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PDGF-AB and FGF-2 (GFs) induce smooth muscle cell (SMC) proliferation which is indispensible for arteriogenesis. While there is common agreement that GFs stimulate SMC proliferation through phosphorylation (P-) of MEK1/2 at Ser218/222, we previously demonstrated that the MEK inhibitors PD98059 and UO126 did not inhibit P-Ser218/222 as originally proposed but caused strong hyperphosphorylation. Here, we demonstrate that GFs increased phosphorylation of MEK1 at Thr292 while UO126 and PD98059 blocked this phosphorylation. This was again surprising since phosphorylation of Thr292 is regarded as a negative feedback loop. Our findings suggest that inhibition of Thr292 phosphorylation in combination with hyperphosphorylation of Ser218/222 serves as an "off" switch of SMC proliferation and potentially of arteriogenesis., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Objective: Collateral artery growth or arteriogenesis is the primary means of the circulatory system to maintain blood flow in the face of major arterial occlusions. Arteriogenesis depends on activation of fibroblast growth factor (FGF) receptors, but relatively little is known about downstream mediators of FGF signaling., Methods and Results: We screened for signaling components that are activated in response to administration of FGF-2 to cultured vascular smooth muscle cells (VSMCs) and detected a significant increase of Rap2 but not of other Ras family members, which corresponded to a strong upregulation of Rap2 and C-Raf in growing collaterals from rabbits with femoral artery occlusion. Small interfering RNAs directed against Rap2 did not affect FGF-2 induced proliferation of VSMC but strongly inhibited their migration. Inhibition of FGF receptor-1 (FGFR1) signaling by infusion of a sulfonic acid polymer or infection with a dominant-negative FGFR1 adenovirus inhibited Rap2 upregulation and collateral vessel growth. Similarly, expression of dominant-negative Rap2 blocked arteriogenesis, whereas constitutive active Rap2 enhanced collateral vessel growth., Conclusions: Rap2 is part of the arteriogenic program and acts downstream of the FGFR1 to stimulate VSMC migration. Specific modulation of Rap2 might be an attractive target to manipulate VSMC migration, which plays a role in numerous pathological processes.

Smooth muscle cells (SMCs) form the backbone of arteries and their proliferation hallmarks collateral vessel growth, a process termed arteriogenesis, as well as pathogenic responses such as restenosis. Since signaling pathways in SMCs are the main targets for therapeutic interventions, we aimed to determine how and to what extent the activation of the ubiquitous MEK-ERK signaling pathway correlates with important in vivo phenomena such as dedifferentiation, nuclear activation and proliferation of SMCs. Specificity of this pathway was monitored using MEK inhibitors UO126 and PD98059 in platelet derived growth factor-AB (PDGF-AB)- and fibroblast growth factor-2 (FGF-2)-stimulated SMCs. PDGF-AB induced a rapid MEK activation followed by phosphorylation of the MEK substrates ERK1/2 while FGF-2 showed a less pronounced and delayed activation. Both growth factors triggered a marked phosphorylation of c-Myc and expression of Egr1. Pretreatment with MEK inhibitors suppressed the activation of the ERK cascade, abolished the down-regulation of desmin and led to cell cycle arrest. However, the reversibility of p27Kip1 down-regulation by UO126 was mainly observed after PDGF-AB stimulation, indicating MEK independent p27Kip1 down-regulation by FGF-2. Surprisingly, treatment of SMCs with UO126 or PD98059 increased the level of MEK phosphorylation in a dose dependent manner at serine residues 217/221 in the presence as well as in the absence of both growth factors. Our results strongly imply that depending on the environmental context phosphorylation of serines 217/221 serves as an "on" as well as an "off " switch., (Copyright 2005 Wiley-Liss, Inc.)

Smooth muscle cells (SMC) and endothelial cells (EC) play a pivotal role in arteriogenesis and atherosclerosis. We evaluated the role of EC on the growth of SMC and neonatal cardiomyocytes (NEO) by using serum-free EC-supernatant (AoCM). Five percent fetal calf serum was used in order to mimic growth effects of blood. EC and SMC purities were 99% as determined by absence or presence of markers such as CD31, desmin, alpha-smooth muscle actin and tropomyosin using immunostaining and FACS analysis. AoCM markedly influenced the morphology of NEO as determined by alpha-actinin staining but showed only little effect on the phenotype of SMC. Protein synthesis after 2 days increased 2.5-fold in SMC and 3.7-fold in NEO as determined by tritium incorporation. The values for serum (2.8 and 2.3-fold, respectively) were comparable. The induction of DNA-synthesis by serum in NEO was twice that of AoCM (3.9-fold). However, proliferative effects of serum and AoCM on SMC differed markedly: Serum induced a 66-fold increase in DNA-synthesis resulting in a 54% higher cell number. DNA-synthesis after AoCM treatment lead to a nonsignificant small increase and no proliferation was detected. Platelet derived growth factor (PDGF-AB), present in blood, induced a 47-fold increase in DNA-synthesis and a 38% increase in cell number. Our data suggest that EC in the absence of physical forces exert strong morphogenic effects on cardiomyocytes but they lack specific effects on smooth muscle cells. In vessels EC might function as a border to isolate SMC from key regulators in blood such as PDGFs.