To gain insight into melanoma pathogenesis, we characterized an insertional mouse mutant, TG3, that is predisposed to develop multiple melanomas. Physical mapping identified multiple tandem insertions of the transgene into intron 3 of Grm1 (encoding metabotropic glutamate receptor 1) with concomitant deletion of 70 kb of intronic sequence. To assess whether this insertional mutagenesis event results in alteration of transcriptional regulation, we analyzed Grm1 and two flanking genes for aberrant expression in melanomas from TG3 mice. We observed aberrant expression of only Grm1. Although we did not detect its expression in normal mouse melanocytes, Grm1 was ectopically expressed in the melanomas from TG3 mice. To confirm the involvement of Grm1 in melanocytic neoplasia, we created an additional transgenic line with Grm1 expression driven by the dopachrome tautomerase promoter. Similar to the original TG3, the Tg(Grm1)EPv line was susceptible to melanoma. In contrast to human melanoma, these transgenic mice had a generalized hyperproliferation of melanocytes with limited transformation to fully malignant metastasis. We detected expression of GRM1 in a number of human melanoma biopsies and cell lines but not in benign nevi and melanocytes. This study provides compelling evidence for the importance of metabotropic glutamate signaling in melanocytic neoplasia.

Ceftobiprole, an anti-methicillin-resistant Staphylococcus aureus broad-spectrum cephalosporin, has activity (MIC for 50% of strains tested, <4 g/ml) against many Pseudomonas aeruginosa strains. A common mechanism of P. aeruginosa resistance to ␤-lactams, including cefepime and ceftazidime, is efflux via increased expression of Mex pumps, especially MexAB. MexXY has differential substrate specificity, recognizing cefepime but not ceftazidime. In ceftobiprole clinical studies, paired isolates of P. aeruginosa from four subjects demonstrated ceftobiprole MICs of 2 to 4 g/ml at baseline but 16 g/ml posttreatment, unrelated to ␤-lactamase levels. Within each pair, the level of mexXY RNA, but not mexAB, mexCD, and mexEF, increased by an average of 50-fold from baseline to posttreatment isolates. Sequencing of the negative regulatory gene mexZ indicated that each posttreatment isolate contained a mutation not present at baseline. mexXY expression as a primary ceftobiprole and cefepime resistance mechanism was further examined in isogenic pairs by using cloned mexXY and mexZ. Expression of cloned mexXY in strain PAO1 or in a baseline isolate increased the ceftobiprole MIC to that for the posttreatment isolate. In contrast, in posttreatment isolates, lowering mexXY expression via introduction of cloned mexZ decreased the ceftobiprole MIC to that for the baseline isolates. Similar changes were observed for cefepime. A spontaneous mutant selectively overexpressing mexXY displayed a fourfold elevation in its ceftobiprole MIC, while overexpression of mexAB, -CD, and -EF had a minimal effect. These data indicate that ceftobiprole, like cefepime, is an atypical ␤-lactam that is a substrate for the MexXY efflux pump in P. aeruginosa.Ceftobiprole is a broad-spectrum, anti-methicillin-resistant Staphylococcus aureus cephalosporin with in vitro microbiological activity against many strains of Pseudomonas aeruginosa (MIC for 50% of strains tested, Յ4 g/ml) (4, 22). Resistance to antipseudomonal cephalosporins can be due to overexpression of the chromosomal AmpC cephalosporinase (17); however, decreased susceptibility of P. aeruginosa to a wide variety of antibiotics can also be attributed to drug efflux (17, 21). In particular, basal expression of the Mex pumps of the resistance nodulation division family appears to be responsible for the intrinsic resistance of P. aeruginosa to multiple antibiotics and for enhanced resistance upon overexpression (21, 23). The efflux pumps MexAB-OprM, MexCD-OprJ, MexEF-OprN, and MexXY-OprM have each been shown to be clinically relevant factors for decreased antibiotic susceptibility, with expression of the operons encoding these pumps subject to regulation (21). Pump overexpression may be due to an assortment of mutations that affect regulation of transcription from the efflux operon, such as mutations in the cognate repressor or in a global regulatory gene, or upstream of the operon, including but not limited to its promoter (21).The MexAB-OprM efflux pump is constitutively expressed in P. aerugi...

