Pulmonary Hypertension - Molecular Pathology of PAH

Molecular Pathology of PAH

The molecular mechanism of arterial pulmonary hypertension (PAH) is not known yet, but it is believed that the epithelial dysfunction results in a decrease in the synthesis of endothelium-derived vasodilators such as nitric oxide and prostacyclin. Moreover, there’s a stimulation of the synthesis of vasoconstrictors such as thromboxane and vascular endothelial growth factor (VEGF). These results in a severe vasoconstriction and smooth muscle and adventitial hypertrophy characteristic of patients with PAH.

In normal conditions, the nitric oxide synthase produces nitric oxide from L-arginine in presence of oxygen. Adenylate-cyclase and gualynate-cyclase are activated in presence of nitric oxide and these enzymes produce cAMP and cGMP respectively. The cGMP is produced by a type of guanylate cyclase (which is a kind of pyrophosphate-liase cyclase): the soluble guanylate cyclase (or sGC), that catalyzes the formation of cGMP from GTP. sGC is a heterodimer made up of one α subunit and one β sub-unit in each chain. It also contains a prosthetic heme group, required for NO binding. The union of NO and sGC produces a conformational enzyme change that stimulates cGMP production.

In the vascular endothelium, cGMP activates cGMP kinase or PKG (protein kinase G), which is a enzyme that belongs to a type of serine/threonine - specific protein kinase. PKG is a dimer composed of two similar polypeptides chains that share a common molecular structure. Each subunit contains a catalytic domain and regulatory domain. GMP-kinase activates potassium channels and subsequently the inhibition of calcium channels. Thus, this process leads to a reduction of intracellular calcium and finally a vasodilation.

Phosphodiesterase type V (PDE5), which is abundant in the pulmonary tissue, is a metalohydrolase that hydrolyzes the cyclic bond of cGMP in the presence of divalent cations (Zn2+). Actually, Zn2+ union is necessary for PDE5 activity. In the N-terminal region (catalytic center) of PDE5 there is an aminoacid sequence (residues 142-526) that joins cGMP. This sequence of PDE5 is divided in two domains; GAF-A and GAF-B; but only GAF-A has the necessary affinity to bind cGMP. This union increases the catalytic activity and it is stabilized by a close serine phosphorylation (performed by a kinase). Consequently, the concentration of cGMP decreases and the vasodilation is stopped.

Patients with PHA produce less NO and others vasodilators and produce more vasoconstrictors. Consequently, this molecular pathway doesn’t work properly and it results in a constant vasoconstriction. For this reason, NO and PDE5 inhibitors such as tadalafil or sildenafil are possible therapies. Tadalafil, for example, causes a vasodilation mediated by nitric oxide in the pulmonary endothelium.