ATM-mediated phosphorylation of the chromatin remodeling enzyme BRG1 modulates DNA double-strand break repair

ATP-dependent chromatin remodeling complexes such as SWI/SNF (SWItch/Sucrose NonFermentable) have been implicated in DNA double-strand break (DSB) repair and damage responses. However, the regulatory mechanisms that control the function of chromatin remodelers in DNA damage response are largely unknown. Here, we show that ataxia telangiectasia mutated (ATM) mediates the phosphorylation of BRG1, the catalytic ATPase of the SWI/SNF complex that contributes to DSB repair by binding γ-H2AX-containing nucleosomes via interaction with acetylated histone H3 and stimulating γ-H2AX formation, at Ser-721 in response to DNA damage. ATM-mediated phosphorylation of BRG1 occurs rapidly and transiently after DNA damage. Phosphorylated BRG1 binds γ-H2AX-containing nucleosomes to form the repair foci. The Ser-721 phosphorylation of BRG1 is critical for binding γ-H2AX-containing nucleosomes and stimulating γ-H2AX formation and DSB repair. BRG1 binds to acetylated H3 peptides much better after phosphorylation at Ser-721 by DNA damage. However, the phosphorylation of Ser-721 does not significantly affect the ATPase and transcriptional activities of BRG1. These results, establishing BRG1 as a novel and functional ATM substrate, suggest that the ATM-mediated phosphorylation of BRG1 facilitates DSB repair by stimulating the association of this remodeler with γ-H2AX nucleosomes via enhancing the affinity to acetylated H3. Our work also suggests that the mechanism of BRG1 stimulation of DNA repair is independent of the remodeler's enzymatic or transcriptional activities.

Phase II study of a polyethoxylated castor oil–free, polymeric micelle formulation of paclitaxel for patients with advanced urothelial cancer previously treated with gemcitabine and platinum

272

Background: Paclitaxel-loaded polymeric micelle (Genexol-PM; G-PM) is a novel polyethoxylated castor oil (Cremophor EL [CrEL])-free formulation of paclitaxel. This multicenter phase II study was designed to evaluate the efficacy and safety of the G-PM monotherapy for the treatment of gemcitabine- and cisplatin-refractory advanced urothelial cancer.

Methods: Patients with advanced urothelial cancer received G-PM 240 mg/m2 intravenously over 3 hours every 3 weeks without premedication as salvage chemotherapy after failure to gemcitabine and cisplatin combination chemotherapy. Intrapatient dose escalation of G-PM to 300 mg/m2 was carried out from the second cycle if the prespecified toxicities were not observed after the first cycle.

Results: Thirty-seven patients were enrolled in this study. Platinum-free interval was less than 6 months in 72% of patients and 64% of patients were categorized into intermediate or poor prognostic group. Overall response rate was 19% including one complete response. When response occurs it is long lasting with a median response duration of 7.8 months (range 1.5∼19.3+). The median progression-free survival was 2.7 months (95% CI, 09-4.6) with a median overall survival of 5.7 months (95% CI, 3.1-8.3). The most common major grade 3-4 non-hematologic toxicity was peripheral neuropathy (sensory type 5.6% and motor type 8.3%) followed by infection (5.6%). Grade 3 or worse hematologic toxicities were observed in only one patient.

Conclusions: G-PM showed modest activity with favorable toxicity profiles when used as salvage therapy after gemcitabine-cisplatin failure in advanced urothelial cancer.

No significant financial relationships to disclose.

Source proximity and residential outdoor concentrations of PM(2.5), OC, EC, and PAHs

We examined the effect of proximity to specific mobile, area, and point sources on the residential outdoor concentrations of fine particulate matter PM (PM(2.5)) and several of its particle components. Integrated (48-h) PM(2.5) samples were collected outside non-smoking residences in Elizabeth, NJ, between summer 1999 and spring 2001. Samples were analyzed for PM(2.5) mass, organic and elemental carbon (OC and EC, respectively), trace elements, particle-phase polycyclic aromatic hydrocarbons (p-PAHs), and other important particle species. Information about the proximity of the study homes to potential mobile and area sources of OC, EC, p-PAHs, sulfur (S), and selenium (Se) (including urban interstate highways, local roadways, the Newark International Airport, the Elizabeth seaport, and a nearby refinery in Linden, NJ) were retrieved from a database that included detailed emissions, meteorological, and geographical data for the study area. The dependence of residential outdoor concentrations on source proximity and on various meteorological parameters was then examined for each species by multiple linear regression analysis. As expected, the predicted ambient air concentrations of all particle species (except S, Se) decreased with increasing distance from the sources. Although the enhancement in PM(2.5) and OC levels outside the study homes closest to primary PM sources was modest (e.g., 1.6 and 2.5 times the background levels 37 m from interstate highways), the elevation of EC and p-PAH concentrations was substantial outside the closest study homes (i.e., about 20 times for p-PAHs 37 m from interstate highways and about 14 times for EC 192 m from the refinery in Linden, NJ). The predicted EC concentrations 192 and 500 m from the oil refinery were 22.8 and 3.0 microgC/m(3), compared with an urban background of 1 microgC/m(3). Thus, emissions from this source might dramatically affect EC exposure for residents living in its close proximity.