John Kanellis

Uric Acid, Hominoid Evolution, and the Pathogenesis of Salt-Sensitivity

Humans have elevated serum uric acid as a result of a mutation in the urate oxidase (uricase) gene that occurred during the Miocene. We hypothesize that the mutation provided a survival advantage because of the ability of hyperuricemia to maintain blood pressure under low-salt dietary conditions, such as prevailed during that period. Mild hyperuricemia in rats acutely increases blood pressure by a renin-dependent mechanism that is most manifest under low-salt dietary conditions. Chronic hyperuricemia also causes salt sensitivity, in part by inducing preglomerular vascular disease. The vascular disease is mediated in part by uric acid-induced smooth muscle cell proliferation with activation of mitogen-activated protein kinases and stimulation of cyclooxygenase-2 and platelet-derived growth factor. Although it provided a survival advantage to early hominoids, hyperuricemia may have a major role in the current cardiovascular disease epidemic.

Decreased allergic lung inflammatory cell egression and increased susceptibility to asphyxiation in MMP2-deficiency

Clearance of recruited immune cells is necessary to resolve inflammatory reactions. We show here that matrix metalloproteinase 2 (MMP2), as part of an interleukin 13 (IL-13)–dependent regulatory loop, dampens inflammation by promoting the egress of inflammatory cells into the airway lumen. MMP2−/− mice showed a robust asthma phenotype and increased susceptibility to asphyxiation induced by allergens. However, whereas the lack of MMP2 reduced the influx of cells into bronchoalveolar lavage (BAL), numerous inflammatory cells accumulated in the lung parenchyma. BAL of MMP2−/− mice lacked normal chemotactic activity, whereas lung inflammatory cells from the same mice showed appropriate chemotactic responses. Thus, MMP2 establishes the chemotactic gradient required for egression of lung inflammatory cells and prevention of lethal asphyxiation.

Uric Acid Causes Vascular Smooth Muscle Cell Proliferation by Entering Cells via a Functional Urate Transporter

Background: Soluble uric acid stimulates vascular smooth muscle cell (VSMC) proliferation by activating mitogen-activated protein kinases, and stimulating COX-2 and PDGF synthesis. The mechanism by which uric acid enters the VSMC is not known. We hypothesized that uric acid enters via transporters similar to that observed in the kidney. Methods: We studied the uptake of uric acid into rat VSMC under polarized and depolarized conditions and in the presence of organic anion transport (OAT) inhibitors (probenecid and benzbromarone) or p-aminohippurate (PAH). We also examined the ability of probenecid to inhibit uric acid-induced VSMC proliferation and monocyte chemoattractant protein-1 (MCP-1) synthesis. Results:14C-Urate uptake was shown in VSMC and was enhanced under depolarized conditions. 14C-Uric acid uptake was inhibited by probenecid and benzbromarone, as well as by unlabelled urate and PAH. Probenecid blocked VSMC proliferation and MCP-1 expression in response to uric acid. VSMC did not express rOAT1-3, rOAT-5 or URAT-1 mRNA by PCR, but did express the voltage-sensitive transporter (UAT) by both PCR and RNase protection assay. Conclusions: Urate enters VSMC by both voltage-sensitive and OAT pathways, and the uptake, cell proliferation and MCP-1 expression can be blocked by OAT inhibitors. The specific transporter(s) responsible for the urate uptake remains to be determined.

Does asymptomatic hyperuricaemia contribute to the development of renal and cardiovascular disease? An old controversy renewed.

