We have shown previously that different chemical classes of small-molecule antagonists of the human chemokine CXCR2 receptor interact with distinct binding sites of the receptor. Although an intracellular binding site for diarylurea CXCR2 antagonists, such as N-(2-bromophenyl)-NЈ-(7-cyano-1H-benzotriazol-4-yl)urea (SB265610), and thiazolopyrimidine compounds was recently mapped by mutagenesis studies, we now report on an imidazolylpyrimidine antagonist binding pocket in the transmembrane domain of CXCR2. Using different CXCR2 orthologs, chimeric proteins, site-directed mutagenesis, and in silico modeling, we have elucidated the binding mode of this antagonist. Our in silico-guided mutagenesis studies indicate that the ligand binding cavity for imidazolylpyrimidine compounds in CXCR2 is located between transmembrane (TM) helices 3 (Phe130 3.36 ), 5 (Ser217 5.44 , Phe220 5.47 ), and 6 (Asn268 6.52 , Leu271 6.55 ) and suggest that these antagonists enter CXCR2 via the TM5-TM6 interface. It is noteworthy that the same interface is postulated as the ligand entry channel in the opsin receptor and is occupied by lipid molecules in the recently solved crystal structure of the CXCR4 chemokine receptor, suggesting a general ligand entrance mechanism for nonpolar ligands to G protein-coupled receptors. The identification of a novel allosteric binding cavity in the TM domain of CXCR2, in addition to the previously identified intracellular binding site, shows the diversity in ligand recognition mechanisms by this receptor and offers new opportunities for the structure-based design of small allosteric modulators of CXCR2 in the future.

Considerable evidence has accumulated indicating that overexpression of P-glycoproteins encoded by the multidrug-resistance (mdr) genes is responsible for the development of collateral resistance to a number of structurally and functionally dissimilar cytotoxic compounds in animal cells. There are three mdr genes (mdr1, mdr2, and mdr3) in the mouse genome and two (MDR1 and MDR2) in the human genome; however, only two mouse genes (mdr1 and mdr3) and one human gene (MDR1) can confer multidrug resistance upon transfection into otherwise drug-sensitive cells. Using RNase protection assay we report here that the steady-state levels of mdr1 and mdr3 messenger RNA were elevated in mouse hepatoma cells treated with dexamethasone (Dex); whereas no induction of mdr2 gene was found. Western blot analyses using anti-mdr1 and anti-mdr3 antibodies revealed that the encoded proteins appeared to be increased, but at much reduced levels. The induction was time and Dex concentration dependent. Nuclear run-on experiments demonstrated that the induction was at least in part by transcriptional control. The induction apparently required new protein synthesis since no increases in mdr1 and mdr3 transcripts was found when cultured cells were simultaneously treated with Dex and cycloheximide. Neither mdr1 nor mdr3 gene was induced in the Dex-treated nonhepatoma cell lines, LMtk- and NIH3T3. Similarly, MDR1 messenger RNA levels were elevated in the Dex-treated human hepatoma line, HepG2, but not in the nonhepatoma, HeLa. This study demonstrated that the hormonal regulation of mdr gene expression is gene and cell type specific.

Cardiac troponin release is generally found in adult athletes after continuous-type endurance exercises or sport competitions. The purpose of this study was to investigate whether the physical stress experienced by adolescents while playing basketball, an intense, intermittent-type sport, could induce transient elevations of the serum cardiac troponin T (cTnT) and I (cTnI). Serum cTnT and cTnI levels in 10 male adolescent players (age 15.0 +/- 0.7 yr) were assessed immediately before and at 2, 4 and 24 h after a game randomly selected from a preseason basketball-training program. At 4 h following the game, serum cTnT levels in four of the ten subjects were above the cutoff of 0.01 ng . ml (-1) for myocardial injury. Two of these four subjects had values higher than the acute myocardial infarction cutoff of 0.05 ng . ml (-1). In three of the four subjects, the serum cTnI was above the cutoff of 0.06 ng . ml (-1) for myocardial injury. Nevertheless, serum cardiac troponins at 24 h had returned to pre-exercise levels. These findings suggest that the physical stress encountered during intense, intermittent-type sports could cause release of cardiac troponins in some adolescents at low risk for cardiac disease.

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