position of guanine in a reaction that inactivates one MGMT molecule for each lesion repaired (reviewed in reference 24). Cells deficient in or depleted of MGMT are therefore sensitive to the mutagenic and cytotoxic effects of carcinogens and chemotherapeutic agents that produce O 6 -alkylguanine lesions (9, 12, 32). MGMT expression has been found to vary widely between normal and tumor tissues (3,10,30) and between individual cells within a tumor (21); additionally, 20% of human tumor cell lines lack MGMT activity altogether (8). Loss of MGMT expression is rarely, if ever, due to deletion, rearrangement, or mutation of the MGMT gene (27,31). This observation, taken together with the relative ease with which MGMT gene expression can be lost in cell culture, suggests that the gene is under epigenetic control (2, 23).The human MGMT gene possesses a CpG island, as defined by Gardiner-Garden and Frommer (13), which extends from approximately nucleotide (nt) 480 to 1480 of the 5Ј end of the gene and spans approximately 500 bases 5Ј and 3Ј of the transcription start site at nt 956 (nucleotide numbering based on references 15 and 26). Inappropriate 5-methylation of CpG cytosines within the MGMT CpG island is a likely epigenetic mechanism of MGMT inactivation. The existence of this mechanism is supported by the demonstration that a cell line lacking MGMT activity (MGMT Ϫ ) was capable of transcribing a human MGMT promoter-chloramphenicol acetyltransferase construct, leading to the conclusion that all of the necessary transcription factors were present in the MGMT Ϫ cell line. Additionally, in vitro methylation of the same MGMT promoter-chloramphenicol acetyltransferase construct prevented its transcription in transient-transfection assays (16). Finally, analysis of the MGMT CpG island promoter in glioma cell lines has provided perhaps the strongest support for CpG methylation and inactivation of the MGMT gene. Linker-mediated PCR analysis of the methylation status of individual CpG cytosines in the MGMT promoter of glioma cell lines with variable levels of MGMT gene expression showed that increasing levels of methylation are associated with corresponding decreases in MGMT expression (6).We have previously reported that the human multiple myeloma cell line 8226/S selected with verapamil and doxorubicin, either in parallel or serially, became phenotypically MGMT Ϫ (11). Reselection of 8226/S with verapamil alone led again to an MGMT Ϫ cell line, which we designated 8226/V. We have used this isogenic model to investigate the role of cytosine methylation in the control of MGMT expression. Analysis of the entire MGMT-associated CpG island, which includes the minimal promoter, the first untranslated exon, and the minimal enhancer (17), indicates that the loss of MGMT expression in 8226/V is associated with a marked increase in methylation of CpG cytosines within discrete regions that bracket the transcription start site. Furthermore, restriction enzyme accessibility assays showed that the MGMT ϩ 8226/S

Friction and wear properties of graphene nanosheets prepared by different processes as solid lubricant on silicon dioxide have been comparatively studied via calibrated atomic force microscopy. The effects of normal load, humidity, and velocity on the friction were also investigated. All kinds of graphene nanosheets possess friction-reduction properties on the nanoscale. Mechanically exfoliated graphene nanosheets exhibit ultralubrication and zero wear under high pressure due to perfect graphitic structure and a hydrophobic surface. Defects in chemical vapor deposited graphene nanosheets decrease the antiwear and friction-reduction capability. The graphene oxide nanosheets (GOS) show the weakest friction-reduction properties on account of destroyed graphitic structure and a hydrophilic surface. The reduced graphene oxide nanosheets (RGOS) possess better friction reduction than GOS by virtue of hydrophobic surface properties. Both RGOS and GOS have weak antiwear properties due to the destroyed graphitic structure. Antiwear properties are correlated strongly with the structure, and friction depends mainly on the structure and surface properties.

The multi-walled carbon nanotubes (MWCNTs) are functionalized by self-assembled surfactant layers after sonication in anionic surfactant sodium dodecyl sulfate (SDS) aqueous solution. The tribological properties of the SDS-functionalized MWCNTs as additive in water-based lubricants were evaluated with a four-ball tester. The results show that the SDSfunctionalized MWCNTs exhibit good anti-wear and friction reduction properties as well as enhanced load-carrying capacity. The maximum non-seizure load (PB value) can be raised about 3-7 times when SDS-functionalized MWCNTs were added into water. The mechanism of SDS-functionalized MWCNT additive was investigated with scanning electron microscopy (SEM), Auger electron spectroscopy (AES), and Raman spectroscopy. These preliminary results show a promise in applications of surfactant functional carbon nanotubes as an additive in water.

Micronutrients, such as beta-carotene and vitamins A and E, are potential chemopreventive agents; however, their concentrations in human target tissues are largely unknown. Because these micronutrients may exert their action at the site of target tissues, the tissue concentrations of the micronutrients need to be determined. In this cross-sectional study, we have measured the concentrations of seven carotenoids, two retinoids, and two tocopherols in paired plasma, buccal mucosal cells (BMC), and skin samples from 96 healthy subjects (ages 26-82 yrs). The plasma-tissue, as well as the diet-plasma and diet-tissue relationships of the micronutrients, and the impact of various potential confounders on the micronutrient concentrations were evaluated. The micronutrient concentrations of plasma and BMC used in the evaluation were the average of three measurements over a one-month period. Our data indicated that 1) the correlations between the plasma and BMC (Spearman r = 0.40-0.91, p < 0.05) and the plasma and skin (r = 0.24-0.75, p < 0.05) concentrations of most micronutrients were significant in all subjects, suggesting that the status of these micronutrients in the BMC and skin may be estimated from their plasma concentrations; 2) the correlations between the diet and plasma/tissue concentrations of the micronutrients were generally not as strong as the plasma-tissue relationships; the diet-plasma and diet-tissue relationships of the carotenoids were particularly poor in the smokers; 3) the plasma and tissue concentrations of most micronutrients were lower in smokers than in nonsmokers and higher in vitamin supplement users than in nonsupplement users; the differences remained significant after adjustment for age, gender, and diet intake estimates; 4) among the seven carotenoids examined, lycopene was unique, because its concentration was not lower in smokers or higher in supplement users but was inversely associated with age.

Graphene and functionalized graphene are promising candidates as ultrathin solid lubricants for dealing with the adhesion and friction in micro- and nanoelectromechanical systems (MEMS and NEMS). Here, the dynamic friction and adhesion characteristics of pristine graphene (PG), graphene oxide (GO), and fluorinated graphene (FG) were comparatively studied using atomic force microscopy (AFM). The friction as a function of load shows nonlinear characteristic on GO with strong adhesion and linear characteristic on PG and FG with relatively weak adhesions. An adhesion enhancement phenomenon that the slide-off force after dynamic friction sliding is larger than the pull-off force is observed. The degree of adhesion enhancement increases with the increasing surface energy, accompanied by a corresponding increase in transient friction strengthening effect. The dynamic adhesion and friction enhancements are attributed to the coupling of dynamic tip sliding and surface hydrophilic properties. The atomic-scale stick-slip behaviors confirm that the interfacial interaction is enhanced during dynamic sliding, and the enhancing degree depends on the surface hydrophilic properties. These findings demonstrate the adhesive strength between the contact surfaces can be enhanced in the dynamic friction process, which needs careful attention in the interface design of MEMS and NEMS.