BRCA1-induced large-scale chromatin unfolding and allele-specific effects of cancer-predisposing mutations

The breast cancer susceptibility gene BRCA1 encodes a protein that has been implicated in multiple nuclear functions, including transcription and DNA repair. The multifunctional nature of BRCA1 has raised the possibility that the polypeptide may regulate various nuclear processes via a common underlying mechanism such as chromatin remodeling. However, to date, no direct evidence exists in mammalian cells for BRCA1-mediated changes in either local or large-scale chromatin structure. Here we show that targeting BRCA1 to an amplified, lac operator–containing chromosome region in the mammalian genome results in large-scale chromatin decondensation. This unfolding activity is independently conferred by three subdomains within the transactivation domain of BRCA1, namely activation domain 1, and the two BRCA1 COOH terminus (BRCT) repeats. In addition, we demonstrate a similar chromatin unfolding activity associated with the transactivation domains of E2F1 and tumor suppressor p53. However, unlike E2F1 and p53, BRCT-mediated chromatin unfolding is not accompanied by histone hyperacetylation. Cancer-predisposing mutations of BRCA1 display an allele-specific effect on chromatin unfolding: 5′ mutations that result in gross truncation of the protein abolish the chromatin unfolding activity, whereas those in the 3′ region of the gene markedly enhance this activity. A novel cofactor of BRCA1 (COBRA1) is recruited to the chromosome site by the first BRCT repeat of BRCA1, and is itself sufficient to induce chromatin unfolding. BRCA1 mutations that enhance chromatin unfolding also increase its affinity for, and recruitment of, COBRA1. These results indicate that reorganization of higher levels of chromatin structure is an important regulated step in BRCA1-mediated nuclear functions

Hepatitis B virus X protein and the estrogen receptor variant lacking exon 5 inhibit estrogen receptor signaling in hepatoma cells

Hepatitis B virus (HBV) X protein (HBx) is considered to play a role in the development of hepatocellular carcinoma (HCC) during HBV infection. HCC was shown to be more prevalent in men than in women. Estrogen, which exerts its biological function through estrogen receptor (ER), can inhibit HBV replication. ERΔ5, an ERα variant lacking exon 5, was found to be preferentially expressed in patients with HCC compared with patients with normal livers. Here, we report the biological role of ERΔ5 and a novel link between HBx and ERα signaling in hepatoma cells. ERΔ5 interacts with ERα in vitro and in vivo and functions as a dominant negative receptor. Both ERα and ERΔ5 associate with HBx. HBx decreases ERα-dependent transcriptional activity, and HBx and ERΔ5 have additive effect on suppression of ERα transactivation. The HBx deletion mutant that lacks the ERα-binding site abolishes the HBx repression of ERα. HBx, ERα and histone deacetylase 1 (HDAC1) form a ternary complex. Trichostatin A, a specific inhibitor of HDAC enzyme, can restore the transcriptional activity of ERα inhibited by HBx. Our data suggest that HBx and ERΔ5 may play a negative role in ERα signaling and that ERα agonists may be developed for HCC therapy

Suppression of Estrogen Receptor Transcriptional Activity by Connective Tissue Growth Factor

Secreted growth factors have been shown to stimulate the transcriptional activity of estrogen receptors (ER) that are responsible for many biological processes. However, whether these growth factors physically interact with ER remains unclear. Here, we show for the first time that connective tissue growth factor (CTGF) physically and functionally associates with ER. CTGF interacted with ER both in vitro and in vivo. CTGF interacted with ER DNA-binding domain. ER interaction region in CTGF was mapped to the thrombospondin type I repeat, a cell attachment motif. Overexpression of CTGF inhibited ER transcriptional activity as well as the expression of estrogen-responsive genes, including pS2 and cathepsin D. Reduction of endogenous CTGF with CTGF small interfering RNA enhanced ER transcriptional activity. The interaction between CTGF and ER is required for the repression of estrogen-responsive transcription by CTGF. Moreover, CTGF reduced ER protein expression, whereas the CTGF mutant that did not repress ER transcriptional activity also did not alter ER protein levels. The results suggested the transcriptional regulation of estrogen signaling through interaction between CTGF and ER, and thus may provide a novel mechanism by which cross-talk between secreted growth factor and ER signaling pathways occurs

miR-1224-3p Promotes Breast Cancer Cell Proliferation and Migration through PGM5-Mediated Aerobic Glycolysis

