Inhibition of Id proteins by a peptide aptamer induces cell-cycle arrest and apoptosis in ovarian cancer cells.

BACKGROUND: Inhibitors of DNA-binding proteins (Id1-4), lacking the basic DNA-binding domain, function as dominant inhibitors of cell-cycle regulators. Overexpression of Id proteins promotes cancer cell proliferation and resistance against apoptosis. Level of Id protein expression, especially of Id1, correlates with poor differentiation, enhanced malignant potential and more aggressive clinical behaviour of ovarian tumours. Although overexpression of Ids has been found and shown to correlate with poor clinical outcome, their inhibition at protein level has never been studied. METHODS: A peptide aptamer, Id1/3-PA7, targeting Id1 and Id3, was isolated from a randomised combinatorial expression library using yeast and mammalian two-hybrid systems. Id1/3-PA7 was fused, expressed and purified with a cell-penetrating protein transduction domain. RESULTS: Intracellular-delivered Id1/3-PA7 colocalised to Id1 and Id3. It induced cell-cycle arrest and apoptosis in ovarian cancer cells ES-2 and PA-1. It activated the E-box promoter and increased the expression level of cyclin-dependent kinase inhibitor (CDKN2A) in a dose-dependent manner that is paralleled by the cleavage of poly-ADP ribose polymerase. These effects were counteracted by ectopically overexpressed Id1 and Id3. CONCLUSION: Id1/3-PA7 could represent an exogenous anti-tumour agent that can significantly trigger cell-cycle arrest and apoptosis in ovarian cancer.

Interference of the dominant negative helix-loop-helix protein ID1 with the proteasomal subunit S5A causes centrosomal abnormalities.

The inhibitor of DNA-binding (ID) proteins are dominant-negative inhibitors of basic helix-loop-helix transcription factors that have multiple functions during development and cellular differentiation. High-level expression of some ID family members has been observed in human malignancies, and in some cases was correlated with poor clinical prognosis. Ectopic ID1 expression extends the life span of primary human epithelial cells, inhibits cellular differentiation and induces centrosome duplication errors, thus suggesting that ID1 may have oncogenic activities. ID1 can bind to the proteasomal subunit S5A/Rpn10, but the biological consequences of the interaction have not been studied in detail. Here, we show that ID1's ability to induce supernumerary centrosomes correlates with S5A binding. Similar to ID1, a fraction of the S5A protein localizes to centrosomal structures. Furthermore, partial depletion of S5A by RNA interference causes accumulation of cells with supernumerary centrosomes. These results are consistent with the model that ID1 dysregulates centrosome homeostasis at least in part by interfering with S5A activities at the centrosome.