Diabetes-specific microvascular disease is a leading cause of blindness, renal failure and nerve damage, and diabetes-accelerated atherosclerosis leads to increased risk of myocardial infarction, stroke and limb amputation. Four main molecular mechanisms have been implicated in glucose-mediated vascular damage. All seem to reflect a single hyperglycaemia-induced process of overproduction of superoxide by the mitochondrial electron-transport chain. This integrating paradigm provides a new conceptual framework for future research and drug discovery.

http://www.chem.uwec.edu/Chem454/BiochemDiabetes.pdf

Biochemistry and molecular cell biology of diabetic complications

Abundant data link hypercholesterolaemia to atherogenesis. However, only recently have we appreciated that inflammatory mechanisms couple dyslipidaemia to atheroma formation. Leukocyte recruitment and expression of pro-inflammatory cytokines characterize early atherogenesis, and malfunction of inflammatory mediators mutes atheroma formation in mice. Moreover, inflammatory pathways promote thrombosis, a late and dreaded complication of atherosclerosis responsible for myocardial infarctions and most strokes. The new appreciation of the role of inflammation in atherosclerosis provides a mechanistic framework for understanding the clinical benefits of lipid-lowering therapies. Identifying the triggers for inflammation and unravelling the details of inflammatory pathways may eventually furnish new therapeutic targets.

Inflammation in atherosclerosis

For a long time, superoxide generation by an NADPH oxidase was considered as an oddity only found in professional phagocytes. Over the last years, six homologs of the cytochrome subunit of the phag...

/pdf/the-nox-family-of-ros-generating-nadph-oxidases-physiology-zjc88zvh59.pdf

The NOX Family of ROS-Generating NADPH Oxidases: Physiology and Pathophysiology

Adiponectin is an adipocyte-derived hormone. Recent genome-wide scans have mapped a susceptibility locus for type 2 diabetes and metabolic syndrome to chromosome 3q27, where the gene encoding adiponectin is located. Here we show that decreased expression of adiponectin correlates with insulin resistance in mouse models of altered insulin sensitivity. Adiponectin decreases insulin resistance by decreasing triglyceride content in muscle and liver in obese mice. This effect results from increased expression of molecules involved in both fatty-acid combustion and energy dissipation in muscle. Moreover, insulin resistance in lipoatrophic mice was completely reversed by the combination of physiological doses of adiponectin and leptin, but only partially by either adiponectin or leptin alone. We conclude that decreased adiponectin is implicated in the development of insulin resistance in mouse models of both obesity and lipoatrophy. These data also indicate that the replenishment of adiponectin might provide a novel treatment modality for insulin resistance and type 2 diabetes.

The fat-derived hormone adiponectin reverses insulin resistance associated with both lipoatrophy and obesity

Experimental work has elucidated molecular and cellular pathways of inflammation that promote atherosclerosis. Unraveling the roles of cytokines as inflammatory messengers provided a mechanism whereby risk factors for atherosclerosis can alter arterial biology, and produce a systemic milieu that favors atherothrombotic events. The discovery of the immune basis of allograft arteriosclerosis demonstrated that inflammation per se can drive arterial hyperplasia, even in the absence of traditional risk factors. Inflammation regulates aspects of plaque biology that trigger the thrombotic complications of atherosclerosis. Translation of these discoveries to humans has enabled both novel mechanistic insights and practical clinical advances.

/pdf/inflammation-in-atherosclerosis-43d3vqivvm.pdf

Inflammation in Atherosclerosis

We would like to offer a different opinion on the ideas presented in the article by Keshava and Caldwell (2006). The authors indicated that their article summarized scientific literature published since an earlier U.S. Environmental Protection Agency (EPA) risk assessment of trichloroethylene (TCE), with an emphasis on the possible role of proliferator-activated receptor α (PPARα) agonism relevant to TCE risk assessment. Interestingly, in the section on recent data on PPARα agonism, Keshava and Caldwell failed to establish any gene expression signature relating TCE and PPARα.

