Molecular mechanisms of DNA damage induced by procarbazine in the presence of Cu(II)

Procarbazine [N-isopropyl-alpha-(2-methylhydrazino)-p-toluamide], a hydrazine derivative, which has been shown to have effective antineoplastic activity, induces cancer in some experimental animals and humans. To clarify a new mechanism for its carcinogenic effect, we examined DNA damage induced by procarbazine in the presence of metal ion, using 32P-5'-end-labeled DNA fragments obtained from the human p53 tumor suppressor gene and the c-Ha-ras-1 protooncogene. Procarbazine plus Cu(II) induced piperidine-labile and formamidopyrimidine-DNA glycosylase-sensitive lesions at the 5'-ACG-3' sequence, complementary to a hotspot of the p53 gene, and the 5'-TG-3' sequence. Catalase partially inhibited DNA damage, suggesting that not only H(2)O(2) but also other reactive species are involved. Procarbazine plus Cu(II) significantly increased the formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine, which was completely inhibited by calatase. Electron spin resonance spin-trapping experiments revealed that methyl radicals were generated from procarbazine and Cu(II). On the basis of these findings, it is considered that procarbazine causes DNA damage through non-enzymatic formation of the Cu(I)-hydroperoxo complex and methyl radicals. In conclusion, in addition to alkylation, oxidative DNA damage may play important roles in not only antitumor effects but also mutagenesis and carcinogenesis induced by procarbazine.     

Mitochondrial reactive oxygen species reduce insulin secretion by pancreatic beta-cells

Pancreatic beta-cells exposed to hyperglycemia produce reactive oxygen species (ROS). Because beta-cells are sensitive to oxidative stress, excessive ROS may cause dysfunction of beta-cells. Here we demonstrate that mitochondrial ROS suppress glucose-induced insulin secretion (GIIS) from beta-cells. Intracellular ROS increased 15min after exposure to high glucose and this effect was blunted by inhibitors of the mitochondrial function. GIIS was also suppressed by H(2)O(2), a chemical substitute for ROS. Interestingly, the first-phase of GIIS could be suppressed by 50 microM H(2)O(2). H(2)O(2) or high glucose suppressed the activity of glyceraldehyde 3-phosphate dehydrogenase (GAPDH), a glycolytic enzyme, and inhibitors of the mitochondrial function abolished the latter effects. Our data suggested that high glucose induced mitochondrial ROS, which suppressed first-phase of GIIS, at least in part, through the suppression of GAPDH activity. We propose that mitochondrial overwork is a potential mechanism causing impaired first-phase of GIIS in the early stages of diabetes mellitus.     

Akt is an endogenous inhibitor toward tumor necrosis factor-related apoptosis inducing ligand-mediated apoptosis in rheumatoid synovial cells

Akt is known to be activated in the rheumatoid synovial tissues. We examined here functional role of Akt during tumor necrosis factor-related apoptosis inducing ligand (TRAIL)-mediated apoptosis in rheumatoid synovial cells. Rheumatoid synovial cells in vitro were rapidly committed to apoptosis in response to TRAIL in mitochondria-dependent manner whereas Akt and extracellular signal-regulated kinase (ERK) were also phosphorylated. TRAIL-mediated apoptosis in synovial cells was significantly increased through inactivation of Akt by LY294002, however, that process was not so changed by adding ERK inhibitor, PD98059. Platelet-derived growth factor (PDGF) clearly phosphorylated both Akt and ERK in synovial cells, and PDGF pretreatment markedly suppressed TRAIL-mediated synovial cell apoptosis. The use of not PD98059 but LY294002 abrogated PDGF-mediated inhibitory effect toward TRAIL-induced apoptosis in synovial cells. The above protective effect of Akt was confirmed by the use of short interfering RNA (siRNA)-directed inhibition of Akt. Our data suggest that Akt is an endogenous inhibitor during TRAIL-mediated synovial cell apoptotic pathway, which may explain that synovial cells in situ of the rheumatoid synovial tissues are resistant toward apoptotic cell death in spite of death receptor expression.     

Effects of different beta73 amino acids on formation of 14-stranded fibers of Hb S versus double-stranded crystals of Hb C-Harlem

