Andras Kapus

Integrity of cell-cell contacts is a critical regulator of TGF-β1-induced epithelial-to-myofibroblast transition: Role for β-catenin

Injury of the tubular epithelium and TGF-β1-induced conversion of epithelial cells to α-smooth muscle actin (SMA)-expressing myofibroblasts are key features of kidney fibrosis. Since injury damages intercellular junctions and promotes fibrosis, we hypothesized that cell contacts are critical regulators of TGF-β1-triggered epithelial-to-mesenchymal transition (EMT). Here we show that TGF-β1 was unable to induce EMT in intact confluent monolayers, but three different models of injury-induced loss of epithelial integrity (subconfluence, wounding, and contact disassembly by Ca2+-removal) restored its EMT-inducing effect. This manifested in loss of E-cadherin, increased fibronectin production and SMA expression. TGF-β1 or contact disassembly alone only modestly stimulated the SMA promoter in confluent layers, but together exhibited strong synergy. Since β-catenin is a component of intact adherens junctions, but when liberated from destabilized contacts may act as a transcriptional co-activator, we investigated its role in TGF-β1-provoked EMT. Contact disassembly alone induced degradation of E-cadherin and β-catenin, but TGF-β1 selectively rescued β-catenin and stimulated the β-catenin-driven reporter TopFLASH. Moreover, chelation of free β-catenin with the N-cadherin cytoplasmic tail suppressed the TGF-β1 plus contact disassembly-induced SMA promoter activation and protein expression. These results suggest a β-catenin-dependent two-hit mechanism in which both an initial epithelial injury and TGF-β1 are required for EMT.

Regulation of Toll-Like Receptor 4 Expression in the Lung Following Hemorrhagic Shock and Lipopolysaccharide

The Toll-like receptor 4 (TLR4) has recently been shown to function as the major upstream sensor for LPS. In this study, a rodent model of lung injury following resuscitated hemorrhagic shock was used to examine the regulation of TLR4 gene and protein expression in vivo and in vitro. Intratracheal LPS alone induced a rapid reduction in whole lung TLR4 mRNA, an effect which is also observed in recovered alveolar macrophages. This effect appeared to be due to a lowering of TLR4 mRNA stability by ∼69%. By contrast, while shock/resuscitation alone had no effect on TLR4 mRNA levels, it markedly altered the response to LPS. Specifically, antecedent shock prevented the LPS-induced reduction in TLR4 mRNA levels. This reversal was explained by the ability of prior resuscitated shock both to prevent the destabilization of TLR4 mRNA by LPS and also to augment LPS-stimulated TLR4 gene transcription compared with LPS alone. Oxidant stress related to shock/resuscitation appeared to contribute to the regulation of TLR4 mRNA, because supplementation of the resuscitation fluid with the antioxidant N-acetylcysteine reversed the ability of shock/resuscitation to preserve TLR4 mRNA levels following LPS. TLR4 protein levels in whole lung mirrored the changes seen for TLR4 mRNA. Considered in aggregate, these data suggest that levels of tlr4 expression are controlled both transcriptionally as well as posttranscriptionally through altered mRNA stability and that antecedent shock/resuscitation, a form of global ischemia/reperfusion, might influence regulation of this gene.

Actin depolymerization-induced tyrosine phosphorylation of cortactin: the role of Fer kinase.

