Calcium pyrophosphate dihydrate crystal-induced inhibition of neutrophil apoptosis: involvement of Bcl-2 family members

Systematic Overview of Solid Particles and Their Host Responses

Crystalline/particulate substances trigger a plethora of signaling events in host cells. The most prominent consequence is the inflammatory reactions that underlie crystal arthropathies, such as gout and pseudogout. However, their impact on our health was underestimated. Recent work on the role of cholesterol crystal in the development of atherosclerosis and the harm of environmental particulates has set up new frontiers in our defense against their detrimental effects. On the other hand, in the last 100 years, crystalline/particulate substances have been used with increasing frequencies in our daily lives as a part of new industrial manufacturing and engineering. Importantly, they have become a tool in modern medicine, used as vaccine adjuvants and drug delivery vehicles. Their biological effects are also being dissected in great detail, particularly with regard to their inflammatory signaling pathways. Solid structure interaction with host cells is far from being uniform, with outcomes dependent on cell types and chemical/physical properties of the particles involved. In this review, we offer a systematic and broad outlook of this landscape and a sage analysis of the complex nature of this topic.

Differential effects of the autophagy inhibitors 3-methyladenine and chloroquine on spontaneous and TNF-α-induced neutrophil apoptosis

Autophagy and apoptosis cooperate to modulate cell survival. Neutrophils are short-lived cells and apoptosis is considered to be the major mechanism of their death. In the present study, we addressed whether autophagy regulates neutrophil apoptosis and investigated the effects of autophagy inhibition on apoptosis of human neutrophils. We first showed that the established autophagy inhibitors 3-methyladenine (MA) and chloroquine (CQ) markedly accelerated spontaneous neutrophil apoptosis as was evidenced by phosphatidylserine exposure, DNA fragmentation and caspase-3 activation. Apoptosis induced by the autophagy inhibitors was completely abrogated by a pan-caspase inhibitor Q-VD-OPh. Unexpectedly, both MA and CQ significantly delayed neutrophil apoptosis induced by TNF-α, although the inhibitors did attenuate late pro-survival effect of the cytokine. The effect was specific for TNF-α because it was not observed in the presence of other inflammation-associated cytokines (IL-1β or IL-8). The autophagy inhibitors did not modulate surface expression of TNF-α receptors in the absence or presence of TNF-α. Both MA and CQ induced a marked down-regulation of a key anti-apoptotic protein Mcl-1 but did not affect significantly the levels of another anti-apoptotic protein Bcl-X(L). Finally, to confirm the effects of the pharmacological inhibition of autophagy by a genetic approach, we evaluated the consequences of siRNA-mediated autophagy suppression in neutrophil-like differentiated HL60 cells. Knockdown of ATG5 in the cells resulted in accelerated spontaneous apoptosis but attenuated TNF-α-induced apoptosis. Together, these data suggest that autophagy regulates neutrophil apoptosis in an inflammatory context-dependent manner and mediates the early pro-apoptotic effect of TNF-α in neutrophils.

Crystals, inflammation, and osteoarthritis

PURPOSE OF REVIEW

Calcium pyrophosphate dihydrate (CPPD) and basic calcium phosphate (BCP) crystals are common components of osteoarthritic joint fluids and tissues. Why these crystals form and how they contribute to joint damage in osteoarthritis remain unclear. With renewed interest in inflammation as a key component of osteoarthritis the role of calcium-containing crystals in this common disease warrants re-examination.

RECENT FINDINGS

There is ample evidence supporting a pathogenic role for inflammation in osteoarthritis, and the innate immune system likely participates in this inflammatory process. Recent work reinforces the almost universal existence of calcium-containing crystals in tissues from patients with end-stage osteoarthritis. Calcium-containing crystals may contribute to inflammation in osteoarthritis tissues through their direct interactions with components of the innate immune system, as well as by inducing or amplifying other inflammatory signals.

SUMMARY

There is increasing evidence that calcium-containing crystals contribute to osteoarthritis and their inflammatory properties may mediate detrimental effects through innate immunity signals. Calcium-containing crystals may thus represent important therapeutic targets in osteoarthritis.

Pathophysiology of articular chondrocalcinosis--role of ANKH

Calcium pyrophosphate (CPP) crystal deposition (CPPD) is associated with ageing and osteoarthritis, and with uncommon disorders such as hyperparathyroidism, hypomagnesemia, hemochromatosis and hypophosphatasia. Elevated levels of synovial fluid pyrophosphate promote CPP crystal formation. This extracellular pyrophosphate originates either from the breakdown of nucleotide triphosphates by plasma-cell membrane glycoprotein 1 (PC-1) or from pyrophosphate transport by the transmembrane protein progressive ankylosis protein homolog (ANK). Although the etiology of apparent sporadic CPPD is not well-established, mutations in the ANK human gene (ANKH) have been shown to cause familial CPPD. In this Review, the key regulators of pyrophosphate metabolism and factors that lead to high extracellular pyrophosphate levels are described. Particular emphasis is placed on the mechanisms by which mutations in ANKH cause CPPD and the clinical phenotype of these mutations is discussed. Cartilage factors predisposing to CPPD and CPP-crystal-induced inflammation and current treatment options for the management of CPPD are also described.