Audesh Bhat

Cell cycle checkpoint defects contribute to genomic instability in PTEN deficient cells independent of DNA DSB repair

Chromosomes in PTEN deficient cells display both numerical as well as structural alterations including regional amplification. We found that PTEN deficient cells displayed a normal DNA damage response (DDR) as evidenced by the ionizing radiation (IR)-induced phosphorylation of Ataxia Telangiectasia Mutated (ATM) as well as its effectors. PTEN deficient cells also had no defect in Rad51 expression or DNA damage repair kinetics post irradiation. In contrast, caffeine treatment specifically increased IR-induced chromosome aberrations and mitotic index only in cells with PTEN, and not in cells deficient for PTEN, suggesting that their checkpoints were defective. Furthermore, PTEN-deficient cells were unable to maintain active spindle checkpoint after taxol treatment. Genomic instability in PTEN deficient cells could not be attributed to lack of PTEN at centromeres, since no interaction was detected between centromeric DNA and PTEN in wild type cells. These results indicate that PTEN deficiency alters multiple cell cycle checkpoints possibly leaving less time for DNA damage repair and/or chromosome segregation as evidenced by the increased structural as well as numerical alterations seen in PTEN deficient cells.

Regulation of nucleotide excision repair through ubiquitination.

Nucleotide excision repair (NER) is the most versatile DNA-repair pathway in all organisms. While bacteria require only three proteins to complete the incision step of NER, eukaryotes employ about 30 proteins to complete the same step. Here we summarize recent studies demonstrating that ubiquitination, a post-translational modification, plays critical roles in regulating the NER activity either dependent on or independent of ubiquitin-proteolysis. Several NER components have been shown as targets of ubiquitination while others are actively involved in the ubiquitination process. We argue through this analysis that ubiquitination serves to coordinate various steps of NER and meanwhile connect NER with other related pathways to achieve the efficient global DNA-damage response.

Rev7/Mad2B plays a critical role in the assembly of a functional mitotic spindle

The spindle assembly checkpoint (SAC) acts as a guardian against cellular threats that may lead to chromosomal missegregation and aneuploidy. Mad2, an anaphase-promoting complex/cyclosome-Cdc20 (APC/CCdc20) inhibitor, has an additional homolog in mammals known as Mad2B, Mad2L2 or Rev7. Apart from its role in Polζ-mediated translesion DNA synthesis and double-strand break repair, Rev7 is also believed to inhibit APC/C by negatively regulating Cdh1. Here we report yet another function of Rev7 in cultured human cells. Rev7, as predicted earlier, is involved in the formation of a functional spindle and maintenance of chromosome segregation. In the absence of Rev7, cells tend to arrest in G2/M-phase and display increased monoastral and abnormal spindles with misaligned chromosomes. Furthermore, Rev7-depleted cells show Mad2 localization at the kinetochores of metaphase cells, an indicator of activated SAC, coupled with increased levels of Cyclin B1, an APCCdc20 substrate. Surprisingly unlike Mad2, depletion of Rev7 in several cultured human cell lines did not compromise SAC activity. Our data therefore suggest that besides its role in APC/CCdh1 inhibition, Rev7 is also required for mitotic spindle organization and faithful chromosome segregation most probably through its physical interaction with RAN.

High glucose impairs insulin signaling via activation of PKR pathway in L6 muscle cells

Double stranded RNA (dsRNA) activated protein kinase R (PKR), a ubiquitously expressed serine/threonine kinase is a key inducer of inflammation, insulin resistance and glucose homeostasis in obesity. Recent studies have demonstrated that PKR can respond to metabolic stress in mice as well as in humans. However the underlying molecular mechanism is not fully understood. The aim of the present study was to examine the effect of high glucose on cultured rat L6 muscle cells and to investigate whether inhibition of PKR could prevent any deleterious effects of high glucose in these cells. PKR expression was determined by immunofluorescence and immunoblotting. The expression of different insulin signaling gene markers were measured by RT-PCR. Oxidative stress and apoptosis were determined by flow cytometry. High glucose treated L6 muscle cells developed a significant increase in PKR expression. Impaired insulin signaling as well as reduced insulin stimulated glucose uptake was observed in high glucose treated L6 muscle cells. A significant increase in reactive oxygen species generation and apoptosis formation was also observed in high glucose treated cultured L6 muscle cells. All these effects of high glucose were attenuated by a selective PKR inhibitor imoxin. Our study demonstrates PKR may have an additive role against the deleterious effects of high glucose in diabetes. Prevention of PKR activation, by safer and specific inhibitors is a therapeutic option in metabolic disorders that needs to be explored further.