The F-actin-binding protein cortactin is an important regulator of cytoskeletal dynamics, and a prominent target of various tyrosine kinases. Tyrosine phosphorylation of cortactin has been suggested to reduce its F-actin cross-linking capability. In the present study, we investigated whether a reciprocal relationship exists, i.e. whether the polymerization state of actin impacts on the cortactin tyrosine phosphorylation. Actin depolymerization by LB (latrunculin B) induced robust phosphorylation of C-terminal tyrosine residues of cortactin. In contrast, F-actin stabilization by jasplakinolide, which redistributed cortactin to F-actin-containing patches, prevented cortactin phosphorylation triggered by hypertonic stress or LB. Using cell lines deficient in candidate tyrosine kinases, we found that the F-actin depolymerization-induced cortactin phosphorylation was mediated by the Fyn/Fer kinase pathway, independent of Src and c-Abl. LB caused modest Fer activation and strongly facilitated the association between Fer and cortactin. Interestingly, the F-actin-binding region within the cortactin N-terminus was essential for the efficient phosphorylation of C-terminal tyrosine residues. Investigating the structural requirements for the Fer-cortactin association, we found that (i) phosphorylation-incompetent cortactin still bound to Fer; (ii) the isolated N-terminus associated with Fer; and (iii) the C-terminus alone was insufficient for binding. Thus the cortactin N-terminus participates in the Fer-cortactin interaction, which cannot be fully due to the binding of the Fer Src homology 2 domain to C-terminal tyrosine residues of cortactin. Taken together, F-actin stabilization prevents cortactin tyrosine phosphorylation, whereas depolymerization promotes it. Depolymerization-induced phosphorylation is mediated by Fer, and requires the actin-binding domain of cortactin. These results define a novel F-actin-dependent pathway that may serve as a feedback mechanism during cytoskeleton remodelling.

Oxidative Stress Reprograms Lipopolysaccharide Signaling via Src Kinase-dependent Pathway in RAW 264.7 Macrophage Cell Line

Oxidative stress generated during ischemia/reperfusion injury has been shown to augment cellular responsiveness. Whereas oxidants are themselves known to induce several intracellular signaling cascades, their effect on signaling pathways initiated by other inflammatory stimuli remains poorly elucidated. Previous work has suggested that oxidants are able to prime alveolar macrophages for increased NF-κB translocation in response to treatment with lipopolysaccharide (LPS). Because oxidants are known to stimulate the Src family of tyrosine kinases, we hypothesized that the oxidants might contribute to augmented NF-κB translocation by LPS via the involvement of Src family kinases. To model macrophage priming in vitro, the murine macrophage cell line, RAW 264.7, was first incubated with various oxidants and then exposed to low dose LPS. These studies show that oxidant stress is able to augment macrophage responsiveness to LPS as evidenced by earlier and increased NF-κB translocation. Inhibition of the Src family kinases by either pharmacological inhibition using PP2 or through a molecular approach by cell transfection with Csk was found to prevent the augmented LPS-induced NF-κB translocation caused by oxidants. Interestingly, while Src kinase inhibition was able to prevent the LPS-induced NF-κB translocation in oxidant-treated macrophages, this strategy had no effect on NF-κB translocation caused by LPS in the absence of oxidants. These findings suggested that oxidative stress might divert LPS signaling along an alternative signaling pathway. Further studies demonstrated that the Src-dependent pathway induced by oxidant pretreatment involved the activation of phosphatidylinositol 3-kinase. Involvement of this pathway appeared to be independent of traditional LPS signaling. Together, these studies provide a novel potential mechanism whereby oxidants might prime alveolar macrophages for altered responsiveness to subsequent inflammatory stimuli and suggest different cellular targets for immunomodulation following ischemia/reperfusion.

Hypertonic preconditioning prevents hepatocellular injury following ischemia/reperfusion in mice: A role for interleukin 10