Keshava and Caldwell (2006) contended that it is difficult to identify a clear pattern of common gene expression changes for TCE and PPARα agonists in general. However, they did not consider numerous reports and reviews (e.g., Klaunig et al. 2003; Peters et al. 2005) illustrating that there are common and reproducible changes in gene expression associated with PPARα agonists. Further, extensive characterization has definitively demonstrated specific, direct targets of PPARα-retinoid X receptor heterodimers (reviewed by Klaunig et al. 2003). Keshava and Caldwell (2006) also did not discuss the possibility that the effect of TCE on gene expression could be mediated by mechanisms independent of PPARα, which likely explains the disparity described in their article. Keshava and Caldwell did not critically discuss the data summarized in their Table 2 (Keshava and Caldwell 2006), failing to note that many of these gene targets have no clear linkage with the PPARα agonist mode of action (MOA) and may be mediated either via different ligand–receptor–coactivator complexes that form on the promoter regions of the regulated genes by secondary events downstream of the initial events associated with PPARα activation, or by mechanisms that are independent of PPARα. In addition, the authors failed to describe the limitations of the various gene array platforms and to correctly interpret the findings in the context of gene targets by other PPARα agonists, especially when more comprehensive data sets exist but were not cited (Anderson et al. 2004a, 2004b).

Keshava and Caldwell (2006) further raised concerns regarding the use of PPARα-null mice to evaluate the MOA of PPARα by indicating that the physiologic differences observed in PPARα-null mice relative to wild-type mice suggest that the null mouse is an inadequate model to study the PPARα MOA. The data they cited, however, appears selective because they failed to mention that liver regeneration in PPARα-null mice is reportedly unchanged compared with wild-type mice (Rao et al. 2002), and age-related, sexually dimorphic obesity has not been observed in congenic PPARα-null mice (Akiyama et al. 2001). Thus, although the null mouse exhibits changes consistent with the critical role of PPARα in modulating fatty acid catabolism, this phenotype does not preclude its application for determining the critical role of this receptor in the MOA of PPARα agonists. Importantly, Keshava and Caldwell (2006) did not comprehensively discuss significant findings a) that PPARα-null mice are refractory to liver tumors induced by two different PPARα agonists (Hays et al. 2005; Peters et al. 1997); b) that they are refractory to increased markers of replicative DNA synthesis and suppression of apoptosis after exposure to numerous PPARα ligands (summarized by Peters et al. 2005); or c) that PPARα-null mice expressing the human PPARα in the liver respond to PPARα agonists by increasing expression of genes encoding proteins that catabolize lipids, but they fail to show increases in markers of cell proliferation and are resistant to liver cancer (Cheung et al. 2004; Morimura et al. 2006). To dismiss these findings through lack of discussion or citation does little to advance our understanding and suggests that Keshava and Caldwell’s article is unbalanced.

Keshava and Caldwell (2006) also misrepresented an earlier review by Klaunig et al. (2003) regarding the MOA of PPARα agonists. Keshava and Caldwell (2006) incorrectly suggested that Klaunig et al. (2003) placed substantial weight on the associative event of peroxisome proliferation with this MOA, when, in fact, peroxisome proliferation was strongly—but not causally—associated, as noted for sustained increased cell proliferation. Keshava and Caldwell (2006) also misconstrued this review (Klaunig et al. 2003), focusing on DNA damage as a possible contributor to the MOA. Citing one manuscript that examined the effect of one, nonspecific PPARα ligand (DHEA) is not sufficient to refute the comprehensive review by Klaunig et al. (2003). Finally, Keshava and Caldwell (2006) also suggested that the effects of PPARα ligands on mitochondrial function are part of the MOA, but they provided no direct evidence to support their contention that PPARα agonists or TCE causes mitochondrial dysfunction.