MurF catalyzes the last cytoplasmic step of bacterial cell wall synthesis and is essential for bacterial survival. Our previous studies used a pharmacophore model of a MurF inhibitor to identify additional inhibitors with improved properties. We now present the characterization of two such inhibitors, the diarylquinolines DQ1 and DQ2. DQ1 inhibited Escherichia coli MurF (50% inhibitory concentration, 24 M) and had modest activity Cell wall biosynthesis and the cell wall structure have long been considered useful targets for antibacterial agents, as demonstrated by antibiotics such as the ␤-lactams and glycopeptides (14, 29). However, of the series of steps catalyzed by the enzymes MurA through MurF that produce UDP-MurNAcpentapeptide, a useful antibiotic, fosfomycin, has been generated only against the MurA target (6, 18), despite extensive screening efforts against all of the enzymes in this pathway (for reviews, see references 8, 11, 14, 20, 29, and 31).MurF catalyzes the last cytoplasmic step of bacterial cell wall biosynthesis, generating UDP-MurNAc-pentapeptide from UDP-MurNAc-tripeptide and D-Ala-D-Ala (37). Previously identified inhibitors of MurF include a nonhydrolyzable ATP analog (1), phosphinate transition state analogs (25), sulfonamides (15, 22), thiazolylaminopyrimidines (4), and 8-hydroxyquinolines (5). These compounds inhibited the purified MurF enzyme but lacked antibacterial activity, presumably due to poor penetration into cells. A pharmacophore model based on the 8-hydroxyquinoline series was used to search for compounds with antibacterial activity, and this process identified several classes of compounds, including a 4-phenylpiperidine derivative (5). This inhibitor had the distinction of being the first inhibitor of the MurF enzyme which appeared to inhibit MurF within Escherichia coli cells.Observations of conditional lethal MurF mutants of E. coli (24) and Staphylococcus aureus (33, 34) are useful for predicting the effects of a MurF inhibitor on bacteria. In E. coli, a temperature-sensitive mutant with a mutated MurF enzyme that possessed Ͻ1% of the relative activity of the wild-type enzyme exhibited (i) cell lysis at the nonpermissive temperature, (ii) the accumulation of UDP-MurNAc-tripeptide (the MurF substrate), and (iii) a decrease in the level of the UDPMurNAc-pentapeptide product (24). In S. aureus, similar effects on the abundance of muropeptide were observed (33). These and other parameters were examined for two new MurF inhibitors, the diarylquinolines DQ1 and DQ2. MATERIALS AND METHODSMurF enzymatic assay. The 50% inhibitory concentrations (IC 50 s) for E. coli MurF were determined as described previously (5).Microbiology studies. All bacterial strains were from the strain collection ofMICs were determined by CLSI broth microdilution assays (9). The checkerboard methodology was used for determination of the MICs of compound DQ2 in combination with vancomycin (10). For growth curve generation, CFU quantitation, and muropeptide analysis, 125-ml cultures of E. coli OC2530 or S. ...

MurF is a key enzyme in the biosynthesis of the bacterial cell wall in both gram-positive and gram-negative bacteria. This enzyme has not been extensively exploited as a drug target, possibly due to the difficulty in obtaining one of the substrates, UDP-MurNAc-L-Ala-␥-D-Glu-meso-diaminopimelate, which is usually purified from bacteria. We have identified putative inhibitors of Escherichia coli MurF by a binding assay, thus bypassing the need for substrate. Inhibition of enzymatic activity was demonstrated in a high-performance liquid chromatography-based secondary assay with UDP-MurNAc-L-Ala-␥-D-Glu-diaminopimelate substrate prepared in a novel way by using muropeptide ligase enzyme to add UDP-MurNAc to synthetic L-Ala-␥-D-Glu-diaminopimelate; the substrate specificity of muropeptide ligase for peptides containing L-Lys in place of diaminopimelate was also investigated. Using the muropeptide ligase-generated MurF substrate, a thiazolylaminopyrimidine series of MurF enzyme inhibitors with 50% inhibitory concentration values as low as 2.5 M was identified.The steps involved in the synthesis of the bacterial cell wall have long been considered to be good targets for antibacterial agents, as evidenced by drugs such as ␤-lactams and vancomycin (18). The MurF enzyme catalyzes the last cytoplasmic step of bacterial cell wall synthesis, the ligation of D-Ala-D-Ala to UDP-MurNAc-tripeptide with the concomitant hydrolysis of ATP. In gram-negative bacteria, the tripeptide is L-Ala-␥-DGlu-meso-A 2 pm (where A 2 pm represents diaminopimelate); in gram-positive bacteria, L-Lys replaces meso-A 2 pm (23).MurF is an attractive antibacterial drug target for several reasons: (i) it carries out an essential step of cell wall biosynthesis as demonstrated by the study of a temperature-sensitive lethal mutation in this gene in Escherichia coli (12); (ii) it is a single-copy gene in both gram-positive and gram-negative bacteria with extensive amino acid sequence conservation, raising the possibility of broad-spectrum inhibitors; and (iii) an earlier step in this pathway, MurA, is the target of the antibacterial drug fosfomycin (9), suggesting that interference with MurF function would likewise disrupt bacterial replication. In addition, normal MurF activity has been shown to be necessary for ␤-lactam resistance in methicillin-resistant Staphylococcus aureus (20).Despite these attractive features, MurF has not been used extensively as a target in high-throughput screening, possibly due to the difficulty in obtaining sufficient quantities of its substrate, UDP-MurNAc-tripeptide. Previous efforts to assay MurF that bypassed the need for substrate included the use of a coupled reaction containing the enzymes MurA, B, C, D, E, and F (8, 24) or permeabilized cells (2). A more direct approach would be an assay to detect compounds that bind to MurF. We have recently reported the use of capillary electrophoresis to identify compounds that bind to E. coli MurF.Similarly, Gu et al. (10) utilized an unspecified affinity selection screening technol...

No abstract