Germline mutations of the BRCA1 gene confer an increased risk for breast cancer and ovarian cancer. To study the contribution of BRCA1 to sporadic cancers, which often exhibit reduced BRCA1 expression, we tested the effect of knocking down BRCA1 on gene expression in human prostate (DU-145) and breast (MCF-7) cancer cells. DNA microarray and confirmatory RNA analyses revealed that BRCA1 small interfering (si) RNA caused down-regulation of multiple genes implicated in the mitotic spindle checkpoint (eg., BUB1B, HEC, and STK6), chromosome segregation (eg., ESPL1, NEK2, and PTTG1), centrosome function (eg., ASPM), cytokinesis (eg., PRC1, PLK, and KNSL2), and the progression into and through mitosis (eg., CDC2, and CDC20). Cells treated with BRCA1-siRNA showed attenuation of the mitotic spindle checkpoint; but not several G2 checkpoints. Finally, BRCA1 knockdown caused the accumulation of multinucleated cells, suggesting a defect in cytokinesis. We conclude that BRCA1 regulates gene expression for orderly mitotic progression.

LINE-1 (L1) is a highly successful autonomous non-LTR retrotransposon and a major force shaping mammalian genomes. Although there are about 600 000 L1 copies covering 23% of the rat genome, full-length rat L1s (L1Rn) with intact open reading frames (ORFs) representing functional master copies for retrotransposition have not been identified yet. In conjunction with studies to elucidate the role of L1 retrotransposons in tumorigenesis, we isolated and characterized 10 different cDNAs from transcribed full-length L1Rn elements in rat chloroleukemia (RCL) cells, each encoding intact ORF1 proteins (ORF1p). We identified the first functional L1Rn retrotransposon from this pool of cDNAs, determined its activity in HeLa cells and in the RCL cell line the cDNAs originated from and demonstrate that it is mobilized in the tumor cell line in which it is expressed. Furthermore, we generated monoclonal antibodies directed against L1Rn ORF1 and ORF2-encoded recombinant proteins, analyzed the expression of L1-encoded proteins and found ORF1p predominantly in the nucleus. Our results support the hypothesis that the reported explosive amplification of genomic L1Rn sequences after their transcriptional activation in RCL cells is based on L1 retrotransposition. Therefore, L1 activity might be one cause for genomic instability observed during the progression of leukemia.

Several human diseases including neurodegenerative disorders and cancer are associated with abnormal accumulation and aggregation of misfolded proteins. Proteins with high tendency to aggregate include the p53 gene product, TAU and alpha synuclein. The potential toxicity of aberrantly folded proteins is limited via their transport into intracellular sub-compartments, the aggresomes, where misfolded proteins are stored or cleared via autophagy. We have identified a region of the acetyltransferase p300 that is highly disordered and displays similarities with prion-like domains. We show that this region is encoded as an alternative spliced variant independently of the acetyltransferase domain, and provides an interaction interface for various misfolded proteins, promoting their aggregation. p300 enhances aggregation of TAU and of p53 and is a component of cellular aggregates in both tissue culture cells and in alpha-synuclein positive Lewy bodies of patients affected by Parkinson disease. Down-regulation of p300 impairs aggresome formation and enhances cytotoxicity induced by misfolded protein stress. These data unravel a novel activity of p300, offer new insights into the function of disordered domains and implicate p300 in pathological aggregation that occurs in neurodegeneration and cancer.

The oncogene amplified in breast cancer 1 (AIB1) is a nuclear receptor coactivator that plays a major role in the progression of various cancers. We previously identified a splice variant of AIB1 called AIB1-⌬4 that is overexpressed in breast cancer. Using mass spectrometry, we define the translation initiation of AIB1-⌬4 at Met 224 of the full-length AIB1 sequence and have raised an antibody to a peptide representing the acetylated N terminus. We show that AIB1-⌬4 is predominantly localized in the cytoplasm, although leptomycin B nuclear export inhibition demonstrates that AIB1-⌬4 can enter and traffic through the nucleus. Our data indicate an import mechanism enhanced by other coactivators such as p300/CBP. We report that the endogenously and exogenously expressed AIB1-⌬4 is recruited as efficiently as full-length AIB1 to estrogen-response elements of genes, and it enhances estrogen-dependent transcription more effectively than AIB1. Expression of an N-terminal AIB1 protein fragment, which is lost in the AIB1-⌬4 isoform, potentiates AIB1 as a coactivator. This suggests a model whereby the transcriptional activity of AIB1 is squelched by a repressive mechanism utilizing the N-terminal domain and that the increased coactivator function of AIB1-⌬4 is due to the loss of this inhibitory domain. Finally, we show, using Scorpion primer technology, that AIB1-⌬4 expression is correlated with metastatic capability of human cancer cell lines.Gene transcription in eukaryotes is a complex and highly regulated process. One of the major controls of gene transcription is exerted by the coregulator family of proteins. These include both corepressors, which dampen transcription, and coactivators, which potentiate transcription. A subgroup of coactivators has been shown to be critical for the malignant progression of cancer and is known as the p160 steroid receptor coactivators (1). One member in particular was identified to be amplified in breast cancer. Amplified in breast cancer 1 (AIB1, SRC-3, NCOA3, ACTR, TRAM-1, pCIP, and RAC3) has been shown to be a gene amplified in breast cancer (2) and is also overexpressed at the mRNA and protein level in various cancers (1, 3, 4). Its role in tumorigenesis is attributed to its ability to coactivate both steroid hormone-and growth factor-dependent transcription (3, 5-7). In several oncogene-driven mouse models (8 -11), reduction of AIB1 levels leads to a decrease in tumorigenesis, and overexpression of AIB1 leads to the formation of various tumors (12). Clinically, AIB1 expression in breast cancer cases is correlated with high HER2 levels, larger tumor size, higher tumor grade, and shorter disease-free survival (13-15). Also, high levels of AIB1 in conjunction with high HER2 levels coincide with reduced disease-free survival in patients treated with tamoxifen, suggesting a role for AIB1 in tamoxifen resistance (16).We had previously identified a splice variant of AIB1, where exon 3 was spliced from the mature mRNA and the resulting protein named AIB1-⌬3 (17). More recently, an additio...