Upregulation of urokinase in alveolar macrophages and lung tissue in response to silica particles

Pulmonary Toxicity of Silica Linked to Its Micro- or Nanometric Particle Size and Crystal Structure: A Review

Silicon dioxide (SiO2) is a mineral compound present in the Earth’s crust in two mineral forms: crystalline and amorphous. Based on epidemiological and/or biological evidence, the pulmonary effects of crystalline silica are considered well understood, with the development of silicosis, emphysema, chronic bronchitis, or chronic obstructive pulmonary disease. The structure and capacity to trigger oxidative stress are recognized as relevant determinants in crystalline silica’s toxicity. In contrast, natural amorphous silica was long considered nontoxic, and was often used as a negative control in experimental studies. However, as manufactured amorphous silica nanoparticles (or nanosilica or SiNP) are becoming widely used in industrial applications, these paradigms must now be reconsidered at the nanoscale (<100 nm). Indeed, recent experimental studies appear to point towards significant toxicity of manufactured amorphous silica nanoparticles similar to that of micrometric crystalline silica. In this article, we present an extensive review of the nontumoral pulmonary effects of silica based on in vitro and in vivo experimental studies. The findings of this review are presented both for micro- and nanoscale particles, but also based on the crystalline structure of the silica particles.

Regulation of epithelial sodium channels in urokinase plasminogen activator deficiency

Epithelial sodium channels (ENaC) govern transepithelial salt and fluid homeostasis. ENaC contributes to polarization, apoptosis, epithelial-mesenchymal transformation, etc. Fibrinolytic proteases play a crucial role in virtually all of these processes and are elaborated by the airway epithelium. We hypothesized that urokinase-like plasminogen activator (uPA) regulates ENaC function in airway epithelial cells and tested that possibility in primary murine tracheal epithelial cells (MTE). Both basal and cAMP-activated Na(+) flow through ENaC were significantly reduced in monolayers of uPA-deficient cells. The reduction in ENaC activity was further confirmed in basolateral membrane-permeabilized cells. A decrease in the Na(+)-K(+)-ATPase activity in the basolateral membrane could contribute to the attenuation of ENaC function in intact monolayer cells. Dysfunctional fluid resolution was seen in uPA-disrupted cells. Administration of uPA and plasmin partially restores ENaC activity and fluid reabsorption by MTEs. ERK1/2, but not Akt, phosphorylation was observed in the cells and lungs of uPA-deficient mice. On the other hand, cleavage of γ ENaC is significantly depressed in the lungs of uPA knockout mice vs. those of wild-type controls. Expression of caspase 8, however, did not differ between wild-type and uPA(-/-) mice. In addition, uPA deficiency did not alter transepithelial resistance. Taken together, the mechanisms for the regulation of ENaC by uPA in MTEs include augmentation of Na(+)-K(+)-ATPase, proteolysis, and restriction of ERK1/2 phosphorylation. We demonstrate for the first time that ENaC may serve as a downstream signaling target by which uPA controls the biophysical profiles of airway fluid and epithelial function.

Type I interferons inhibit the generation of tumor-associated macrophages

Tumor-associated macrophages (TAM) are very abundant in tumors and are thought to play a major role in promoting tumor growth. The generation of TAM is positively regulated by several cytokines, including colony stimulating factor-1 (CSF-1) and monocyte chemoattractant protein-1 (CCL2). However, endogenous factors that suppress the generation of TAM within tumors have not been previously identified. An earlier study showed that endogenously produced type I interferons (IFN) suppressed tumor growth via their effects on hematopoietic cells rather than through direct effects on tumor cells. Therefore, we used mouse tumor models to investigate the effects of endogenously produced type I IFNs on the generation of TAM. We found using immunohistochemistry and flow cytometry that TAM density was significantly increased in tumors of mice lacking the type I IFN receptor (IFN-alpha/betaR(-/-) mice) compared to wild type mice. Moreover, the increase in TAM density was associated with a significant increase in tumor growth rate and angiogenesis. The phenotype of TAM was similar in IFN-alpha/betaR(-/-) mice and wild type mice and tumors in both mice produced similar amounts of CSF-1 and CCL2. However, in vitro assays indicated that low concentrations of type I IFNs significantly inhibited the generation of bone marrow macrophages in response to CSF-1. These findings indicate that endogenously produced type I IFNs suppress the generation of TAM, which may in turn account for inhibition of tumor growth and angiogenesis.