Genetic Diversity Among Indian Coffee Cultivars Determined via Molecular Markers

Information on genetic diversity of different commercial coffee cultivars grown in India during the last 90 years is scarce. In the present study, the genetic diversity of selected coffee cultivars, along with some advanced breeding lines, was evaluated using random amplified polymorphic DNA (RAPD), inter-simple sequence repeat (ISSR), and sequence-related amplified polymorphism (SRAP) markers. The average polymorphism information content (PIC) or the genetic diversity of ISSR primers (0.365) was higher than that for both RAPD (0.222) and SRAP primers (0.346). The average resolving power (Rp) of SRAP primers (13.29) was slightly lower than that of RAPD (13.60) but higher than that of ISSR primers (11.23). The genetic similarity among various cultivars using Jaccards similarity coefficients ranged from 0.74 to 0.99 using RAPD, 0.48 to 0.94 using SRAP marker, and 0.57 to 0.95 using ISSR systems. Based on the marker analysis, all 24 coffee cultivars were clustered into two major groups, of which one is represented by cultivars belonging to C. arabica and the other group represented by C. canephora. Of the three marker systems used, the SRAP marker system was more informative and amplified several cultivar-specific fragments. The use of molecular markers can speed up genetic improvement of coffee cultivars.

Imoxin attenuates high fructose‐induced oxidative stress and apoptosis in renal epithelial cells via downregulation of protein kinase R pathway

Double‐stranded RNA (dsRNA)‐activated protein kinase R (PKR), a ubiquitously expressed serine/threonine kinase, is a key inducer of inflammation, insulin resistance, and glucose homeostasis in obesity. Recent studies have demonstrated that PKR can respond to metabolic stress in mice as well as in humans. However, the underlying molecular mechanism is not fully understood. The aim of this study was to examine the effect of high fructose (HF) in cultured renal tubular epithelial cells (NRK‐52E) derived from rat kidney and to investigate whether inhibition of PKR could prevent any deleterious effects of HF in these cells. PKR expression was determined by immunofluorescence staining and Western blotting. Oxidative damage and apoptosis were measured by flow cytometry. HF‐treated renal cells developed a significant increase in PKR expression. A significant increase in reactive oxygen species generation and apoptosis was also observed in HF‐treated cultured renal epithelial cells. All these effects of HF were attenuated by a selective PKR inhibitor, imoxin (C16). In conclusion, our study demonstrates PKR induces oxidative stress and apoptosis, is a significant contributor involved in vascular complications and is a possible mediator of HF‐induced hypertension. Inhibition of PKR pathway can be used as a therapeutic strategy for the treatment of cardiovascular and metabolic disorders.

Alagebrium attenuates methylglyoxal induced oxidative stress and AGE formation in H9C2 cardiac myocytes.

AIM Diabetes mellitus associated cardiovascular complications are a leading cause of morbidity and mortality worldwide. Methylglyoxal (MG) is a reactive ketoaldehyde and a byproduct of glucose metabolism and an inducer of advanced glycation endproducts (AGEs). Alagebrium (ALA) is an AGEs crosslink breaker, however, the effects of ALA on MG levels and its consequences in cultured rat cardiomyocytes are not known. The aim of the present study was to examine the effect of high glucose and MG on cultured rat cardiomyocytes and to investigate whether ALA could prevent any deleterious effects of high glucose and MG in these cells. MAIN METHODS MG levels were determined by HPLC. The expression of different genes was measured by RT-PCR. Oxidative stress and AGEs formation was determined by DCF probe and immunocytochemistry respectively. KEY FINDINGS High glucose- and MG treated- cardiomyocytes developed a significant increase in MG, and the expression for caspase-3, Bax, RAGE and NF-KB, which were all attenuated after pretreatment with ALA. A significant increase in reactive oxygen species generation and AGEs formation in high glucose- and MG treated- cultured cardiomyocytes was also observed, which was attenuated after pretreatment with ALA. SIGNIFICANCE ALA may have a preventive role against the deleterious effects of high glucose and MG in the heart. Prevention of dicarbonyl-induced AGEs, by safer and specific scavengers of MG is an attractive therapeutic option.

