Transforming growth factor-beta1-induced activation of the Raf-MEK-MAPK signaling pathway in rat lung fibroblasts via a PKC-dependent mechanism

In fibroblasts transforming growth factor-beta1 (TGF-beta1) regulates cell proliferation and turnover of macromolecular components of the extracellular matrix. Here, intracellular signaling events in growth-inhibited embryonic rat lung fibroblasts (RFL-6) upon stimulation with TGF-beta1 were investigated. TGF-beta1 rapidly induced the activation of c-Raf-1, MEK-1, and MAPK p42 and p44. The activation of this pathway by TGF-beta1 did not depend on autocrine platelet-derived growth factor (PDGF) or basic fibroblast growth factor (bFGF). Inhibition of the binding of growth factors to their tyrosine kinase receptors did not affect MAPK activation by TGF-beta1. Ras activation by TGF-beta1 was significantly lower compared to the activation by PDGF or bFGF. The intracellular transduction of the TGF-beta1 signal was completely suppressed by depletion or inhibition of protein kinase C (PKC). It is shown that calcium-dependent isoforms of PKC are required for MAPK activation by TGF-beta1.

Loss of caveolin expression in type I pneumocytes as an indicator of subcellular alterations during lung fibrogenesis

Caveolin is a major structural protein of caveolae, also known as plasmalemmal vesicles, which are particularly abundant in type I pneumocytes and capillary endothelial cells of lung parenchyma. Here we demonstrate that caveolin expression in the alveolar epithelium of rats and mini pigs is strikingly downregulated after irradiation-induced lung injury. Indirect immunoperoxidase staining with polyclonal anti-caveolin antibodies, confirmed by double fluorescence studies with type I cell-specific monoclonal anti-cytokeratin antibodies or lectins, revealed a dramatic loss of caveolin immunoreactivity in type I pneumocytes. In contrast, caveolin expression increased in endothelial cells. Immunoblotting of lung homogenates from normal and irradiated rats using specific anti-caveolin antibodies confirmed the presence of caveolin in normal tissue and its marked decrease of expression in fibrotic tissue. The loss of caveolin as an important structural protein of caveolae in alveolar epithelial cells may be an early indicator of serious type I cell injury during fibrogenesis. The increase of caveolin immunoreactivity in endothelia of blood vessels may indicate that different types of caveolae and/or different regulatory mechanisms of caveolin expression exist.

Local production of interleukin-4 during radiation-induced pneumonitis and pulmonary fibrosis in rats: macrophages as a prominent source of interleukin-4

Fibrosis of lung tissue is a frequent and serious consequence of radiotherapy of mammary carcinoma. The pathogenesis of radiation-induced pulmonary fibrosis remains unclear. Cytokines such as transforming growth factor beta (TGFbeta) and interleukin-4 (IL-4) have been reported to stimulate collagen synthesis in fibroblasts in vitro. The aim of this study was to document the presence of IL-4 during the development of post-irradiation lung fibrosis. Right lungs of male Fischer rats were irradiated with a single dose of 20 Gy and IL-4 expression in the irradiated lungs was monitored for a period of three months. IL-4 gene transcription as determined by ribonuclease protection assay (RPA) as well as IL-4 synthesis as shown by Western blotting increased in the irradiated lungs reaching a plateau concentration within 3 weeks after irradiation. Enhanced IL-4 production was still detected at day 84 after irradiation. The cellular origin of IL-4 was analyzed by in situ hybridization and two-color immunofluorescence on lung tissue sections and on cytospin preparations of leukocytes obtained from bronchoalveolar lavages. These experiments revealed a substantial IL-4 production by macrophages during development of post-irradiation lung fibrosis.

Transforming growth factor-beta1 induces activation of Ras, Raf-1, MEK and MAPK in rat hepatic stellate cells

The transdifferentiation of hepatic stellate cells into myofibroblast-like cells and the proliferation of the transdifferentiated cells are controlled by TGF-beta1. Little is known about the intracellular signal transducers of TGF-beta1. In this paper we show that in cultured hepatic stellate cells TGF-beta1 induces activation of Ras, Raf-1, MEK and MAPK p42 and p44. The activation of MAPK depends on the activation of MEK. Our data exclude that the observed effects are mediated by a bFGF or PDGF autocrine loop.

Upregulation of gap junction protein connexin43 in alveolar epithelial cells of rats with radiation-induced pulmonary fibrosis

The degree of immunoreactive connexin43 (C x 43) in rat lung was evaluated during the development of radiation-induced pulmonary fibrosis in rat by a double immunofluorescence technique using polyclonal antisera to Cx43 and monoclonal antibodies to cytokeratins on cryostat sections. In normal rat lungs, Cx43 was detected in pneumocytes type II and I, in large blood vessel endothelia, in peribronchial smooth muscle cells, and in some peribronchial and perivascular interstitial cells. As early as 1 week after irradiation, enhanced immunoreactivity for Cx43 in the epithelial cells was detected. In severely injured lungs (about 3 months after irradiation), Cx43 was found also in the cytoplasm of type II pneumocytes. These findings were confirmed by western blot data. Western blot analysis also revealed increased phosphorylation of Cx43. It remains to be investigated whether the increased content of Cx43 in irradiated rat lung may be due to an enhanced number of gap junctions between type I and II alveolar epithelial cells.

