Tadayoshi Karasawa

Crucial role of a long-chain fatty acid elongase, Elovl6, in obesity-induced insulin resistance

Insulin resistance is often associated with obesity and can precipitate type 2 diabetes. To date, most known approaches that improve insulin resistance must be preceded by the amelioration of obesity and hepatosteatosis. Here, we show that this provision is not mandatory; insulin resistance and hyperglycemia are improved by the modification of hepatic fatty acid composition, even in the presence of persistent obesity and hepatosteatosis. Mice deficient for Elovl6, the gene encoding the elongase that catalyzes the conversion of palmitate to stearate, were generated and shown to become obese and develop hepatosteatosis when fed a high-fat diet or mated to leptin-deficient ob/ob mice. However, they showed marked protection from hyperinsulinemia, hyperglycemia and hyperleptinemia. Amelioration of insulin resistance was associated with restoration of hepatic insulin receptor substrate-2 and suppression of hepatic protein kinase C ε activity resulting in restoration of Akt phosphorylation. Collectively, these data show that hepatic fatty acid composition is a new determinant for insulin sensitivity that acts independently of cellular energy balance and stress. Inhibition of this elongase could be a new therapeutic approach for ameliorating insulin resistance, diabetes and cardiovascular risks, even in the presence of a continuing state of obesity.

Critical role of caspase-1 in vascular inflammation and development of atherosclerosis in Western diet-fed apolipoprotein E-deficient mice

OBJECTIVE Recent investigations have suggested that the inflammasome plays a role in the development of vascular inflammation and atherosclerosis; however, its precise role remains controversial. We produced double-deficient mice for apolipoprotien E (Apoe) and caspase-1 (Casp1), a key component molecule of the inflammasome, and investigated the effect of caspase-1 deficiency on vascular inflammation and atherosclerosis. METHODS AND RESULTS Atherosclerotic plaque areas in whole aortas and aortic root of Western diet (WD)-fed Apoe(-/-)Casp1(-/-) mice were significantly reduced compared to those in Apoe(-/-) mice. The amount of macrophages and vascular smooth muscle cells in the plaques was also reduced in Apoe(-/-)Casp1(-/-) mice. No significant differences in plasma lipid profiles and body weight change were observed between these mice. Expression of interleukin (IL)-1β in the plaques as well as plasma levels of IL-1β, IL-1α, IL-6, CCL2, and TNF-α, in Apoe(-/-)Casp1(-/-) mice were lower than those in Apoe(-/-) mice. In vitro experiments showed that calcium phosphate crystals induced caspase-1 activation and secretion of IL-1β and IL-1α in macrophages. CONCLUSION Our findings suggest that caspase-1 plays a critical role in vascular inflammation and atherosclerosis, and that modulation of caspase-1 could be a potential target for prevention and treatment of atherosclerosis.

Inflammasome Activation by Mitochondrial Oxidative Stress in Macrophages Leads to the Development of Angiotensin II–Induced Aortic Aneurysm

Objective—Abdominal aortic aneurysm (AAA) is considered a chronic inflammatory disease; however, the molecular basis underlying the sterile inflammatory response involved in the process of AAA remains unclear. We previously showed that the inflammasome, which regulates the caspase-1–dependent interleukin-1&bgr; production, mediates the sterile cardiovascular inflammatory responses. Therefore, we hypothesized that the inflammasome is a key mediator of initial inflammation in AAA formation. Approach and Results—Apoptosis-associated speck-like protein containing a caspase recruitment domain is highly expressed in adventitial macrophages in human and murine AAA tissues. Using an established mouse model of AAA induced by continuous infusion of angiotensin II in Apoe–/– mice, NLR family pyrin domain containing 3 (NLRP3), apoptosis-associated speck-like protein containing a caspase recruitment domain, and caspase-1 deficiency in Apoe–/– mice were shown to decrease the incidence, maximal diameter, and severity of AAA along with adventitial fibrosis and inflammatory responses significantly, such as inflammatory cell infiltration and cytokine expression in the vessel wall. NLRP3, apoptosis-associated speck-like protein containing a caspase recruitment domain, and caspase-1 deficiency in Apoe–/– mice also reduced elastic lamina degradation and metalloproteinase activation in the early phase of AAA formation. Furthermore, angiotensin II stimulated generation of mitochondria-derived reactive oxygen species in the adventitial macrophages, and this mitochondria-derived reactive oxygen species generation was inhibited by NLRP3, apoptosis-associated speck-like protein containing a caspase recruitment domain, and caspase-1 deficiency. In vitro experiments revealed that angiotensin II stimulated the NLRP3 inflammasome activation and subsequent interleukin-1&bgr; release in macrophages, and this activation was mediated through an angiotensin type I receptor/mitochondria-derived reactive oxygen species–dependent pathway. Conclusions—Our results demonstrate the importance of the NLRP3 inflammasome in the initial inflammatory responses in AAA formation, indicating its potential as a novel therapeutic target for preventing AAA progression.

