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Strassheim et al. Vessel Plus 2018;2:29 I http://dx.doi.org/10.20517/2574-1209.2018.44 Page 5 of 22
acting via 5-HT -G coupled and 5-HT 2A/2B -G coupled GPCRs, stimulates VSMC proliferation via the
q
1B
i
activation of the transcription factor GATA-4 and increase of cytokine generation from leukocytes, such
[46]
as dendritic cells . TxA level in PH is elevated due to up-regulation of thromboxane-A synthase . In-
[48]
2
creased presence of inflammatory cytokines, such as TNFa and IFNg, stimulates ET1 release from VSMC,
believed to be an important source of the vasoconstrictor ET-1 in PH. This effect of cytokines and ET1 is
[49]
antagonized by the PGI -IP axis .
2
GPCR ligand-dependent vasodilator response
In contrast to vasoconstrictors, several vasodilators are decreased in PH, promoting vasoconstriction in
pulmonary vascular system. Apelin, the ligand for CVD protective GPCR (APJ), modestly falls in PH pa-
tients (1.25 ng/mL vs. 0.89 ng/mL, P = 0.037) [50-52] . Decreased PGI synthase (PGIS) in ECs also plays a role
2
in vasodilation and inflammation [45,46] .

Increased activity of vasoconstrictor GPCRs
GPCR activity is frequently altered in diseases via internalization, phosphorylation, and expression levels.
In lung, increased activity of TxA and its G -coupled GPCR (TP) occurs via palmitoylation of TP and in-
q
2
creasing the proportion of the active receptor at the plasma membrane, consistent with pathophysiological
action of TP in PH [53-56] . Similarly, increased expression of other GPCRs involved in PH pathogenesis has
been noted for ET1 (ET ) and serotonin receptors, 5-HT R and 5-HT R in COPD-PH patients [54,55,57,58] .
A
1B
2B
Decreased activity of vasodilator GPCRs
In PH, decreased serum concentrations of PGI is accompanied by decrease in levels of the receptor IP,
2
reducing the effectiveness of PGI2 therapy . Similarly, chronic stimulation of PGI -IP axis, occurring
[59]
2
with prostacyclin therapy in PH patients, is likely to even further down regulate the PGI -IP axis via het-
2
erologous desensitization, compounding a pathogenic situation [60-62] . GPCRs such as IP, which increase
cAMP-PKA, frequently exert anti-inflammatory effects, inhibiting key pro-inflammatory/pro-proliferative
transcription factors, including NF-kB [63,64] , Hippo pathway transcription factors Yaz-Taz (co-factors for
[65]
the pro-proliferative transcription factor TEAD1) and, no doubt, many others . Induction of anti-in-
[66]
flammatory/anti-proliferative PPARg is also another mechanism, by which PGI acts . PPARg, along with
2
sibling, transcription factors PPARb/δ all are protective in PH and other cardiovascular diseases [34,66-71] . The
induction of PPARg activity by PGI was once thought to be a direct binding event to the PPARg, but it now
2
appears to occur by indirect mechanism. Activation of PKA or p38MAPK by PGI -IP stimulates the cAMP
2
response element-binding protein (CREB) by phosphorylation. Activated CREB turns on the transcription-
al co-activator, peroxisome proliferator-activated receptor gamma co-activator 1a (PGC1a) gene, increases
[71]
PGC1a activity and stimulates PPARg, leading to protective anti-inflammatory effects Molecular targets
[72]
of PPARg include inhibition of NF-kB and hypoxic activation of HIF-1a . HIF-1a is clearly important in
VSMC proliferation occurring in PH, as it helps the cell switch to a glycolytic/Warburg metabolic pheno-
type and has been connected to the increased expression of Ca entry channel, TRPC6, both aiding VSMC
2+
proliferation [73-76] . Targeted KO of HIF-1a inhibitor protein, prolyl-hydroxylase domain containing protein
2 (PHD2), reduced O -driven proteolysis of HIF-1a, thereby increasing HIF-1a-dependent proliferation of
2
[76]
VSMC . There are 3 PHD (PHD1-3) enzymes, which in presence of O hydroxylate proline residues, 402
2
and 564, ultimately resulting in the proteolysis of HIF-1a. A small molecule drug, R59949, a PDH inhibitor,
[76]
has shown potential to combat PH in the hypoxic mouse model .
Post-receptor mechanisms leading to increased vasoconstrictor GPCR response
In VSMC, Angiotensin II (Ang II) up regulates G expression, thereby increasing the activation of PLCb
i
2+
and mobilization of Ca , further enhancing vasoconstriction and proliferation by a post-receptor mecha-
[77]
nism . Of the PH pre-clinical therapeutics, RhoA-ROCK inhibitor, fasudil and statins both act at post
GPCR level [78,79] . Statins, such as simvastatin, can work in combination with sildenafil, the cGMP-PDE in-
hibitor, likely an important feature of any new therapy. Although some studies reported no drug combina-