Nox family NADPH oxidases in mechano-transduction: mechanisms and consequences

Abstract

Significance: The majority of cells in a multi-cellular organism are continuously exposed to ever-changing physical forces. Mechano-transduction links these events to appropriate reactions of the cells involving stimulation of signaling cascades, reorganization of the cytoskeleton and alteration of gene expression.

Recent advances: Mechano-transduction alters the cellular redox balance and the formation of reactive oxygen species (ROS). Nicotine amide adenine dinucleotide reduced form (NADPH) oxidases of the Nox family are prominent ROS generators and thus, contribute to this stress-induced ROS formation.

Critical issues: Different types and patterns of mechano-stress lead to Nox-dependent ROS formation and Nox-mediated ROS formation contributes to cellular responses and adaptation to physical forces. Thereby, Nox enzymes can mediate vascular protection during physiological mechano-stress. Despite this, over-activation and induction of Nox enzymes and a subsequent substantial increase in ROS formation also promotes oxidative stress in pathological situations like disturbed blood flow or extensive stretch.

Future directions: Individual protein targets of Nox-mediated redox-signaling will be identified to better understand the specificity of Nox-dependent ROS signaling in mechano-transduction. Nox-inhibitors will be tested to reduce cellular activation in response to mechano-stimuli.

FIG. 1.

The Nox family of NADPH oxidases. ROS formation occurs at the large catalytic Nox or Duox protein, which forms homo- or heterodimers in the plasma membrane. With the exception of Nox4, activation processes are required to allow Nox-dependent ROS formation. Whereas for Nox5 and the Duox enzymes, an increase in intracellular calcium is sufficient to stimulate the enzyme, Nox1 and Nox2 require a complex interaction with cytosolic proteins. Although Nox4 is constitutively active, it appears that TGF β1 further stimulates its enzymatic activity. NADPH, nicotine amide adenine dinucleotide reduced form; Nox, NADPH oxidase; PKA, protein kinase A; PKC, protein kinase C; ROS, reactive oxygen species; TGF, transforming growth factor. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars

FIG. 2.

Mechano-stimulation and subsequent ROS formation in a blood vessel. The figure attempts to relate different patterns of mechano-stimulation to ROS formation. ROS production appears to increase in response to cyclic stretch but the response to static stretch is incompletely studied. During the initiation and after cessation of flow, ROS production transiently increases, whereas stationary oscillatory flow results in a stable increase in ROS. The response to unidirectional flow with superimposed oscillation is variable and obviously depends on the intensity of the individual component. ???, ROS production under this condition has not been systematically studied.

FIG. 3.

Interactions of hormone and stretch-induced ROS production in blood vessels. Numerous vasoconstrictors induce and activate Nox proteins, which contribute to events that directly or indirectly increase vascular tone or filling and thus, tangential vascular wall stress. The subsequent stretch, in turn, activates signaling cascades that also stimulate Nox activation. Thus, Nox-dependent ROS production somehow appears to contribute but also to arise from increased vascular strain. GFR, glomerular filtration rate; S1P, sphingosine-1-phosphate; SMC, smooth muscle cell.

FIG. 4.

Nox-dependent ROS production in response to oscillatory flow. Oscillatory flow leads to an oscillating displacement of the apical and basal cell membrane and imposes altering forces on integrins, the molecules that largely connect the cell to the extracellular matrix. In endothelial cells, subsequently, Rac1 activation occurs which stimulates Nox2 and leads to the generation of bone morphogenic protein 4 (BMP4), a strong inducer of Nox1. Upon continuation of oscillatory flow, also Nox1 is activated by Rac1. The resulting massive ROS formation increases the activity of proinflammatory transcription factors, such as AP-1 and NFκB, which enhance the expression of adhesion molecules and induction of cyclooxygenase 2 (Cox2). AP-1, activator protein 1; ICAM, intercellular adhesion molecule; NFκB, nuclear factor kappa B. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars

FIG. 5.

Attenuation of Nox-dependent ROS formation by laminar flow under steady state conditions. Laminar flow increases the expression of eNOS and CuZnSOD, which limits O₂⁻ availability. NO also decreases Rac-1 activity and Nox2 expression. Laminar flow also activates PKA which increases eNOS activity and via promoting the interaction with 14-3-3 proteins leads to an inhibition of NoxA1. eNOS, endothelial nitric oxidase synthase; NO, nitric oxide; PKA, protein kinase A. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars

FIG. 6.

Potential role of Nox4 in endothelial flow signaling. Shear stress activates TGF-β signaling by incompletely understood mechanisms. TGF-β activates and induces Nox4. The subsequent H₂O₂ formation via p38 MAP kinase leads to eNOS phosphorylation and increases NO production. H₂O₂ also induces the MAP kinase phosphatase-1 (MKP-1). Whereas NO decreases Nox4 expression, MKP-1 attenuates the Nox4-mediate p38 MAP kinase activation. Thus, via two negative feed-back loops the systems limits its activity. MAP kinase, mitogen-activated protein kinase. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars