Jani Lappalainen

BACKGROUND: Chronic inflammation of the arterial wall is a key element in the pathogenesis of atherosclerosis, yet the factors that trigger and sustain the inflammation remain elusive. Inflammasomes are cytoplasmic caspase-1-activating protein complexes that promote maturation and secretion of the proinflammatory cytokines interleukin(IL)-1beta and IL-18. The most intensively studied inflammasome, NLRP3 inflammasome, is activated by diverse substances, including crystalline and particulate materials. As cholesterol crystals are abundant in atherosclerotic lesions, and IL-1beta has been linked to atherogenesis, we explored the possibility that cholesterol crystals promote inflammation by activating the inflammasome pathway. PRINCIPAL FINDINGS: Here we show that human macrophages avidly phagocytose cholesterol crystals and store the ingested cholesterol as cholesteryl esters. Importantly, cholesterol crystals induced dose-dependent secretion of mature IL-1beta from human monocytes and macrophages. The cholesterol crystal-induced secretion of IL-1beta was caspase-1-dependent, suggesting the involvement of an inflammasome-mediated pathway. Silencing of the NLRP3 receptor, the crucial component in NLRP3 inflammasome, completely abolished crystal-induced IL-1beta secretion, thus identifying NLRP3 inflammasome as the cholesterol crystal-responsive element in macrophages. The crystals were shown to induce leakage of the lysosomal protease cathepsin B into the cytoplasm and inhibition of this enzyme reduced cholesterol crystal-induced IL-1beta secretion, suggesting that NLRP3 inflammasome activation occurred via lysosomal destabilization. CONCLUSIONS: The cholesterol crystal-induced inflammasome activation in macrophages may represent an important link between cholesterol metabolism and inflammation in atherosclerotic lesions.

Abstract Serum amyloid A (SAA) is an acute-phase protein, the serum levels of which can increase up to 1000-fold during inflammation. SAA has a pathogenic role in amyloid A-type amyloidosis, and increased serum levels of SAA correlate with the risk for cardiovascular diseases. IL-1β is a key proinflammatory cytokine, and its secretion is strictly controlled by the inflammasomes. We studied the role of SAA in the regulation of IL-1β production and activation of the inflammasome cascade in human and mouse macrophages, as well as in THP-1 cells. SAA could provide a signal for the induction of pro–IL-1β expression and for inflammasome activation, resulting in secretion of mature IL-1β. Blocking TLR2 and TLR4 attenuated SAA-induced expression of IL1B, whereas inhibition of caspase-1 and the ATP receptor P2X7 abrogated the release of mature IL-1β. NLRP3 inflammasome consists of the NLRP3 receptor and the adaptor protein apoptosis-associated speck-like protein containing CARD (a caspase-recruitment domain) (ASC). SAA-mediated IL-1β secretion was markedly reduced in ASC−/− macrophages, and silencing NLRP3 decreased IL-1β secretion, confirming NLRP3 as the SAA-responsive inflammasome. Inflammasome activation was dependent on cathepsin B activity, but it was not associated with lysosomal destabilization. SAA also induced secretion of cathepsin B and ASC. In conclusion, SAA can induce the expression of pro–IL-1β and activation of the NLRP3 inflammasome via P2X7 receptor and a cathepsin B-sensitive pathway. Thus, during systemic inflammation, SAA may promote the production of IL-1β in tissues. Furthermore, the SAA-induced secretion of active cathepsin B may lead to extracellular processing of SAA and, thus, potentially to the development of amyloid A amyloidosis.

Strenuous exercise induces oxidative stress and modification of intracellular proteins. Exercise training, however, upregulates endogenous antioxidant defenses and heat shock protein (HSP) expression. In diabetes, perturbations in the endogenous antioxidant and HSP protection have been reported. The aim of this study was to examine the effect of 8 wk of endurance training on HSP expression and oxidative stress markers in the skeletal muscle, heart, and liver of streptozotocin-induced diabetic (SID) and nondiabetic control rats. Induction of diabetes decreased HSP72 expression in heart, liver, and vastus lateralis muscles. SID increased heme oxygenase-1, an oxidative stress-inducible HSP, in liver, red gastrocnemius muscle, and vastus lateralis muscle and glucose-regulated protein 75 in liver. SID increased HSP90 levels in the heart, but levels decreased in the liver. Diabetes induced oxidative stress marker protein carbonyl levels and tissue inflammation. Although endurance training increased the expression of HSP72 in all of the tissues examined, this induction was less pronounced in diabetic rats than in nondiabetic controls. Furthermore, endurance training induced the activation and expression of transcriptional regulator heat shock factor-1 only in nondiabetic control animals. In summary, diabetes may increase susceptibility to oxidative damage and impair HSP protection, but endurance training may offset some of the adverse effects of diabetes by upregulating tissue HSP expression. Our results suggest that diabetes impairs HSP protection, possibly via transcriptionally mediated mechanisms.

The heat shock proteins (HSPs), originally identified as heat-inducible gene products, are a highly conserved family of proteins that respond to a wide variety of stress. Although HSPs are among the most abundant intracellular proteins, they are expressed at low levels under normal physiological conditions, and show marked induction in response to various stressors. HSPs function primarily as molecular chaperones, facilitating the folding of other cellular proteins, preventing protein aggregation, or targeting improperly folded proteins to specific pathways for degradation. By modulating inflammation, wound debris clearance, cell proliferation, migration and collagen synthesis, HSPs are essential for normal wound healing of the skin. In this review, our goal is to discuss the role and clinical implications of HSP with respect to skin wound healing and diabetes. The numerous defects in the function of HSPs associated with diabetes could contribute to the commonly observed complications and delayed wound healing in diabetics. Several physical, pharmacological and genetic approaches may be considered to address HSP-directed therapies both in the laboratory and in the clinics.