The only in vivo evidence for a role
The only in vivo evidence for a role of DDR2 in fibrotic disease showed that DDR2 knockout mice exhibited exaggerated severity of chronic hepatic fibrosis. Although a previous in vitro study has demonstrated that DDR2 promotes lung fibroblast proliferation and migration, till now it remains unclear whether and how DDR2 contributes to the pathogenesis of pulmonary fibrosis. In this study, we initially found that DDR2 mutant mice were refractory to induction of experimental lung fibrosis. Furthermore in vitro studies showed that DDR2 can synergize the actions of both TGF-β and fibrillar collagen to stimulate lung fibroblasts to undergo myofibroblastic changes and vascular endothelial growth factor (VEGF) expression. We also confirmed that DDR2-targeting strategies potently inhibited the further progression of established lung fibrosis. Thus, our data suggest the potential of DDR2 as a therapeutic target for treatment of pulmonary fibrosis.
Discussion Based on the data from this study, we arrive at the following hypotheses regarding the role of DDR2 in lung fibrillogenesis (). In the early phase of fibrotic reaction, the induced Phos-tag of DDR2 in lung fibroblasts by TGF-β can synergize with TGF-β-initiated signals to specifically trigger the activation of p38 and Akt in an activation-independent manner, which accelerates the cell phenotypic transition toward myofibroblasts. When the fibrotic process is already established, the abundant fibrillar collagens within fibrotic foci may gradually augment the activation of myofibroblasts via DDR2/ERK axis in a positive feedback manner. In both phases, DDR2 promotes the oversynthesis of ECM components as well as the secretion of VEGF, resulting in massive fibrosis and angiogenesis. The persistence of myofibroblasts in injured tissues can trigger a relentless scarring process and finally cause fibrosis. Although the in vivo correlation of DDR2 with myofibroblast abundance has been previously documented in other models,, it remains poorly understood how DDR2 modulates the activation of this type of cells. A major finding of this study is that DDR2 expression deficiency hampers TGF-β1 induction of lung myofibroblast phenotypes. However, an intriguing phenomenon is that under resting conditions, DDR2-null lung fibroblasts express a higher level of α-SMA than wild-type ones, which was not observed in DDR2 mutant mice. In the light of the recent report that DDR2 silencing retards fibroblast migration, we infer that the upregulation of basal α-SMA expression caused by DDR2 deficiency may reflect a cellular mechanism to compensate for the complete loss of key migration molecules in vitro. Although the unresponsiveness of Smad activity to DDR2 has been documented in some types of cells,, its impact on non-Smad pathways under TGF-β stimulation has not been explored. We revealed that DDR2 could specifically strengthen the TGF-β-induced activation of p38 and Akt in lung fibroblasts. Because the cascades of both p38 and Akt were well established as key intermediates of lung fibroblast–myofibroblast differentiation as well of experimental lung fibrosis,,,,, our results suggest that DDR2 downregulation might not only suppress TGF-β-induced initiation of fibrotic reaction in the lung but also theoretically avoid undesirable potential side effects caused by pan-TGF-β blockade (autoimmunity and carcinogenesis). In addition, our disclosure of their crosstalk will also shed light on investigation of DDR2 function under other physiological or pathological situations where TGF-β plays a central role. Notably, the promoting effect of DDR2 on TGF-β signaling was shown to have nothing to do with the activation status of DDR2. This complies with the fact that DDR2 constitutively interacts with its coreceptor in other type of cells. Our ongoing study is to find the molecules that are responsible for determining the selectivity of DDR2 on TGF-β non-Smad pathway. It was recently shown that the IPF patient-derived ECM could stimulate lung fibroblasts to express genes encoding ECM proteins detected in IPF tissue, strongly supporting the concept that the abnormal ECM components accumulated within fibrotic lung might act as positive-feedback stimuli for the development of lung fibrosis. Compared with normal lung matrices, IPF lung was enriched in several types of collagens, with at least collagen types I and III being reported to activate DDR2. Our identification of the inducible effects of collagen I/DDR2 cascades on lung myofibroblast formation implies that DDR2 might be an attractive molecular target for the interruption of the fibrillar collagen-induced positive feedback loop events. This conjecture can be partially supported by our results that DDR2 knockdown from day 21, a time point with obvious established scar, can almost completely inhibit the further disorder of lung structure. Even collagen I was reported to induce DDR1 expression in human lung fibroblasts, this process was demonstrated to rely on the activation of DDR2 signaling.