A new strategy for the reversible attachment of methoxypoly(ethylene glycol) (mPEG) to an amino-containing substrate is described. The strategy is based on formation of a benzyl carbamate linkage substituted with a disulfide in the para or ortho position. While being stable under nonreducing conditions, the dithiobenzyl (DTB) urethane linkage is susceptible to cleavage by mild thiolysis with cysteine resulting in release of the parent amino component of the conjugate in its original form. The method is exemplified by preparation of mPEG-DTB-alcohol, its activation and attachment to distearoylphosphatidylethanolamine (DSPE). The resulting lipopolymer incorporates into liposomes, which are capable of losing their polymer coating under conditions approximating those existing in vivo. Implications for drug delivery are briefly discussed.
MutS homologs, identified in nearly all bacteria and eukaryotes, include the bacterial proteins MutS1 and MutS2 and the eukaryotic MutS homologs 1 to 7, and they often are involved in recognition and repair of mismatched bases and small insertion/deletions, thereby limiting illegitimate recombination and spontaneous mutation. To explore the relationship of MutS2 to other MutS homologs, we examined conserved protein domains. Fundamental differences in structure between MutS2 and other MutS homologs suggest that MutS1 and MutS2 diverged early during evolution, with all eukaryotic homologs arising from a MutS1 ancestor. Data from MutS1 crystal structures, biochemical results from MutS2 analyses, and our phylogenetic studies suggest that MutS2 has functions distinct from other members of the MutS family. A mutS2 mutant was constructed in Helicobacter pylori, which lacks mutS1 and mismatch repair genes mutL and mutH. We show that MutS2 plays no role in mismatch or recombinational repair or deletion between direct DNA repeats. In contrast, MutS2 plays a significant role in limiting intergenomic recombination across a range of donor DNA tested. This phenotypic analysis is consistent with the phylogenetic and biochemical data suggesting that MutS1 and MutS2 have divergent functions.MutS homologs (MSH) have been identified in most prokaryotic and all eukaryotic organisms examined. Prokaryotes have two homologs (MutS1 and MutS2), whereas seven MSH proteins (MSH1 to MSH7) have been identified in eukaryotes (16,19,23). The homodimer MutS1 and heterodimers MSH2-MSH3 and MSH2-MSH6 are primarily involved in mitotic mismatch repair, whereas MSH4-MSH5 is involved in resolution of Holliday junctions during meiosis (1, 64). All members of the MutS family contain the highly conserved Walker A/B ATPase domain (16), and many share a common mechanism of action. MutS1, MSH2-MSH3, MSH2-MSH6, and MSH4-MSH5 dimerize to form sliding clamps, and recognition of specific DNA structures or lesions results in ADP/ATP exchange (27,45,49,64).The function of the second prokaryotic homolog, MutS2, is unknown. Sequence analyses reveal fundamental differences between MutS2 and other MutS family members (19). MutS2 proteins contain a conserved C-terminal domain of ϳ250 amino acid residues not found in other MutS homologs and lack the conserved N-terminal region present in most of the other MutS family members (43). According to one hypothesis, MutS2 is more closely related to the meiotic recombination proteins MSH4 and MSH5, while MutS1 is more closely related to MSH2,. This hypothesis suggests a gene duplication event early in the evolution of MutS, resulting in the two main MutS lineages, with MSH4 and MSH5 branching with MutS2 and MSH2, -3, and -6 branching with MutS1. Consistent with this hypothesis, MutS2 has been shown to not play a role in mismatch repair (13,59,69). However, arguing against this hypothesis is the lack of homology between MutS2 and MSH4-MSH5. According to another hypothesis, all eukaryotic MutS homologs evolved from one ancesto...
