member of the Ig superfamily, is bound to the cell membrane by a glycosylphosphatidylinositol (GPI) anchor. We demonstrate that following anchor cleavage by phospholipase C, the reactivity of the solubilized Thy-1 with several mAbs is lost, and its reactivity with polyclonal anti-Thy-1 Abs is markedly decreased. Hence, solubilized Thy-1 cannot be detected by a range of mAbs. In contrast, enzymatic cleavage of biotinylated Thy-1 yields an intact solubilized protein that can be detected by streptavidin. These results exclude a possible proteolytic degradation of solubilized Thy-1 and suggest that the marked decrease in Thy-1 immunoreactivity following delipidation is due to conformational changes in the Thy-1 protein. We further demonstrate that addition of phospholipase C to preformed Ab-Ag complexes causes dissociation and removal of Thy-1 from the complex, indicating that delipidation of Thy-1 induces a conformational change in Thy-1 that is sufficient to dissociate bound Ab. The possibility should therefore be considered that the GPI anchor affects the conformation of a protein to which it is linked.