Ischemia/reperfusion (I/R) of the liver occurs in many clinical scenarios including trauma, elective surgery, and transplantation. Events initiated by this process can lead to inflammation in the liver, culminating in local injury as well as distant organ dysfunction. Recent studies have suggested that hypertonic saline exerts anti‐inflammatory effects, which may be beneficial in preventing organ injury. In the present study, we examine the effect of hypertonic saline on the development of liver inflammation following I/R in both rat and mouse models. Hypertonic pretreatment was shown to prevent liver enzyme release concomitant with a reduction in liver neutrophil sequestration. Hypertonic saline appeared to exert this effect by inhibiting liver tumor necrosis factor α (TNF‐α) generation, an effect that culminated in reduced liver adhesion molecule expression. Hypertonic saline pretreatment was shown to augment liver interleukin 10 (IL‐10) expression following I/R, as a potential mechanism underlying its anti‐inflammatory effect. To examine the role of IL‐10 in the protective effect of hypertonic saline on liver I/R injury, we used a murine model of I/R. In wild type mice, hypertonic pretreatment similarly prevented liver injury induced by I/R. However, in IL‐10 knockout animals, hypertonic pretreatment was unable to prevent the liver enzyme release, TNF‐α generation, or neutrophil sequestration induced by I/R. In conclusion, these findings define a novel mechanism responsible for the anti‐inflammatory effects of hypertonic saline and also suggest a potential clinical role for hyperosmolar solutions in the prevention of liver injury associated with I/R. Supplementary material for this article can be found on the HEPATOLOGY website (http://interscience.wiley.com/jpages/0270‐9139/suppmat/index.html). (HEPATOLOGY 2004;40:211–220.)

Differential binding to dorsal and ventral cell surfaces of fibroblasts: effect on collagen phagocytosis

Matrix remodeling by phagocytic fibroblasts is essential for growth and development but the regulatory processes are undefined. We evaluated the impact of spreading on the binding step of collagen phagocytosis with a novel culture system that more closely replicates phagocytosis in vivo than previous models. 3T3 cells were plated on collagen-coated beads, thereby loading only ventral surfaces (adhesion with spreading), or were allowed to spread on collagen films and then loaded with beads on their dorsal surfaces (adhesion without spreading). Ventral surfaces bound three-fold more beads than dorsal surfaces which was accompanied by accelerated phagosomal maturation. Arp3 and cortactin, markers of the actin-associated spreading machinery, strongly accumulated around ventrally but not dorsally loaded beads, suggesting that spreading contributes to enhanced binding of ventral surfaces. Further, ventral surfaces exhibited two-fold more free alpha2beta1 integrins, the major collagen receptors. Notably, compared to cells spread on collagen substrates, spreading cells exhibited a three-fold higher alpha2beta1 mobile fraction which was correlated with limited engagement of ventral receptors by actin filaments. Thus integrin ligation by actin filaments regulates the mobility of collagen receptors which in turn mediates the enhanced binding of collagen beads on spreading surfaces.

Integrity of Cell-Cell Contacts Is a Critical Regulator of TGF-β1-Induced Epithelial-to-Myofibroblast Transition

Injury of the tubular epithelium and TGF-β1-induced conversion of epithelial cells to α-smooth muscle actin (SMA)-expressing myofibroblasts are key features of kidney fibrosis. Since injury damages intercellular junctions and promotes fibrosis, we hypothesized that cell contacts are critical regulators of TGF-β1-triggered epithelial-to-mesenchymal transition (EMT). Here we show that TGF-β1 was unable to induce EMT in intact confluent monolayers, but three different models of injury-induced loss of epithelial integrity (subconfluence, wounding, and contact disassembly by Ca2+-removal) restored its EMT-inducing effect. This manifested in loss of E-cadherin, increased fibronectin production and SMA expression. TGF-β1 or contact disassembly alone only modestly stimulated the SMA promoter in confluent layers, but together exhibited strong synergy. Since β-catenin is a component of intact adherens junctions, but when liberated from destabilized contacts may act as a transcriptional co-activator, we investigated its role in TGF-β1-provoked EMT. Contact disassembly alone induced degradation of E-cadherin and β-catenin, but TGF-β1 selectively rescued β-catenin and stimulated the β-catenin-driven reporter TopFLASH. Moreover, chelation of free β-catenin with the N-cadherin cytoplasmic tail suppressed the TGF-β1 plus contact disassembly-induced SMA promoter activation and protein expression. These results suggest a β-catenin-dependent two-hit mechanism in which both an initial epithelial injury and TGF-β1 are required for EMT.