Tpl2 is required for VEGF-A-stimulated signal transduction and endothelial cell function

Abstract

New blood vessel sprouting (angiogenesis) and vascular physiology are fundamental features of metazoan species but we do not fully understand how signal transduction pathways regulate diverse vascular responses. The vascular endothelial growth factor (VEGF) family bind membrane-bound receptor tyrosine kinases (VEGFRs), which trigger multiple signal transduction pathways and diverse cellular responses. We evaluated whether the MAP3K family member and proto-oncoprotein Tpl2 (MAP3K8) regulates basal and VEGF-A-stimulated signal transduction in endothelial cells. Notably, stimulation with exogenous VEGF-A increased Tpl2 mRNA levels and consequently de novo protein synthesis. Depletion of Tpl2 levels reveals a role in both basal and VEGF-A-stimulated endothelial cell responses, including endothelial-leukocyte interactions, monolayer permeability and new blood vessel formation. Under basal conditions, Tpl2 modulates a signal transduction cascade resulting in phosphorylation of a nuclear transcription factor (ATF-2) and altered endothelial gene expression, a pathway previously identified as crucial in VEGF-dependent vascular responses. Loss of Tpl2 expression or activity impairs signal transduction through Akt, eNOS and ATF-2, broadly impacting on endothelial function. Our study now provides a mechanism for Tpl2 as a central component of signal transduction pathways in the endothelium.

ATF-2 and Tpl2 regulation of endothelial cell cycle progression and apoptosis

“…We have previously identified a signalling nexus between VEGFR2, the MAP3K family member Tpl2, and a transcription factor, ATF-2. In previous studies on VEGF-Astimulated and ATF-2-regulated endothelial gene expression on VCAM-1 [15] and Tpl2 [16], we find that maximal VEGF-A165-stimulated, ATF-2-regulated VCAM-1 synthesis occurs 8 h after VEGF-A addition. Under such conditions, there is complexity in decreased VEGFR2 levels caused by increased ubiquitination, endocytosis and degradation [15].…”

“…There is link between VEGF-A-stimulated ATF-2 phosphorylation and ATF-2 levels [15,16]. Activation of the p53 tumour suppressor is known to increase levels of p21, a negative cell cycle regulator, thus potentially impacting additional cell cycle modifiers including Cyclin D1 [18][19][20].…”

“…The MAP3K family member and oncoprotein, Tpl2, which is implicated in lung carcinogenesis [21] regulates VEGF-A-stimulated angiogenesis [22] and transduces signals from the cytosol to the nucleus in endothelial cells [16]. To assess whether Tpl2 is functionally linked to levels of ATF-2 and cell cycle regulators such as p53, we asked whether RNAi altered protein expression ( Fig.…”

“…One possibility is that signalling through the canonical MAPK pathway leads to Tpl2 phosphorylation, activation and translocation into the nucleus in a manner analogous to ERK1/2. It has been reported that Tpl2 activity is required for phosphorylation of nuclear factors including ATF-2 [16,32], implying a functional role in nuclear gene expression. Importantly, a direct nuclear role of Tpl2 has also been reported [30].…”

Tumor progression locus 2 (TPL2) in tumor-promoting Inflammation, Tumorigenesis and Tumor Immunity

“…TPL2 also plays a crucial role in growth factor and chemokine-induced tumor angiogenesis. Recently, it was shown that angiogenic factors such as VEGF, EGF and CXCL1 induce TPL2 activation, which in turn modulates endothelial-leukocyte interactions, monolayer permeability and new blood vessel formation through the regulation of Akt, endothelial NOS (eNOS) and ATF-2 60 , 61 . Moreover, TPL2 serves as a downstream effector of CXCL1, bFGF and EGF, where it activates the transcription factors C/EBPβ, NF-κB and AP-1 that induce VEGF expression, thereby promoting tumor angiogenesis 19 , 61 .…”