Metabolic reprogramming of aerobic glycolysis is a hallmark of cancer cells. Regulators of aerobic glycolysis have become targets for cancer diagnosis and therapy. However, the regulators of aerobic glycolysis in breast cancer development have not been well elucidated. Here, we show that the phosphoglucomutase (PGM) family member PGM5 promotes conversion of glucose-1-phosphate (G1P) into glucose-6-phosphate (G6P) and inhibits breast cancer cell proliferation and migration through regulating aerobic glycolysis. In breast cancer patients, PGM5 is significantly downregulated, and its low expression is a predictor of poor prognosis. MicroRNA-1224-3p (miR-1224-3p) inhibits the PGM5 level through directly targeting its 3’-untranslated region and suppresses PGM5-mediated breast cancer cell proliferation, migration, and glycolytic function. Moreover, the miR-1224-3p/PGM5 axis regulates the expression of cell cycle- and apoptosis-related genes and the markers of epithelial-mesenchymal transition (EMT), a process involved in migration and metastasis of cancer cells. Taken together, our results indicate that miR-1224-3p/PGM5 axis plays important roles in breast cancer cell proliferation, migration, and aerobic glycolysis and may be a potential target for breast cancer therapy

Ligand-independent activation of estrogen receptor α by XBP-1

The estrogen receptor (ER) is a member of a large superfamily of nuclear receptors that regulates the transcription of estrogen-responsive genes. Several recent studies have demonstrated that XBP-1 mRNA expression is associated with ERα status in breast tumors. However, the role of XBP-1 in ERα signaling remains to be elucidated. More recently, two forms of XBP-1 were identified due to its unconventional splicing. We refer to the spliced and unspliced forms of XBP-1 as XBP-1S and XBP-1U, respectively. Here, we report that XBP-1S and XBP-1U enhanced ERα-dependent transcriptional activity in a ligand-independent manner. XBP-1S had stronger activity than XBP-1U. The maximal effects of XBP-1S and XBP-1U on ERα transactivation were observed when they were co-expressed with full-length ERα. SRC-1, the p160 steroid receptor coactivator family member, synergized with XBP-1S or XBP-1U to potentiate ERα activity. XBP-1S and XBP-1U bound to the ERα both in vitro and in vivo in a ligand-independent fashion. XBP-1S and XBP-1U interacted with the ERα region containing the DNA-binding domain. The ERα-interacting regions on XBP-1S and XBP-1U have been mapped to two regions, including the N-terminal basic region leucine zipper domain (bZIP) and the C-terminal activation domain. The bZIP-deleted mutants of XBP-1S and XBP-1U completely abolished ERα transactivation by XBP-1S and XBP-1U. These findings suggest that XBP-1S and XBP-1U may directly modulate ERα signaling in both the absence and presence of estrogen and, therefore, may play important roles in the proliferation of normal and malignant estrogen-regulated tissues

GATA1 Promotes Gemcitabine Resistance in Pancreatic Cancer through Antiapoptotic Pathway

Gemcitabine-based chemotherapy is the first-line treatment for pancreatic cancer. However, chemoresistance is a major obstacle to drug efficacy, leading to poor prognosis. Little progress has been achieved although multiple mechanisms are investigated. Therefore, effective strategies are urgently needed to overcome drug resistance. Here, we demonstrate that the transcription factor GATA binding protein 1 (GATA1) promotes gemcitabine resistance in pancreatic cancer through antiapoptotic pathway. GATA1 is highly expressed in pancreatic ductal adenocarcinoma (PDAC) tissues, and GATA1 status is an independent predictor of prognosis and response to gemcitabine therapy. Further investigation demonstrates GATA1 is involved in both intrinsic and acquired gemcitabine resistance in PDAC cells. Mechanistically, we find that GATA1 upregulates Bcl-XL expression by binding to its promoter and thus induces gemcitabine resistance through enhancing Bcl-XL mediated antiapoptosis in vitro and in vivo. Moreover, in PDAC patients, Bcl-XL expression is positively correlated with GATA1 level and predicts clinical outcomes and gemcitabine response. Taken together, our results indicate that GATA1 is a novel marker and potential target for pancreatic cancer. Targeting GATA1 combined with Bcl-XL may be a promising strategy to enhance gemcitabine response