NLRP3 Regulates Neutrophil Functions and Contributes to Hepatic Ischemia–Reperfusion Injury Independently of Inflammasomes

Inflammation plays a key role in the pathophysiology of hepatic ischemia–reperfusion (I/R) injury. However, the mechanism by which hepatic I/R induces inflammatory responses remains unclear. Recent evidence indicates that a sterile inflammatory response triggered by I/R is mediated through a multiple-protein complex called the inflammasome. Therefore, we investigated the role of the inflammasome in hepatic I/R injury and found that hepatic I/R stimuli upregulated the inflammasome-component molecule, nucleotide-binding oligomerization domain–like receptor (NLR) family pyrin domain–containing 3 (NLRP3), but not apoptosis-associated speck–like protein containing a caspase recruitment domain (ASC). NLRP3−/− mice, but not ASC−/− and caspase-1−/− mice, had significantly less liver injury after hepatic I/R. NLRP3−/− mice showed reduced inflammatory responses, reactive oxygen species production, and apoptosis in I/R liver. Notably, infiltration of neutrophils, but not macrophages, was markedly inhibited in the I/R liver of NLRP3−/− mice. Bone marrow transplantation experiments showed that NLRP3 not only in bone marrow–derived cells, but also in non-bone marrow–derived cells contributed to liver injury after I/R. In vitro experiments revealed that keratinocyte-derived chemokine–induced activation of heterotrimeric G proteins was markedly diminished. Furthermore, NLRP3−/− neutrophils decreased keratinocyte-derived chemokine–induced concentrations of intracellular calcium elevation, Rac activation, and actin assembly formation, thereby resulting in impaired migration activity. Taken together, NLRP3 regulates chemokine-mediated functions and recruitment of neutrophils, and thereby contributes to hepatic I/R injury independently of inflammasomes. These findings identify a novel role of NLRP3 in the pathophysiology of hepatic I/R injury.

ASC in renal collecting duct epithelial cells contributes to inflammation and injury after unilateral ureteral obstruction.

Inflammation plays a crucial role in the pathophysiological characteristics of chronic kidney disease; however, the inflammatory mechanisms underlying the chronic kidney disease process remain unclear. Recent evidence indicates that sterile inflammation triggered by tissue injury is mediated through a multiprotein complex called the inflammasome. Therefore, we investigated the role of the inflammasome in the development of chronic kidney disease using a murine unilateral ureteral obstruction (UUO) model. Inflammasome-related molecules were up-regulated in the kidney after UUO. Apoptosis-associated speck-like protein containing a caspase recruitment domain deficiency significantly reduced inflammatory responses, such as inflammatory cell infiltration and cytokine expression, and improved subsequent renal injury and fibrosis. Furthermore, apoptosis-associated speck-like protein containing a caspase recruitment domain was specifically up-regulated in collecting duct (CD) epithelial cells of the UUO-treated kidney. In vitro experiments showed that extracellular adenosine triphosphate (ATP) induced inflammasome activation in CD epithelial cells through P2X7-potassium efflux and reactive oxygen species-dependent pathways. These results demonstrate the molecular basis for the inflammatory response in the process of chronic kidney disease and suggest the CD inflammasome as a potential therapeutic target for preventing chronic kidney disease progression.

Role of NLRP3 Inflammasomes for Rhabdomyolysis-induced Acute Kidney Injury

Rhabdomyolysis is one of the main causes of community-acquired acute kidney injury (AKI). Although inflammation is involved in the pathogenesis of rhabdomyolysis-induced AKI (RIAKI), little is known about the mechanism that triggers inflammation during RIAKI. Recent evidence has indicated that sterile inflammation triggered by tissue injury can be mediated through multiprotein complexes called the inflammasomes. Therefore, we investigated the role of NLRP3 inflammasomes in the pathogenesis of RIAKI using a glycerol-induced murine rhabdomyolysis model. Inflammasome-related molecules were upregulated in the kidney of RIAKI. Renal tubular injury and dysfunction preceded leukocyte infiltration into the kidney during the early phase of RIAKI, and they were markedly attenuated in mice deficient in NLRP3, ASC, caspase-1, and interleukin (IL)-1β compared with those in wild-type mice. No difference in leukocyte infiltration was observed between wild-type and NLRP3-deficient mice. Furthermore, NLRP3 deficiency strikingly suppressed the expression of renal injury markers and inflammatory cytokines and apoptosis of renal tubular cells. These results demonstrated that NLRP3 inflammasomes contribute to inflammation, apoptosis, and tissue injury during the early phase of RIAKI and provide new insights into the mechanism underlying the pathogenesis of RIAKI.

Sterol regulatory element-binding protein-1 determines plasma remnant lipoproteins and accelerates atherosclerosis in low-density lipoprotein receptor-deficient mice.