SUMMARY:  Recent studies in both humans and experimental animals have led to renewed interest in uric acid and its association with hypertension, cardiovascular events and renal disease progression. This has also refuelled a longstanding debate regarding the precise role of this ubiquitous breakdown product of purine metabolism in these disease processes. Various lines of evidence suggest that uric acid may have a direct role in the pathogenesis of hypertension and vascular disease. Regardless of this possibility, it is apparent that serum uric acid levels serve as a powerful ‘biomarker’ or independent predictor of prognosis and outcome in certain renal, cardiovascular and cerebrovascular diseases. Whether these outcomes can be improved by specifically treating asymptomatic hyperuricaemia remains inadequately resolved at this stage. Data from various animal studies suggests that lowering uric acid levels may be of benefit, but the crucial human studies are still lacking. This review will examine some of the recent evidence supporting a causal and contributory role for uric acid in cardiovascular and renal disease. How clarification of the role of uric acid may guide future treatment strategies will also be discussed.

Modulation of Inflammation by Slit Protein In Vivo in Experimental Crescentic Glomerulonephritis

A basic conservation of cell migration guidance mechanisms in the nervous and immune systems was proposed when Slit, known for its role in axon guidance, was found to inhibit chemokine-induced leukocyte chemotaxis in vitro. These studies examined the role of Slit2 in modulating inflammation in vivo. In a rat model of glomerulonephritis, endogenous glomerular Slit2 expression fell after disease induction, and its inhibition during the early disease period accelerated inflammation. Ex vivo glomerular leukocytes showed decreased chemokine and chemoattractant-induced chemotaxis in response to Slit2, suggesting an anti-inflammatory role for glomerular Slit2. In contrast to the effect of inhibition, glomerulonephritis was ameliorated by systemic Slit2 administration. Slit2 treatment improved disease histologically and also improved renal function when given early in the disease course. Leukocytes harvested from rats receiving Slit2 showed decreased monocyte chemoattractant protein-1 (MCP)-1-mediated migration, consistent with a peripheral Slit2 effect. In keeping with this functional alteration, Slit2-mediated inhibition of RAW264.7 cell chemotaxis was associated with decreased levels of active cdc42 and Rac1, implicating GTPases in leukocyte Slit2 signaling. These findings suggest a role for endogenous Slit2 in the inhibition of chemoattractant-mediated signals, demonstrate a potentially important anti-inflammatory effect for Slit2 in vivo, and provide further evidence for conserved mechanisms guiding the process of migration in distinct cell types.

Inhibition of p38 mitogen-activated protein kinase augments progression of remnant kidney model by activating the ERK pathway.

p38, a mitogen-activated protein kinase, is a major intracellular signaling molecule involved in inflammation. To test the hypothesis that p38 mediates renal disease progression, we administered a novel p38 alpha inhibitor, NPC31169, to rats with remnant kidneys (RKs). RK rats showed increased p38 activation at 9 weeks (by p38 kinase assay), which was blocked by the inhibitor. In contrast to our expectation, treatment with the NPC31169 resulted in worse renal function, more proteinuria, and more severe glomerulosclerosis and tubulointerstitial injury. p38 inhibition resulted in marked cell proliferation in RK rats, with more proliferating tubular cells, myofibroblasts, and macrophages. In contrast, p38 suppression resulted in less tubular cell apoptosis. Interestingly, Western blot demonstrated increased ERK1/2 phosphorylation in p38-treated rats. No histological changes were observed in p38 inhibited sham-operated rats. Our findings indicate that, whereas blocking p38 usually shows benefit in inflammatory disease, in this model p38 inhibition resulted in accelerated renal progression. We conclude that blocking p38-dependent inflammation may have resulted in enhanced proliferation and increased ERK1/2 activation, and thereby explains the worse renal lesions observed.

