Signal transduction events involved in TPA downregulation of SP-A gene expression

Silica nanoparticle exposure inhibits surfactant protein A and B in A549 cells through ROS-mediated JNK/c-Jun signaling pathway

Exposure to silica nanoparticles (SiNPs) is related to the dysregulation of pulmonary surfactant that maintains lung stability and function. Nevertheless, there are limited studies concerning the interaction and influence between SiNPs and pulmonary surfactant, and the damage and mechanism are still unclear. Herein, we used A549 cells to develop an in vitro model, with which we investigated the effect of SiNPs exposure on the expression of pulmonary surfactant and the potential regulatory mechanism. The results showed that SiNPs were of cytotoxicity in regarding of reduced cell viability and promoted the production of excessive reactive oxygen species (ROS). Additionally, the JNK/c-Jun signaling pathway was activated, and the expression of surfactant protein A (SP-A) and surfactant protein B (SP-B) was decreased. After the cells being treated with N-acetyl-L-cysteine (NAC), we found that the ROS content was effectively downregulated, and the expression of proteins related to JNK and c-Jun signaling pathways was suppressed. In contrast, the expression of SP-A and SP-B was enhanced. Furthermore, we treated the cells with JNK inhibitor and c-Jun-siRNA and found that the expression of protein related to JNK and c-Jun signaling pathways, as well as SP-A and SP-B, changed in line with that of NAC treatment. These findings suggest that SiNPs exposure can upregulate ROS and activate the JNK/c-Jun signaling pathway in A549 cells, thereby inhibiting the expression of SP-A and SP-B proteins.

Toll-like receptor 2-mediated sequential activation of MyD88 and MAPKs contributes to lipopolysaccharide-induced sp-a gene expression in human alveolar epithelial cells

Surfactant proteins (SPs) produced by pulmonary epithelial cells participate in the regulation of sepsis-induced acute lung injury. Our previous study has shown that lipopolysaccharide (LPS), a Gram-negative bacterial outer membrane component, can regulate sp-a gene expression in human lung carcinoma type II epithelial A549 cells. This study was further designed to evaluate the signal-transducing mechanisms of LPS-induced sp-a gene expression. Exposure of A549 cells to LPS induced SP-A mRNA and protein production in time-dependent manners. Application of toll-like receptor 2 (TLR2) siRNA into A549 cells decreased the levels of this receptor and simultaneously inhibited LPS-induced SP-A mRNA expression. Sequentially, LPS enhanced phosphorylation of mitogen-activated protein kinase (MEK) 4 and c-Jun NH(2) terminal kinase 1 (JNK1) in time-dependent manners. Application of TLR2 siRNA decreased LPS-enhanced phosphorylation of MEK4 and JNK1. After knocking-down the translation of MyD88 by RNA interference, the LPS-triggered MEK4 phosphorylation was attenuated. Consequently, LPS augmented the translocation of c-Jun from the cytoplasm to nuclei without affecting c-Fos. Pretreatment of A549 cells with SP600125, an inhibitor of JNK1, significantly lowered LPS-induced SP-A mRNA production. Analyses of an electrophoretic mobility shift assay and a reporter gene further showed that LPS increased the transactivation activity of AP-1 in A549 cells. Therefore, the present study demonstrates that LPS can induce sp-a gene expression in human type II epithelial A549 cells through TLR2-mediated sequential activation of MyD88-MEK4-JNK1-AP-1.

Inhibition of TPA-induced cyclooxygenase-2 (COX-2) expression by apigenin through downregulation of Akt signal transduction in human keratinocytes