Pharmacological evaluation of novel alagebrium analogs as methylglyoxal scavengers in vitro in cardiac myocytes and in vivo in SD rats

BACKGROUND Methylglyoxal (MG) is a byproduct of glucose metabolism and an inducer of advanced glycation end products (AGEs). AGEs are implicated in the pathogenesis of diabetes as well as hypertension. Most of the currently available MG scavengers are non-specific and have other effects as well. Alagebrium (ALA), developed by Alteon Corporation is a MG scavenger. Thus the aim of the present study was to investigate the potential of novel ALA analogs as possible MG scavengers and whether they could prevent any deleterious effects of MG. METHODS AND RESULTS MG levels were measured by HPLC. The different biochemical and molecular parameters were measured by assay kits, RT-PCR and immunocytochemistry. Out of the 15 ALA analogs tested in vitro, compound no. 13 was found to be an effective inhibitor of MG in a concentration and time dependent manner. Compound no. 13 significantly attenuated the MG levels in vitro in MG treated cultured H9C2 cardiomyocytes as well as in vivo in MG treated SD rats. MG induced oxidative stress and apoptosis were attenuated by pretreatment of H9C2 cardiac myocytes with compound no. 13. MG induced cardiac hypertrophy and apoptosis were also attenuated by treating MG treated SD rats with compound no. 13. CONCLUSION Our results indicate compound 13 as an effective inhibitor of MG in vitro in cultured cardiomyocytes and in vivo in SD rats and thus it may prove very useful in blocking the multiple deleterious effects of MG, including AGEs and vascular complications of diabetes.

Protein kinase R and the metabolic syndrome

Metabolic syndrome greatly increases the risk for developing metabolic and cardiovascular disorders and has reached epidemic proportions globally. Despite recent advances in medical science, scientific understandings on the root mechanisms of metabolic syndrome are still not fully understood, and such insufficient knowledge contributes to the relative lack of effective treatments for such diseases. Protein Kinase R (PKR) is a serine threonine kinase activated during various stress conditions. Activation of PKR can increase reactive oxygen product generation, cause oxidative stress and inflammation. In this review we discuss the potential role of PKR in metabolic syndrome, pathways activated by it and the interrelationship between pathways activated, modes of propagation if one of the pathways is inhibited or activated. Specific and effective inhibitors of PKR are being developed and can become potential treatment for metabolic syndrome and prevent many diseases.

Pharmacological evaluation of novel PKR inhibitor indirubin-3-hydrazone in-vitro in cardiac myocytes and in-vivo in wistar rats

Aims: Double stranded protein kinase R cellular response is associated with various stress signals such as nutrients, endoplasmic stress, cytokines and mechanical stress. Increased PKR activity has been observed under diabetic and cardiovascular disease conditions. Most of the currently available PKR inhibitors are non‐specific and have other effects as well. Thus, the aim of the present study was to examine the effect of novel PKR inhibitor indirubin‐3‐hydrazone (IHZ) in cultured rat H9C2 cardiomyocytes and wistar rats. Materials and methods: PKR expression was determined by Q‐PCR, immunofluorescence and immunoblotting. The expression of different gene markers for apoptosis was measured by RT‐PCR. Apoptosis and oxidative stress were determined by flow cytometry. KEY FINDINGS: High glucose (HG) treated H9C2 cardiomyocytes and high fructose (HF) treated wistar rats developed a significant increase in PKR expression. A significant increase in apoptosis and generation of reactive oxygen species was also observed in HG treated H9C2 cells and HF treated rats. Reduced vacuole formation and prominent nuclei were also observed in high glucose treated cells. Cardiac hypertrophy and increased fibrosis were observed in HF treated rats. All these effects of HG and HF were attenuated by novel PKR inhibitor, indirubin‐3‐hydrazone. Significance: Our results indicate IHZ as an effective inhibitor of PKR in vitro and in‐vivo, thus it may prove very useful in blocking the multiple harmful effects of PKR.