Ras-dependent and -independent pathways target the mitogen-activated protein kinase network in macrophages

Mitogen-activated protein kinases (MAPKs) are activated upon a variety of extracellular stimuli in different cells. In macrophages, colony-stimulating factor 1 (CSF-1) stimulates proliferation, while bacterial lipopolysaccharide (LPS) inhibits cell growth and causes differentiation and activation. Both CSF-1 and LPS rapidly activate the MAPK network and induce the phosphorylation of two distinct ternary complex factors (TCFs), TCF/Elk and TCF/SAP. CSF-1, but not LPS, stimulated the formation of p21ras. GTP complexes. Expression of a dominant negative ras mutant reduced, but did not abolish, CSF-1-mediated stimulation of MEK and MAPK. In contrast, activation of the MEK kinase Raf-1 was Ras independent. Treatment with the phosphatidylcholine-specific phospholipase C inhibitor D609 suppressed LPS-mediated, but not CSF-1-mediated, activation of Raf-1, MEK, and MAPK. Similarly, down-regulation or inhibition of protein kinase C blocked MEK and MAPK induction by LPS but not that by CSF-1. Phorbol 12-myristate 13-acetate pretreatment led to the sustained activation of the Raf-1 kinase but not that of MEK and MAPK. Thus, activated Raf-1 alone does not support MEK/MAPK activation in macrophages. Phosphorylation of TCF/Elk but not that of TCF/SAP was blocked by all treatments that interfered with MAPK activation, implying that TCF/SAP was targeted by a MAPK-independent pathway. Therefore, CSF-1 and LPS target the MAPK network by two alternative pathways, both of which induce Raf-1 activation. The mitogenic pathway depends on Ras activity, while the differentiation signal relies on protein kinase C and phosphatidylcholine-specific phospholipase C activation.

Lipopolysaccharide induces activation of the Raf-1/MAP kinase pathway. A putative role for Raf-1 in the induction of the IL-1 beta and the TNF-alpha genes

Bacterial LPS is a potent macrophage activator. The early steps in LPS signal transduction involve the tyrosine phosphorylation and activation of a number of kinases of the src family, and inhibition of this pathway causes a severe impairment in the production of the cytokines TNF-alpha and IL-1 beta. We find that LPS-induced macrophages activation also involves the Raf-1 kinase, a key component in mitogenic signal transduction. Treatment of BAC-1.2F5 macrophages with LPS causes phosphorylation and activation of Raf-1. This is paralleled by the stimulation of MEK-1 and MAP-kinase activity and by the phosphorylation of the transcription factor Elk-1, a nuclear target of MAP-kinase. Activation of the Raf/MAP-kinase pathway was inhibited upon pretreatment of the cells with genistein, a tyrosine kinase inhibitor. Raf-1 must thus lie downstream of tyrosine kinase in LPS signal transduction. However, Raf-1 is not a direct substrate of a LPS-induced tyrosine kinase, because Raf-1 immunoisolated from LPS-induced cells contains only phosphoserine. This resembles the situation after CSF-1-stimulation of macrophages, in which Raf-1 clearly transduces a signal generated by the CSF-1 receptor kinase, but is phosphorylated exclusively in serine. Phosphopeptide maps of Raf-1 immunoprecipitated from LPS- or CSF-1-treated cells are indistinguishable, suggesting that these agents activate Raf-1 by similar mechanisms. Finally, v-raf-infected BAC-1.2F5 macrophages were found to constitutively express low levels of IL-1 beta and TNF-alpha. These data argue that Raf-1 functions downstream of tyrosine kinases in LPS-mediated macrophage activation and cytokine production.

Lipopolysaccharide induces activation of the Raf-1/MAP kinase pathway. A putative role for Raf-1 in the induction of the IL-1 beta and the TNF-alpha genes

Bacterial LPS is a potent macrophage activator. The early steps in LPS signal transduction involve the tyrosine phosphorylation and activation of a number of kinases of the src family, and inhibition of this pathway causes a severe impairment in the production of the cytokines TNF-alpha and IL-1 beta. We find that LPS-induced macrophages activation also involves the Raf-1 kinase, a key component in mitogenic signal transduction. Treatment of BAC-1.2F5 macrophages with LPS causes phosphorylation and activation of Raf-1. This is paralleled by the stimulation of MEK-1 and MAP-kinase activity and by the phosphorylation of the transcription factor Elk-1, a nuclear target of MAP-kinase. Activation of the Raf/MAP-kinase pathway was inhibited upon pretreatment of the cells with genistein, a tyrosine kinase inhibitor. Raf-1 must thus lie downstream of tyrosine kinase in LPS signal transduction. However, Raf-1 is not a direct substrate of a LPS-induced tyrosine kinase, because Raf-1 immunoisolated from LPS-induced cells contains only phosphoserine. This resembles the situation after CSF-1-stimulation of macrophages, in which Raf-1 clearly transduces a signal generated by the CSF-1 receptor kinase, but is phosphorylated exclusively in serine. Phosphopeptide maps of Raf-1 immunoprecipitated from LPS- or CSF-1-treated cells are indistinguishable, suggesting that these agents activate Raf-1 by similar mechanisms. Finally, v-raf-infected BAC-1.2F5 macrophages were found to constitutively express low levels of IL-1 beta and TNF-alpha. These data argue that Raf-1 functions downstream of tyrosine kinases in LPS-mediated macrophage activation and cytokine production.