Apigenin is a nonmutagenic bioflavonoid that has been shown to be an inhibitor of mouse skin carcinogenesis induced by the two-stage regimen of initiation and promotion with dimethylbenzanthracene (DMBA) and 12-O-tetradecanoylphorbol-13-acetate (TPA). These DMBA/TPA-induced squamous cell carcinomas overexpress cyclooxygenase-2 (COX-2). Cyclooxygenases are key enzymes required for prostaglandin (PG) synthesis, converting the arachidonic acid (AA) released by phospholipase A2 into prostaglandins. A large body of evidence indicates that the inducible form of cyclooxygenase, COX-2, is involved in tumor promotion and carcinogenesis in a wide variety of tissue types, including colon, breast, lung, and skin. In the present study, we have determined that apigenin inhibited the TPA-induced increase in COX-2 protein and mRNA in the human keratinocyte cell line; HaCaT. The induction of COX-2 elicited by TPA correlated with increased activation of Akt kinase and cell treatment with the PI3 kinase inhibitor, LY294002, blocked TPA induction of COX-2. In cells treated with TPA and apigenin, the inhibition of COX-2 expression correlated with inhibition of Akt kinase activation. Apigenin-mediated inhibition of TPA-induced COX-2 expression was reversed by transient transfection with constitutively active Akt (CA-Akt). Chemical inhibitors of MEK (PD98059), p38 (SB202190), but not JNK (SP600125) blocked TPA induction of COX-2 although apigenin did not inhibit TPA-mediated COX-2 expression through these pathways. The TPA-induced release of AA from HaCaT cells was also inhibited by cell treatment with apigenin. These data show that apigenin inhibits TPA-mediated COX-2 expression by blocking signal transduction of Akt and that apigenin also blocks AA release, which may contribute to its chemopreventive activity.

Differences in the translation efficiency and mRNA stability mediated by 5'-UTR splice variants of human SP-A1 and SP-A2 genes

Surfactant protein A (SP-A) plays an important role in host defense, modulation of inflammatory processes, and surfactant-related functions of the lung. The human SP-A (hSP-A) locus consists of two functional genes, SP-A1 and SP-A2. Several hSP-A 5'-untranslated region (UTR) splice variants for each gene have been characterized and shown to be translated in vitro and in vivo. In this report, we investigated the role of hSP-A 5'-UTR splice variants on SP-A production and molecular mechanisms involved. We used in vitro transient expression of hSP-A 5'-UTR constructs containing luciferase as the reporter gene and quantitative real-time PCR to study hSP-A 5'-UTR-mediated gene expression. We found that 1) the four (A'D', ABD, AB'D', and A'CD') 5'-UTR splice variants under study enhanced gene expression, by increasing luciferase activity from 2.5- to 19.5-fold and luciferase mRNA from 4.3- to 8.8-fold compared with the control vector that lacked hSP-A 5'-UTR; 2) all four 5'-UTR splice variants studied regulated mRNA stability. The ABD variant exhibited the lowest rate of mRNA decay compared with the other three constructs (A'D', AB'D', and A'CD'). These three constructs also exhibited significantly lower rate of mRNA decay compared with the control vector; 3) based on the indexes of translational efficiency (luciferase activity/mRNA), ABD and AB'D' exhibited higher translational efficiency compared with the control vector, whereas the translational efficiency of each A'D' and A'CD' was lower than that of the control vector. These findings indicate that the hSP-A 5'-UTR splice variants play an important role in both SP-A translation and mRNA stability.

Role of protein kinase C in the monocyte-derived macrophage-mediated biodegradation of polycarbonate-based polyurethanes

Polycarbonate-polyurethanes (PCNUs) elicit a foreign body reaction during the initial tissue contact, partly mediated by the respiratory burst in monocytes, during which protein kinase C (PKC) activates NADPH (nicotinamide adenine dinucleotide phosphate) oxidase. Using an in vitro cell system, monocytes were differentiated into monocyte-derived macrophages (MDMs) and then reseeded onto three PCNUs (HDI431, HDI321, or MDI321): hexane (HDI) or 4,4-methylene bis-phenyl (MDI) diisocyanates synthesized with poly(1,6-hexyl 1,2-ethyl carbonate) diol (PCN) and 14C-labeled butanediol (BD) in the ratios 4:3:1 or 3:2:1 (diisocyanate/PCN/BD). MDM-mediated degradation was assessed by radiolabel release in the presence of a PKC activator (phorbol myristate acetate), inhibitor (H7), and a catalase/peroxidase inhibitor (NaN3). Activating PKC decreased biodegradation and esterase activity in MDMs on HDI431 and HDI321 but not MDI321, whereas H7 and NaN3 inhibited the MDM degradation of MDI321 only. Pretreatment of the PCNUs with H2O2 inhibited esterase-mediated radiolabel release from HDI431 and HDI321 but stimulated radiolabel release from MDI321. The difference in the effect of H2O2 on the HDI versus MDI PCNUs contributes to explaining the effect of PKC activation on material degradation. Understanding the mechanism by which this pathway is linked to PCNU chemistry may assist in designing materials with tailored biodegradation rates.