Silica Particles Enhance Peripheral Thrombosis

RATIONALE

Inflammation and thrombosis are related via interactions between leukocytes, platelets, the vasculature, and the coagulation system. However, the mechanisms behind these interactions remain poorly understood.

OBJECTIVES

We have investigated the effects of the well known pulmonary inflammation induced by silica for the development of peripheral thrombogenicity in a hamster model of thrombosis. In addition, the consequences of pulmonary macrophage and circulating monocyte and neutrophil depletion on the thrombogenicity were investigated.

METHODS

Silica particles (2-200 mug/hamster) were intratracheally instilled, and experimental thrombosis in photochemically induced femoral vein lesions was assessed 24 hours later, in association with cellular infiltration in the lung.

MEASUREMENTS AND MAIN RESULTS

Intratracheally instilled silica particles (20 and 200 mug/hamster) triggered pulmonary inflammation, together with stimulation of peripheral platelet-rich thrombus formation. Both the selective depletion of lung macrophages by intratracheal administration of clodronate liposomes and the combined depletion of circulating monocytes and neutrophils by intraperitoneal injection of cyclophosphamide significantly reduced silica-induced influx of macrophages and neutrophils in bronchoalveolar lavage, and reduced peripheral thrombogenicity. Silica-induced lung inflammation was accompanied by increased neutrophil elastase levels in bronchoalveolar lavage and in plasma. Specific neutrophil elastase inhibition in the lung did not affect lung inflammation but reduced peripheral thrombogenicity.

CONCLUSION

These findings uncover pulmonary macrophage-neutrophil cross-talk releasing neutrophil elastase into the blood circulation. Elastase, triggering activation of circulating platelets, may then predispose platelets to initiate thrombotic events on mildly damaged vasculature.

Modulation of proteolytic potential and differentiation by CNTF and BDNF in two mouse neuroblastoma clones: relation to invasion

The effect of CNTF and BDNF on a proteolytic complement instrumental to invasion and on differentiation was studied in two murine neuroblastoma clones, N1 and N7. At the membrane level, gelatinase MMP-2--mainly the activated form--was restrained by CNTF and BDNF to a residual 34% with both factors; membrane-type 1 MMP was down-regulated to 50% (10 h) and 34% (24 h) with both factors; and urokinase-type plasminogen activator was restrained mainly by BDNF to 70%. In the medium, the two gelatinases MMP-2 and MMP-9 were mainly in zymogen form: only MMP-2 was restrained in N1 cells, while only MMP-9 was restrained in N7 cells by both factors, single or in combination. These effects were paralleled by the induction of neurite outgrowth, which was more stimulated in the less differentiated clone. These dose-dependent and transient effects make CNTF and BDNF ideal candidates for constraining the potentially invasive behavior of nervous system tumors.

Matrix metalloproteinases 2, 9, and 13, and tissue inhibitors of metalloproteinases 1 and 2 in experimental lung silicosis

Exposure to silica induces granulomatous lung inflammation evolving to fibrosis through yet unclear pathogenic mechanisms. We examined the expression of extracellular matrix remodeling molecules: collagenase 3, gelatinases A and B, and TIMP-1 and TIMP-2 in experimental lung silicosis. Rats were instilled with 50 mg of silica and sacrificed after 15 and 60 d. At 60 d a significant increase in lung collagen content was found (170.2 +/- 34.4 versus 88.2 +/- 20.8 microgram/mg in controls, p = 0.01). Gelatin zymography of bronchoalveolar lavage fluid (BALF) from 15 and 60 d revealed bands of progelatinase A and progelatinase B, and lung tissue zymograms showed in addition, the active gelatinase A form at 15 d. By in situ hybridization and immunohistochemistry, early silicotic granulomas exhibited intense staining for all matrix metalloproteinases (MMPs) and TIMPs assayed. Labeling was restricted inside granulomas and surrounding areas. Late silicotic granulomas at 60 d showed lower MMP expression than did early lesions, and in highly fibrotic nodules scarce signal was usually found. TIMP-1 and TIMP-2 showed a moderate reduction in 60-d silicotic nodules. These findings suggest that an imbalance in the expression of MMPs and TIMPs may be implicated in extracellular matrix remodeling and basement membrane disruption during experimental lung silicosis.