In summary, Keshava and Caldwell (2006) missed an excellent opportunity to critically and objectively examine the data that support or refute the role of PPARα in TCE-induced effects. In our opinion, their article did not advance our understanding of the MOA of PPARα agonists or TCE.

PPARα and Effects of TCE

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors (NRs) that regulate genes involved in lipid and glucose metabolism. PPAR activity is regulated by interactions with cofactors and of interest are cofactors with ubiquitin ligase activity. The E6-associated protein (E6-AP) is an E3 ubiquitin ligase that affects the activity of other NRs, although its effects on PPARs have not been examined. E6-AP inhibited the ligand-independent transcriptional activity of PPARα and PPARβ, with marginal effects on PPARγ, and decreased basal mRNA levels of PPARα target genes. Inhibition of PPARα activity required the ubiquitin ligase function of E6-AP, but occurred in a proteasome-independent manner. PPARα interacted with E6-AP, and in mice treated with PPARα agonist clofibrate, mRNA and protein levels of E6-AP were increased in wildtype, but not in PPARα null mice, indicating a PPARα-dependent regulation. These studies suggest coordinate regulation of E6-AP and PPARα, and contribute to our understanding of the role of PPARs in cellular metabolism.

/pdf/researcharticle-regulation-of-peroxisome-proliferator-kzrqmmc4mm.pdf

ResearchArticle Regulation of Peroxisome Proliferator-Activated Receptors by E6-Associated Protein

Determination of Protein-Protein Interactions and the Motifs That Mediate Them

Effect of the antihypertensive drug hydralazine on mineral metabolism in the rat.

The peroxisomal proliferator-activated receptor β (δ) (PPARβ) is a nuclear hormone receptor that is ubiquitously expressed and that regulates the transcription of genes involved in lipid metabolism. A homozygous PPARβ-null mouse has been developed in which the ligand-binding domain of the PPARβ re- ceptor is disrupted. Analysis of brains from these animals shows that female null mice have 24 and 17% increases in plas- menylethanolamine and phosphatidylserine and a 9% decrease in the level of phosphatidylinositol when compared to controls. The phospholipid changes found in female null mice were asso- ciated with increased levels of esterified 18:1n-9, 20:1n-9, 20:4n-6, and 22:5n-3 FA in plasmenylethanolamine, 20:1n-9 in phosphatidylinositol, and 18:0, 18:1n-9, 18:3n-6, 20:1n-9, and 20:4n-6 in phosphatidylserine. Increased levels of esterified 18:1n-9, 18:2n-6, 18:3n-6, and 20:1n-9 were also found in the phosphatidylethanolamine fraction despite its cellular content remaining unchanged. Brain phospholipid content in male PPARβ-null mice did not differ from controls, but increased lev- els of 20:1n-9 in the phosphatidylinositol and 18:1n-9 in the phosphatidylserine fractions were observed. No changes were found in the content of brain cholesterol, TAG, and FFA in either female or male PPARβ-null mice. These data suggest that PPARβ is involved in maintaining FA and phospholipid levels in adult female mouse brain and provide strong evidence that suggests a

Targeted Disruption of the Nuclear Hormone Receptor, Peroxisomal Proliferator-Activated Receptor β (δ), Results in Distinct Gender Differences in Mouse Brain Phospholipid and Esterified FA Levels

Jeffrey M. Peters

Papers

PPARα and Effects of TCE

ResearchArticle Regulation of Peroxisome Proliferator-Activated Receptors by E6-Associated Protein

Determination of Protein-Protein Interactions and the Motifs That Mediate Them

Effect of the antihypertensive drug hydralazine on mineral metabolism in the rat.

Targeted Disruption of the Nuclear Hormone Receptor, Peroxisomal Proliferator-Activated Receptor β (δ), Results in Distinct Gender Differences in Mouse Brain Phospholipid and Esterified FA Levels