Electroporation figured prominently as an effective nonviral gene delivery approach for its balance on the transfection efficiency and cell viability, no restrictions of probe or cell type, and operation simplicity. The commercial electroporation systems have been widely adopted in the past two decades while still carry drawbacks associated with the high applied electric voltage, unsatisfied delivery efficiency, and/or low cell viability. By adding highly conductive gold nanoparticles (AuNPs) in electroporation solution, we demonstrated enhanced electroporation performance (i.e. better DNA delivery efficiency and higher cell viability) on mammalian cells from two different aspects: the free, naked AuNPs reduce the resistance of the electroporation solution so that the local pulse strength on cells was enhanced; targeting AuNPs (e.g., Tf-AuNPs) were brought to the cell membrane to work as virtual microelectrodes to porate cells with limited area from many different sites. The enhancement was confirmed with leukemia cells in both a commercial batch electroporation system and a home-made flow-through system using pWizGFP plasmid DNA probes. Such enhancement depends on the size, concentration, and the mixing ratio of free AuNPs/Tf-AuNPs. An equivalent mixture of free AuNPs and Tf-AuNPs exhibited the best enhancement with the transfection efficiency increased 2-3 folds at minimum sacrifice of cell viability. This new delivery concept, the combination of nanoparticles and electroporation technologies, may stimulate various in vitro and in vivo biomedical applications which rely on the efficient delivery of nucleic acids, anticancer drugs, or other therapeutic materials.
The DNA-damaging drug doxorubicin (Dox) induces cell senescence at concentrations significantly lower than those required for induction of apoptosis. At low Dox concentrations, tumor suppressor p53 is activated, which enhances the expression of p21 Waf1/Cip1 (p21). At high concentrations, Dox activates p53 leading to apoptosis without enhancing p21 expression. The underlying mechanisms and factors that govern the differential effects of Dox in inducing senescence and apoptosis are unclear. Here, we report that the DNA methyltransferase (DNMT) DNMT3a was upregulated by Dox especially at concentrations that induced apoptosis in HCT116 colorectal cancer cells, and this process was regulated by p53. Meanwhile, p21 expression was significantly upregulated at senescence-inducing concentrations and kept low on treatment with apoptosis-inducing concentrations of Dox. The differential expression of DNMT3a and p21 in response to Dox suggests that DNMT3a may be a key factor in switches between Dox-induced senescence and apoptosis. Moreover, when DNMT3a was silenced, treatment of HCT116 cells with apoptosis-inducing concentration of Dox increased the percentage of cells undergoing senescence, accompanied by upregulation of p21. Contrarily, senescence-inducing concentration of Dox promoted apoptosis rate, and p21 expression was repressed. Surprisingly, no changes in DNA methylation status at p21 promoter were detected at either ranges of Dox, although DNMT3a and HDAC1 were recruited to p21 promoter at apoptosis-inducing Dox concentration, where they were present in the same complex. Overall, these data demonstrate that DNMT3a impacts the expression of p21 and plays a role in determining the Dox-induced senescence and apoptosis in HCT116 cells.When proliferating cells encounter a genotoxic stress, induced by, for instance the chemotherapeutic drugs, the cell cycle must be arrested immediately to ensure DNA integrity. This event is usually followed by a decision of whether the cells remain arrested in the cell cycle for initiating DNA repair, or execute the apoptotic program. The anthracycline antibiotic agent doxorubicin (Dox) has been used as a chemotherapeutic drug for >40 years for the treatment of a variety of malignancies. 1 Despite the extensive and long-standing clinical utilization of Dox, the mechanism of its action remains uncertain and has long been an issue of controversy. Dox can induce at least 3 distinct types of cell death, i.e., senescence, apoptosis, and necrosis, in a concentration-dependent manner. Specific molecular markers such as p21, activated caspase-3 and activated Akt, were associated with these death modes. 2 At low concentrations, the predominant effect of Dox is to initiate senescence, whereas at high Dox concentrations, tumor cells may undergo cell death.The p53 tumor suppressor is a transcription factor that is involved in the cellular DNA damage response, causing either G1 arrest or apoptosis. 3 p53 plays an important role in Doxinduced senescence and apoptosis. At low concentrations of Dox, p...
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