Although a specific CTGF receptor has not yet been identified, CTGF appears to perform many of its functions through integrins, heparin sulfate-containing proteoglycans, and the low-density lipoprotein receptor-related protein (LRP).
38 In chondrocytes, for example, CTGF regulates extracellular matrix, and both the expression and signaling properties of integrin α5 via ERK1/2 signaling.
39 CTGF activates the MAPK signaling pathway after binding to the lipoprotein receptor-related protein (LRP) via a heparan sulfate proteoglycan dependent process.
40 41 In renal myofibroblasts, CTGF induces tyrosine phosphorylation of the cytoplasmic domain of LRP and activates the ERK1/2 signaling pathway.
24 42 The MAPK are a family of serine-threonine protein kinases that are activated by a number of extracellular stimuli. ERK (p42/44
mapk), p38
mapk, and JNK are three major subfamilies of MAPK that mediate downstream effects such as cellular proliferation, differentiation, and apoptosis through activation of appropriate transcription factors.
43 44 Since activation of the MAPK pathways are dependent in part on the cell type, we performed experiments to determine whether any of these pathways were involved in matrix regulation resulting from CTGF stimulation of ARPE-19 cells by either inhibiting CTGF with either a neutralizing anti-CTGF antibody or validated blocking agents of these pathways after CTGF stimulation, and evaluating the production of fibronectin, laminin, and MMP-2 as a functional outcome. Our experiments support a role for both the ERK (p42/p44
mapk) and p38
mapk signaling pathways since we demonstrated phosphorylation of ERK1/2 and p38
mapk by Western blot analysis after exogenous CTGF stimulation of ARPE-19 cells. In addition, the specific pathway was clarified using specific inhibitors of the MAPK pathway. After stimulation with CTGF, the expression of fibronectin and MMP-2 were suppressed by the ERK1/2 inhibitor, PD98059, but not by the p38
mapk inhibitor, SB203580. These results suggest that CTGF induces signaling through the ERK1/2 pathway to increase fibronectin and MMP-2 production. On the other hand, laminin appears to be regulated through both the ERK1/2 and p38
mapk pathways since induction after CTGF stimulation was suppressed by both inhibitors. These results indicate crosstalk between these two signaling pathways. Yosimichi et al. have found that inhibition of one pathway can induce the induction of the other pathway.
45