Hb S (alpha(2)beta(2)(6Glu-->Val)) forms polymers, while Hb C-Harlem (alpha(2)beta(2)(6Glu-->Val,73Asp-->Asn)) forms crystals upon oversaturation. Since the only difference between the two is the beta73 amino acid, it follows that this site is a critical determinant in promoting either polymerization or crystallization. Beta73 Asp in Hb S forms a hydrogen bond with beta4 Thr, while beta73 Asn in Hb C-Harlem may inhibit this interaction as well as increase the hydrophobicity at the EF helix beta6 Val acceptor sites. Two new beta73 Hb S variants (beta73 His and Leu) were constructed and analyzed to define other amino acids facilitating formation of Hb S-like polymers versus Hb C-Harlem-like crystals. The two variants that were chosen were expected to either (1) enhance formation of the beta73-beta4 hydrogen bond (beta73 His) or (2) inhibit it and increase the hydrophobicity of the EF helix beta6 Val acceptor sites (beta73 Leu). beta73 His Hb S formed fibers but at a lower concentration than Hb S, while beta73 Leu Hb S formed crystals but at a higher concentration than Hb C-Harlem. The solubility of beta73 His Hb S was (1)/(7) of that of Hb S, while the solubility of beta73 Leu Hb S was similar to that of Hb C-Harlem. The delay time prior to polymer or crystal formation depended on Hb concentration. The delay time for beta73 His Hb S was 10(5)-fold shorter than that for Hb S, while that for beta73 Leu Hb S was 10(5)-fold longer in 1.0 M phosphate buffer. NMR results indicate beta73 amino acid changes induce alteration in the beta-chain heme pocket region, while CD results indicate no change in the helical content of the variants. These results suggest that enhancing the beta73-beta4 hydrogen bond and/or induced changes in the heme pocket by the beta73 Asp to His change facilitate formation of Hb S-like fibers. Our results also suggest that removal of the beta73-beta4 hydrogen bond and enhancing the hydrophobicity of the EF helix beta6 Val acceptor sites by the beta73 Asp to Leu or Asn changes delay nuclei formation and facilitate formation of Hb C-Harlem-like crystals.     

Characterization of a novel thermostable O-acetylserine sulfhydrylase from Aeropyrum pernix K1

An O-acetylserine sulfhydrylase (OASS) from the hyperthermophilic archaeon Aeropyrum pernix K1, which shares the pyridoxal 5'-phosphate binding motif with both OASS and cystathionine beta-synthase (CBS), was cloned and expressed by using Escherichia coli Rosetta(DE3). The purified protein was a dimer and contained pyridoxal 5'-phosphate. It was shown to be an enzyme with CBS activity as well as OASS activity in vitro. The enzyme retained 90% of its activity after a 6-h incubation at 100 degrees C. In the O-acetyl-L-serine sulfhydrylation reaction, it had a pH optimum of 6.7, apparent K(m) values for O-acetyl-L-serine and sulfide of 28 and below 0.2 mM, respectively, and a rate constant of 202 s(-1). In the L-cystathionine synthetic reaction, it showed a broad pH optimum in the range of 8.1 to 8.8, apparent K(m) values for L-serine and L-homocysteine of 8 and 0.51 mM, respectively, and a rate constant of 0.7 s(-1). A. pernix OASS has a high activity in the L-cysteine desulfurization reaction, which produces sulfide and S-(2,3-hydroxy-4-thiobutyl)-L-cysteine from L-cysteine and dithiothreitol.     

Structural basis of calcification inhibition by alpha 2-HS glycoprotein/fetuin-A. Formation of colloidal calciprotein particles

Genetic evidence from mutant mice suggests that alpha(2)-HS glycoprotein/fetuin-A (Ahsg) is a systemic inhibitor of precipitation of basic calcium phosphate preventing unwanted calcification. Using electron microscopy and dynamic light scattering, we demonstrate that precipitation inhibition by Ahsg is caused by the transient formation of soluble, colloidal spheres, containing Ahsg, calcium, and phosphate. These "calciprotein particles" of 30-150 nm in diameter are initially amorphous and soluble but turn progressively more crystalline and insoluble in a time- and temperature-dependent fashion. Solubilization in Ahsg-containing calciprotein particles provides a novel conceptual framework to explain how insoluble calcium precipitates may be transported and removed in the bodies of mammals. Mutational analysis showed that the basic calcium phosphate precipitation inhibition activity resides in the amino-terminal cystatin-like domain D1 of Ahsg. A structure-function analysis of wild type and mutant forms of cystatin-like domains from Ahsg, full-length fetuin-B, histidine-rich glycoprotein, and kininogen demonstrated that Ahsg domain D1 is most efficient in inhibiting basic calcium phosphate precipitation. The computer-modeled domain structures suggest that a dense array of acidic residues on an extended beta-sheet of the cystatin-like domain Ahsg-D1 mediates efficient inhibition.     

CrkL directs ASAP1 to peripheral focal adhesions

Searching for proteins in platelets that can interact with the N-terminal SH3 domain of CrkL (using a combination of a pull-down assay followed by mass spectrometry), we have found that human platelets express an ADP-ribosylation factor (Arf)-specific GTPase-activating protein (GAP), ASAP1, as a CrkL-binding protein. In spreading platelets, most endogenous ASAP1 is localized at peripheral focal adhesions. To determine the physiologic significance of the CrkL-ASAP1 association, we overexpressed CrkL, ASAP1, or both in combination in COS7 cells. Unlike endogenous ASAP1 in platelets, overexpressed ASAP1 showed diffuse cytoplasmic distribution. However, when co-expressed with wild-type CrkL, both endogenous and expressed ASAP1 accumulated at CrkL-induced focal adhesions. An SH2-mutated CrkL, which cannot localize at focal adhesions, failed to recruit ASAP1 into focal adhesions. Thus, CrkL appears to be a lynchpin between ASAP1 and peripheral focal adhesions.