Prolonged treatment of porcine pulmonary artery with nitric oxide decreases cGMP sensitivity and cGMP-dependent protein kinase specific activity

Newborn rat response to single vs. combined cGMP-dependent pulmonary vasodilators

Inhaled nitric oxide (NO) and other cGMP- or cAMP-dependent pulmonary vasodilators are often used in combination for the treatment of the persistent pulmonary hypertension of the newborn syndrome. There is in vitro evidence to indicate that NO downregulate the pulmonary vascular response to cGMP-dependent agonists raising concern as to whether a synergistic effect is observed when employing a combined strategy in newborns. Hypothesizing that a synergistic effect is absent, we evaluated newborn and juvenile rat pulmonary arteries to determine the individual and combined vasodilatory effect of cGMP- and cAMP-dependent agonists. In precontracted near-resistance pulmonary arteries, the addition of sildenafil reduced vasorelaxation response to NO donor S-nitroso-N-acetyl penicillamine (SNAP). A similar decrease in SNAP-induced vasodilation was observed in arteries pretreated with BAY 41-2272 (10(-9) M), a soluble guanylate cyclase stimulator cGMP, and its downstream protein kinase activator. cGMP also reduced the vasorelaxant response to the cAMP-dependent forskolin. Inhibition of endogenous vascular NO generation enhanced SNAP-induced relaxation. The present data suggest that the mechanism involved in the cGMP desensitization to other relaxant agonists involves downregulation of the small heat shock protein HSP20 and is evident in rat pulmonary and systemic vascular smooth muscle cells. In newborn rats with chronic hypoxia-induced pulmonary hypertension, the combination of sildenafil and inhaled NO resulted in a lesser reduction in pulmonary vascular resistance compared with their individual effect. These data suggest that clinical exposure to one cGMP-dependent pulmonary vasodilator may affect the response to other cGMP- or cAMP-mediated agonists.

Transcriptional and post-transcriptional regulation of cGMP-dependent protein kinase (PKG-I): pathophysiological significance

The ability of the endothelium to produce nitric oxide, which induces generation of cyclic guanosine monophosphate (cGMP) that activates cGMP-dependent protein kinase (PKG-I), in vascular smooth muscle cells (VSMCs), is essential for the maintenance of vascular homeostasis. Yet, disturbance of this nitric oxide/cGMP/PKG-I pathway has been shown to play an important role in many cardiovascular diseases. In the last two decades, in vitro and in vivo models of vascular injury have shown that PKG-I is suppressed following nitric oxide, cGMP, cytokine, and growth factor stimulation. The molecular basis for these changes in PKG-I expression is still poorly understood, and they are likely to be mediated by a number of processes, including changes in gene transcription, mRNA stability, protein synthesis, or protein degradation. Emerging studies have begun to define mechanisms responsible for changes in PKG-I expression and have identified cis- and trans-acting regulatory elements, with a plausible role being attributed to post-translational control of PKG-I protein levels. This review will focus mainly on recent advances in understanding of the regulation of PKG-I expression in VSMCs, with an emphasis on the physiological and pathological significance of PKG-I down-regulation in VSMCs in certain circumstances.

Differential roles of cAMP and cGMP in megakaryocyte maturation and platelet biogenesis

The cyclic nucleotides cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) regulate the activity of protein kinase A (PKA) and protein kinase G (PKG), respectively. This process helps maintain circulating platelets in a resting state. Here we studied the role of cAMP and cGMP in the regulation of megakaryocyte (MK) differentiation and platelet formation. Cultured, platelet-producing MKs were differentiated from fetal livers harvested from 13.5 days postcoital mouse embryos. MK development was accompanied by a dramatic increase in cAMP production and expression of soluble guanylate cyclase, PKG, and PKA as well as their downstream targets vasodilator-stimulated phosphoprotein (VASP) and MENA. Stimulation of prostaglandin E(1) receptor/adenylyl cyclase or soluble guanylate cyclase/PKG in cultured MKs increased VASP phosphorylation, indicating that these components share a common signaling pathway. To dissect out the role of cyclic nucleotides in MK differentiation, cAMP/PKA and cGMP/PKG signaling were alternately blocked in cultured MKs. Down-regulation of cAMP pathway effectors decreased MK numbers and ploidy. Notably, cGMP levels increased at the beginning of MK development and returned to basal levels in parallel with MK maturation. However, inhibition of cGMP pathway effectors had no effect on MK development. In addition, platelet release from mature MKs was enhanced by cGMP and inhibited by cAMP. Our data suggest that cAMP plays an important role in MK differentiation, while cAMP and cGMP have opposite effects on platelet production. Identifying the signaling pathways that underpin MK development and proplatelet formation will provide greater insights into thrombopoiesis and may potentially yield useful therapeutic targets.

Increased degradation of MYPT1 contributes to the development of tolerance to nitric oxide in porcine pulmonary artery

Myosin phosphatase target subunit 1 (MYPT1) is the regulatory subunit of myosin light chain phosphatase (MLCP). It plays a critical role in vasodilatation induced by cGMP-elevating agents such as nitric oxide (NO). The present study was performed to determine the role of MYPT1 in the development of tolerance of the pulmonary artery to NO. Incubation of isolated porcine pulmonary arteries for 24 or 48 h with DETA NONOate (DETA NO) significantly reduced protein levels of MYPT1 and the leucine zipper-positive (LZ+) isoform of MYPT1 but not that of PP1cdelta. The extent of reduction in total MYPT1 protein level was comparable to that of MYPT1 (LZ+). The decrease in MYPT1 protein caused by 48-h DETA NO incubation was prevented by ODQ, an inhibitor of guanylyl cyclase, and by inhibitors of proteasomes (MG-132 and lactacystin) but was not affected by the inhibitor of protein synthesis, cycloheximide. A reduction in MYPT1 protein was also obtained with 8-bromo-cGMP, but this was prevented by Rp-8-bromo-PET-cGMP [inhibitor of cGMP-dependent protein kinase (PKG)]. Incubation for 48 h with DETA NO also reduced dephosphorylation of myosin light chain and relaxation of the artery in response to DETA NO, which was prevented by MG-132. These results suggest that the reduction in MYPT1 protein contributes to the development of tolerance of pulmonary arteries to NO. This may result from increased degradation of MYPT1 after prolonged PKG activation.