The major finding of this study is the discovery of a novel EC survival pathway involving TXNIP-PARP1–mediated activation of VEGFR2. Specifically, we show that under basal conditions, TXNIP and PARP1 bind to each other and are located in the nucleus (). In response to inflammatory or oxidative stress stimuli (TNF or H2O2, respectively), PARP1 activity is stimulated and poly-ADP ribosylation of PARP1 maintains TXNIP in the nucleus. In contrast, inhibition of PARP1 with PJ34 disrupts PARP1-TXNIP binding, enabling translocation of TXNIP to the PM. At the PM, TXNIP promotes VEGFR2 activation and antiapoptotic signaling. These findings describe a novel signaling pathway for the VEGFR2 that involves nuclear to PM shuttling of TXNIP.
The present study provides 2 new findings important for understanding PARP1 biology. First, TXNIP is required for the antiapoptotic effect of PARP1 inhibition in EC through activation of the VEGFR2. PARP1 was originally shown to play a critical role in the cellular response to oxidative stress-induced DNA breaks.30
Although PARP1 protects cells from genotoxic stress, it appears that in chronic situations, PARP1 may contribute to the pathogenesis of several diseases. Specifically, PARP1 inhibition was shown to be beneficial in inflammatory diseases,6,8
post–myocardial infarction remodeling,5
PARP1 inhibition has been proposed to be beneficial by 2 mechanisms. When PARP1 is highly activated (eg, high dose of H2
), it consumes large amounts of cellular NADPH and, by reducing cellular energy bioavailability, promotes cell death. Furthermore, we and others have shown that PARP1 inhibition activated prosurvival signals: Akt in hepatic carcinoma cells and VEGFR2 in EC.26,31
The second finding pertinent to PARP1 biology is that PARP1 binding to TXNIP and PARP1 catalytic activity regulate TXNIP subcellular localization. These results are in agreement with previous publications that demonstrated that PARP1 regulated p53 and NFkB function by direct protein-protein interaction and/or poly-ADP ribosylation.2,4
We report that PARP1 is bound to TXNIP in the nucleus under basal conditions. On PARP1 depoly-ADP ribosylation, TXNIP is exported from the nucleus to the PM. Thus, PARP1 has a new role in the nuclear retention of TXNIP.
The present study provides further support for the hypothesis that TXNIP has important functions as an α-arrestin in addition to its role as the endogenous inhibitor of TRX.15,16
Accumulating data demonstrate both TRX-dependent and TRX-independent functions of TXNIP that involve the regulation of metabolism,23
and mitochondrial function.22,34
We show that TXNIP, as an α-arrestin, promotes PM signaling that is not linked to its function as the inhibitor of TRX. Specifically, we show that as a result of PARP1 inhibition, TXNIP stimulates transactivation of the VEGFR2, which is a key receptor for EC prosurvival signaling through regulation of the activity of proteins such as eNOS and Akt.26,35
It appears likely that TXNIP mediates transactivation of the VEGFR2 in response to stress, based on several recent studies. First, TXNIP acts as a scaffold to mediate TRX translocation to VEGFR227
and potentially regulate redox dependent activation, such as the recruitment of tyrosine phosphatases. Second, TXNIP, through its ITIM, PPxY, and SH3 domains, may facilitate a direct interaction with modulators of PM tyrosine kinase receptors, such as Grb2 and SHP2.25
The exact mechanism of TXNIP-mediated VEGFR2 activation is an important area for future investigation. Finally, the present study points to an important difference between α- and β-arrestins. The function of the β-arrestin family of proteins is well established and mediates primarily PM to cytosol and/ or nucleus communication.24
We report the novel finding that TXNIP, an α-arrestin protein, mediates nuclear to PM signaling. This suggests evolution of α-arrestins in mammals beyond the role of yeast arrestin-related trafficking adaptors in cargo transport from PM to endocytotic vesicles.36
Finally, the present data strongly support a critical role for TXNIP in the endothelial stress response. Specifically, we propose that TXNIP orchestrates multiple cellular response mechanisms that enable EC to survive acute stress long enough to mount an inflammatory response. For example, a traumatic injury that disrupts blood flow would cause disturbed flow patterns that induce TXNIP. Induction of TXNIP leads to inhibition of TRX, resulting in excessive ROS, protein oxidation, and the release of the proinflammatory protein ASK1. As a result, ASK1 activates nuclear signaling that leads to expression of VCAM-1.16,37
A similar stress response also occurs in EC exposed to inflammatory cytokines and hyperglycemia.38,39
Furthermore, TXNIP is involved in inflammasome activation and IL1β maturation, recruiting additional inflammatory cells.40
This central role for TXNIP in the response to injury and inflammation suggest that drugs that regulate its expression may have broad therapeutic applications.