Page 17 - Read Online

Page 12 of 14 Lust et al. Vessel Plus 2021;5:7 I http://dx.doi.org/10.20517/2574-1209.2020.52
[27]
myocardial infarction, and essential hypertension . In vascular smooth muscle cells, the prostacyclin (IP)
receptor is usually coupled to the G-protein subunit Gαs that is connected to a signaling cascade, which
[28]
promotes relaxation and a decrease in vascular tone . Consequently, prostacyclin synthase knockout
[29]
mice develop hypertension . For PGE2, the signaling pathways become even more complex, since PGE2
may activate at least four different receptors (EP1-EP4) coupled to several subtypes of G-proteins, which
drive multiple stimulatory as well as inhibitory effects in vascular cells . For instance, activation of EP
[30]
receptors in endothelial cells has been shown to trigger pro-inflammatory responses, MMP-9 expression,
[30]
and angiogenesis . In line with these observations, injury-induced arterial hyperplasia appears to be
[23]
controlled through the COX-2-PGE2-EP3 signaling axis , while knockout of microsomal prostaglandin
[31]
E2 synthase-1 inhibits this process .
Although supplementary corresponding analyses are lacking for venous diseases, the association of varicose
[13]
[14]
vein development with accentuated COX expression , increased PGE2 levels and decreased PGI2 levels
have been observed previously. Under these conditions, diclofenac may interfere with the activity of COX,
[32]
with a preference for COX-2 due to its pharmacological mechanism of action . As a consequence, lower
PGI2 and PGE2 levels would result in dampened EP-receptor-mediated signaling that is closely connected
with MAP kinases. This relationship may explain the reduced ERK1/2 phosphorylation in pressure-exposed
diclofenac-treated veins indicating a diminished cellular activation that is also reflected by the lower
abundance of active MMP-2. Consequently, in vivo, diclofenac may hamper the stress-induced proliferation
and proteolytic capacity of venous cells, attenuating or delaying varicose-like venous remodeling, as
evidenced in this study.

One of the limiting factors for the translational impact of our study is the utility of a mouse model, which
only mimics certain aspects of venous remodeling processes in humans. As such, although occlusion- or
reflux-mediated venous wall stresses induce comparable cellular responses in mice and humans [5,18] , and
venous harvesting and conduits in mice bear resemblance with venous diseases in men; their size, structure,
and overall rate of remodeling nevertheless differ significantly. Moreover, small animals are not generally
exposed to drastic changes in hydrostatic pressure levels even if they shortly stay in an erect position. For
instance, normal supine venous pressures of 5-10 mmHg in humans are comparable to the values measured
in mice (~ 6 mmHg); and under experimental conditions, the pressure in mouse veins may increase
three- to four-fold. In contrast, in the human venous vasculature, pressure levels of up to 24 mmHg
in the supine position and up to 120 mmHg in the erect posture, have been measured in association with
[33]
venous diseases .

On the molecular level however, many cellular responses in the remodeling veins are comparable between
the two species, considering their proteolytic, transcriptional, or proliferative activity [5,6,17,18] . In mice,
these parameters are influenced directly by the increase in the wall stress (thus indicating its relevance
as a trigger for venous remodeling); in humans, analysis is usually focused on veins that are in later
stages of the venous diseases progression, where the aforementioned parameters may also be affected by
additional determinants such as inflammatory processes. As our mouse model does not mimic aspects
associated with late-stage venous diseases progression, the impact of diclofenac on venous inflammation
and edema formation, specifically, would be better addressed in another context. However, considering its
broad spectrum of activity, inhibition of COX by diclofenac may indeed provide an additional benefit in
attenuating inflammatory responses as well as the associated sensations of pain, which is characteristic of
venous diseases as has been shown for acute varicophlebitis or acute superficial thrombophlebitis of the
lower limbs . Although the influence of diclofenac on early stages of venous remodeling or diseases has
[34]
not been analyzed in detail, it may diminish not only inflammation and thrombosis, but may also dampen
the prostanoid-mediated proteolytic activity and proliferation in the vessel wall, which slowly and steadily
has the potential to spur further venous disease development.