Smooth muscle cells were harvested after washing three times with cold PBS in homogenization buffer A (50 mm Tris-HCl, pH 7.5, 1.5 mm EDTA, 1 mm dithiothreitol, 10 μg/ml aprotinin, 5 μg/ml pepstatin, 20 μg/ml leupeptin, 1 mmbenzamidine, 0.2 mm sodium vanadate, 25 mmsodium fluoride). All cells were maintained in a humid incubator at 5% CO2 and 37 °C and were grown either on 6-well or 100-mm plates. PKG I-deficient mice were phenotypically indistinguishable from those reported previously ( 1 ). Mouse aortic smooth muscle cells from wild-type and PKG I-deficient mice were prepared by enzymatic digestion as described ( 10 ) and were used in the first passage.

PKG I-deficient mice were generated by homologous recombination in embryonic stem (ES) cells. Human uterine smooth muscle cells (Clonetics, San Diego, CA) were cultured in Dulbecco’s modified Eagle’s medium supplemented with 10% FBS and characterized as smooth muscle by expression of smooth muscle α-actin. We also utilized intact tissues and isolated cells from mice having disruptions in the PKG I gene.

These include human uterine and mouse aortic smooth muscle cells. More recently, a 50-70% increase in the activity of partially purified recombinant bovine PDE5 was reported after phosphorylation in vitro ( 7 ). However, no significant changes in PDE activity were detectable in studies designed to directly test this proposal.

The molecular mechanisms for regulation of PDE5 activity are not fully understood, particularly in intact tissues. Disruption of the PKG I gene also caused erectile dysfunction in mice ( 2 ). In male mice the ability to reproduce was greatly diminished, and no relaxation was induced by nerve stimulation of the corpus cavernosum. Nitric oxide, nitrovasodilators, and natriuretic peptides act as relaxants, regulating smooth muscle tone by direct activation of guanylyl cyclase, leading to the elevation of cGMP and activation of cGMP-dependent protein kinase (PKG).

These results strongly suggest that a major regulatory pathway for control of PDE5 phosphorylation and activity in intact smooth muscle is via PKG-dependent phosphorylation of PDE5. As with the human cells, no phosphorylation was seen in the mouse cells in response to 8-Br-cAMP. Nitric oxide and endogenous nitrovasodilators regulate smooth muscle tone by elevation of cGMP and activation of cyclic GMP-dependent protein kinase (PKG).

2. Ramachandran, I.R., et al., Myeloid-derived suppressor cells regulate growth of multiple myeloma by inhibiting T cells in bone marrow. Interestingly, MDSCs were not detected in any of the patients at baseline in both blood and marrow and this correlated with the lack of clinical response. Amongst these pathways, myeloid derived suppressor cells (MDSCs) play a critical role in suppressing immune function through upregulation of iNOS and arginase-1 (Arg1).

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