Impaired Neutrophil Mechanoregulation by Fluid Flow: A Potential Contributing Factor for Microvascular Dysfunction in Obesity - The Mechanobiology of Obesity and Related Diseases - page 206

4.2 Neutrophil Responses to Fluid Shear Stress
Neutrophils exposed to shear stress levels typically found in the macro- and micro-
circulations display attributes of cell deactivation independent of cell adhesion
[ 93 ]. For example, both adherent and non-adherent (i.e., suspended) neutrophils
sense and respond similarly to shear stress by retracting, or minimizing their
projection of, pseudopods [ 99 , 100 ]. Notably, a key feature of this shear response
is its time-dependent nature. Shear-induced pseudopod retraction typically requires
*30 s of flow exposure before their first detection, with maximal reductions
occurring within 2 min of shear onset [ 11 ]. Other attributes of neutrophil deacti-
vation include F-actin depolymerization, cell detachment under a flow field (for
migrating cells), cleavage of CD18 integrins off the cell surface, and attenuation of
phagocytic activity [ 93 ]. In parallel, shear stress exposure also reportedly enhances
caspase 3-dependent apoptosis [ 101 ], in line with the relatively short lifespan
(18-24 h) of these cells as they passively circulate in the physiologic bloodstream.
Notably, the cell-deactivating effects of shear stress mechanotransduction are
consistent with its role as an anti-inflammatory mediator of neutrophil activity.
Additional evidence of the anti-inflammatory effects of shear stress comes from
in vitro and in vivo studies demonstrating that while low levels of cell agonists (e.g.,
\1 lM fMLP) do not affect shear-mediated neutrophil pseudopod retraction, these
same agonists above threshold levels (e.g., [1 lM fMLP) abolish it [ 98 - 100 ].
Intuitively, this is logical since the blood chemistry should be able to override the
mechanobiological response when neutrophil activation is required, such as during
acute inflammation. Recently, we also showed a similar relationship between fMLP
concentration and shear-related cleavage of CD18 integrins by HL60 neutrophil-
like cells (Fig. 4 )[ 102 ].
The underlying mechanisms associated with the cell-deactivating effects of
shear stress, however, remain to be elucidated, but they offer a possible source of
biological targets for therapeutic interventional strategies in dealing with chronic
inflammation. Reportedly, reactive oxygen species, such as superoxide, interfere
with the neutrophil shear response [ 11 , 98 ]. This suggests that oxidative stress
contributes to the blocking effects of threshold concentrations of cell agonists (e.g.,
fMLP) on flow-induced pseudopod retraction. Nitric oxide (NO) signaling path-
ways have also been implicated in the pseudopod retraction responses of neutro-
phils to shear. Interestingly, NO enhances neutrophil pseudopod retraction in
response to shear and is capable of offsetting the blocking effects of cell agonists
(e.g., fMLP and PAF) [ 99 ]. In contrast, blocking NO synthase activity in neu-
trophils has no impact on shear-induced pseudopod retraction [ 99 ]. Thus, exoge-
nous NO sources may have regulatory impact on neutrophil shear responses.
It has also been reported that neutrophil shear responses involve remodeling of
cytoskeletal F-actin in leukocytes via an effect on signaling by the Ras superfamily
of small guanine triphosphate (GTP)-binding proteins (see prior reviews [ 69 , 103 ]).
This class of signaling molecules includes Rac1, Rac2, and cdc42, three GTPases
that are involved in pseudopod projection processes. RhoA also plays a critical role