Editorial Comment—Elevated Uric Acid and Ischemic Stroke: Accumulating Evidence That It Is Injurious and Not Neuroprotective

Hyperuricemia was first associated with hypertension and cardiovascular disease in 1879.1 Since that time, many have attributed this association to a simple clustering of hyperuricemia with well-established cardiovascular risk factors, and an elevated serum uric acid level by itself has generally been regarded as insignificant or incidental. A recent reanalysis of the Framingham study concluded that hyperuricemia was not an independent risk factor for cardiovascular events after controlling for these other associated factors.2 Nevertheless, other studies have found an elevated serum uric acid level to be an independent risk factor for cardiovascular and renal disease (reviewed elsewhere3). In particular, several studies have reported that hyperuricemia is an independent predictor of stroke in diabetic subjects,4 individuals with isolated systolic hypertension,5,6 and the general population.7 A new study reported in this issue of Stroke 8 examining 2498 subjects admitted with acute stroke found that the admission serum uric acid also independently predicted worse outcome and a higher rate of repeated stroke or other cardiovascular event. Others have also reported that a higher uric acid level in patients with acute stroke is associated with poorer outcome.9 …

A study of VEGF and its receptors in two rat models of proteinuria

Background: The high level of expression of vascular endothelial growth factor (VEGF) in normal podocyte foot processes suggests that VEGF has an important role in maintaining normal glomerular function. While altered VEGF expression occurs in many glomerular diseases, a direct role for VEGF in the pathogenesis of proteinuria has not been demonstrated. Methods: Expression of VEGF and its receptors (VEGFR-1 and VEGFR-2) was examined in passive Heymann nephritis (PHN) and puromycin aminonucleoside nephrosis (PAN), by immunohistochemistry, in situ hybridization, Northern and Western blotting. Inhibition of VEGF in the PAN model was performed by administration of a blocking antibody. Results: In both models, glomeruli showed upregulation of VEGF and VEGF receptors compared to control animals. VEGF mRNA was increased most significantly (5-fold) at day 5 after induction of PHN, prior to the onset of proteinuria, with persistent upregulation (3-fold) at day 21. Increased VEGF mRNA was also seen in PAN, but it was less marked. In situ hybridization and immunohistochemistry localized VEGF predominantly to podocytes. Increased expression of VEGFR-1 and VEGFR-2 protein was seen in glomerular endothelial cells of PHN and PAN rats by immunohistochemistry, as was VEGFR-2 mRNA by in situ hybridization. Upregulation of VEGFR-1 by endothelial cells was more striking in the PAN model than PHN. Administration of a blocking antibody to rats with PAN did not affect proteinuria, creatinine clearance or sodium excretion. Conclusion: The expression of VEGF and its receptors is significantly increased in the PHN and PAN rat models of proteinuria suggesting a role for VEGF in the disease process. VEGF may have an important role in promoting glomerular repair in a variety of glomerular diseases.

Upregulation of heparin‐binding epidermal growth factor‐like growth factor and osteopontin in experimental hydronephrosis

SUMMARY This study examined the expression of heparin‐binding epidermal growth factor‐like growth factor (HB‐EGF) and osteopontin in unilateral ureteral obstruction (UUO) in the rat, a model of obstructive uropathy. HB‐EGF mRNA was upregulated 5.5‐fold at 4 h post‐obstruction (P < 0.05) and 4.5‐fold after 12 h (P < 0.05). Immunohistochemical staining for HB‐EGF demonstrated an increase in protein in the distended tubules. To determine what effects increased HB‐EGF might have in the obstructed kidney, we attempted to determine whether HB‐EGF upregulates osteopontin and α‐smooth muscle actin (α‐SMA) in the tubular line NRK‐52E. Both of these molecules are increased in UUO. Osteopontin mRNA was upregulated in NRK‐52E cells after 24, 48 and 72 h HB‐EGF stimulation. In contrast, HB‐EGF caused a downregulation of α‐SMA protein by Western blot in NRK‐52E cells. When a blocking mAb against secreted HB‐EGF was administered, however, there was no effect on osteopontin mRNA levels or immunohistochemical staining for α‐smooth muscle actin. These data suggest that the action of HB‐EGF in UUO may be to increase osteopontin and reduce α‐smooth muscle actin expression by tubular epithelial cells by an autocrine or intracrine mechanism. By reducing α‐SMA expression, HB‐EGF may also act to maintain epithelial cell morphology in this model.