Objective—Sterol regulatory element–binding protein-1 (SREBP-1) is nutritionally regulated and is known to be a key transcription factor regulating lipogenic enzymes. The goal of this study was to evaluate the roles of SREBP-1 in dyslipidemia and atherosclerosis. Methods and Results—Transgenic mice that overexpress SREBP-1c in the liver and SREBP-1-deficient mice were crossed with low-density lipoprotein receptor (LDLR)–deficient mice, and the plasma lipids and atherosclerosis were analyzed. Hepatic SREBP-1c overexpression in LDLR-deficient mice caused postprandial hypertriglyceridemia, increased very-low-density lipoprotein (VLDL) cholesterol, and decreased high-density lipoprotein cholesterol in plasma, which resulted in accelerated aortic atheroma formation. Conversely, absence of SREBP-1 suppressed Western diet–induced hyperlipidemia in LDLR-deficient mice and ameliorated atherosclerosis. In contrast, bone marrow-specific SREBP-1 deficiency did not alter the development of atherosclerosis. The size of nascent VLDL particles secreted from the liver was increased in SREBP-1c transgenic mice and reduced in SREBP-1-deficient mice, accompanied by upregulation and downregulation of phospholipid transfer protein expression, respectively. Conclusion—Hepatic SREBP-1c determines plasma triglycerides and remnant cholesterol and contributes to atherosclerosis in hyperlipidemic states. Hepatic SREBP-1c also regulates the size of nascent VLDL particles.

Nanosilica-induced placental inflammation and pregnancy complications: Different roles of the inflammasome components NLRP3 and ASC.

Abstract Despite the increasing commercial use of nanoparticles, little is known about their effects on placental inflammation and pregnancy complications. In this study, nanosilica (NS) particles upregulated the inflammasome component nucleotide-binding oligomerization domain-like receptor (NLR) family pyrin domain-containing 3 (NLRP3) and induced placental inflammation and reactive oxygen species (ROS) generation, resulting in pregnancy complications. Furthermore, NS-induced pregnancy complications were markedly improved in Nlrp3−/− mice but not in component apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC)-deficient (Asc−/−) mice, indicating the independence of NLRP3 inflammasomes. Pregnancy complications in Nlrp3−/− and Asc−/− mice phenotypes were dependent on the balance between interleukin (IL)-1α and IL-10. NS-induced pregnancy complications were completely prevented by either inhibition of ROS generation or forced expression of IL-10. Our findings provide important information about NS-induced placental inflammation and pregnancy complications and the novel pathophysiological roles of NLRP3 and ASC in pregnancy.

Role of NLRP3 Inflammasomes in Atherosclerosis

Inflammation with macrophage infiltration is a key feature of atherosclerosis. Although the mechanisms had been unclear, emerging evidence unveiled that NLRP3 inflammasomes, which regulate caspase-1 activation and subsequent processing of pro-IL-1β, trigger vascular wall inflammatory responses and lead to progression of atherosclerosis. NLRP3 inflammasomes are activated by various danger signals, such as cholesterol crystals, calcium phosphate crystals, and oxidized low-density lipoprotein in macrophages, to initiate inflammatory responses in the atherosclerotic lesion. Recent studies have further clarified the regulatory mechanisms and the potential therapeutic agents that target NLRP3 inflammasomes. In this study, we reviewed the present state of knowledge on the role of NLRP3 inflammasomes in the pathogenesis of atherosclerosis and discussed the therapeutic approaches that target NLRP3 inflammasomes.

NLRP3 Protein Deficiency Exacerbates Hyperoxia-induced Lethality through Stat3 Protein Signaling Independent of Interleukin-1β

Background: The role of NLRP3 inflammasomes in hyperoxic acute lung injury (HALI) remains unclear. Results: NLRP3 deficiency exacerbated lethality and diminished Stat3 activation caused by inflammatory cells in a murine HALI model. Conclusion: NLRP3 regulates Stat3 activation by affecting inflammatory cell infiltration independent of IL-1β. Significance: These findings demonstrate the novel role of NLRP3 in Stat3-mediated protective effects against HALI. Supplemental oxygen inhalation is frequently used to treat severe respiratory failure; however, prolonged exposure to hyperoxia causes hyperoxic acute lung injury (HALI), which induces acute respiratory distress syndrome and leads to high mortality rates. Recent investigations suggest the possible role of NLRP3 inflammasomes, which regulate IL-1β production and lead to inflammatory responses, in the pathophysiology of HALI; however, their role is not fully understood. In this study, we investigated the role of NLRP3 inflammasomes in mice with HALI. Under hyperoxic conditions, NLRP3−/− mice died at a higher rate compared with wild-type and IL-1β−/− mice, and there was no difference in IL-1β production in their lungs. Under hyperoxic conditions, the lungs of NLRP3−/− mice exhibited reduced inflammatory responses, such as inflammatory cell infiltration and cytokine expression, as well as increased and decreased expression of MMP-9 and Bcl-2, respectively. NLRP3−/− mice exhibited diminished expression and activation of Stat3, which regulates MMP-9 and Bcl-2, in addition to increased numbers of apoptotic alveolar epithelial cells. In vitro experiments revealed that alveolar macrophages and neutrophils promoted Stat3 activation in alveolar epithelial cells. Furthermore, NLRP3 deficiency impaired the migration of neutrophils and chemokine expression by macrophages. These findings demonstrate that NLRP3 regulates Stat3 signaling in alveolar epithelial cells by affecting macrophage and neutrophil function independent of IL-1β production and contributes to the pathophysiology of HALI.