Regulation of pulmonary microvascular endothelial cell cyclic adenosine monophosphate by adenylyl cyclase: implications for endothelial barrier function

Functional coupling of TRPV4, IK, and SK channels contributes to Ca(2+)-dependent endothelial injury in rodent lung

Our previous work has shown that the increased lung endothelial permeability response to 14,15-epoxyeicosatrienoic acid (14,15-EET) in rat lung requires Ca(2+) entry via vanilloid type-4 transient receptor potential (TRPV4) channels. Recent studies suggest that activation of TRPV4 channels in systemic vascular endothelium prolongs agonist-induced hyperpolarization and amplifies Ca(2+) entry by activating Ca(2+)-activated K(+) (KCa) channels, resulting in vessel relaxation. Activation of endothelial KCa channels thus has potential to increase the electrochemical driving force for Ca(2+) influx via TRPV4 channels and to amplify permeability responses to TRPV4 activation in lung. To examine this hypothesis, we used Western blot analysis, electrophysiological recordings, and isolated-lung permeability measurements to document expression of TRPV4 and KCa channels and the potential for functional coupling. The results show that rat pulmonary microvascular endothelial cells express TRPV4 and 3 KCa channels of different conductances: large (BK), intermediate (IK), and small (SK3). However, TRPV4 channel activity modulates the IK and SK3, but not the BK, channel current density. Furthermore, the TRPV4-mediated permeability response to 14,15-EET in mouse lung is significantly attenuated by pharmacologic blockade of IK and SK3, but not BK, channels. Collectively, this functional coupling suggests that endothelial TRPV4 channels in rodent lung likely form signaling microdomains with IK and SK3 channels and that the integrated response dictates the extent of lung endothelial injury caused by 14,15-EET.

TRPV4 calcium entry and surface expression attenuated by inhibition of myosin light chain kinase in rat pulmonary microvascular endothelial cells

In previous studies, blockade or gene deletion of either myosin light chain kinase (MLCK) or the mechanogated transient receptor potential vanilloid 4 (TRPV4) channel attenuated mechanical lung injury. To determine their effects on calcium entry, rat pulmonary microvascular endothelial cells (RPMVEC) were labeled with fluo-4 and calcium entry initiated with the TRPV4 agonist, 4α-phorbol 12, 13-didecanoate (4αPDD). Mean calcium transients peaked at ∼25 sec and persisted ∼500 sec. The 4αPDD response was essentially abolished in calcium-free media, or after pretreatment with the MLCK inhibitor, ML-7. ML-7 also attenuated the 4αPDD-induced inward calcium current measured directly using whole-cell patch clamp. Pretreatment with dynasore, an inhibitor of dynamin produced an initial calcium transient followed by a 4αPDD transient of unchanged peak intensity. Automated averaging of areas under the curve (AUC) of calcium transients in individual cells indicated total calcium activity with a relationship between treatment groups of ML-7 + 4αPDD < 4αPDD only < dynasore + 4αPDD. Measurement of biotinylated surface TRPV4 protein indicated a significant reduction after ML-7 pretreatment, but no significant change with dynasore treatment. RPMVEC monolayer electrical resistances were decreased by only 3% with 10 μmol/L 4αPDD and the response was dose-related. Dynasore alone produced a 29% decrease in resistance, but neither ML-7 nor dynasore affected the subsequent 4αPDD resistance response. These studies suggest that MLCK may inhibit mechanogated calcium responses through reduced surface expression of stretch activated TRPV4 channels in the plasma membrane.

Control of intracellular trafficking of ICAM-1-targeted nanocarriers by endothelial Na+/H+ exchanger proteins

Targeting nanocarriers (NC) loaded by antioxidant enzymes (e.g., catalase) to endothelial cell adhesion molecules (CAM) alleviates oxidative stress in the pulmonary vasculature. However, antioxidant protection is transient, since CAM-targeted catalase is internalized, delivered to lysosomes, and degraded. To design means to modulate the metabolism and longevity of endothelial cell (EC)-targeted drugs, we identified and manipulated cellular elements controlling the uptake and intracellular trafficking of NC targeted to ICAM-1 (anti-ICAM/NC). BAPTA, thapsigargin, amiloride, and EIPA inhibited anti-ICAM/NC uptake by EC and actin rearrangements induced by anti-ICAM/NC (required for uptake), suggesting that member(s) of Na(+)/H(+) exchanger family proteins (NHE) regulate these processes. Consistent with this hypothesis, an siRNA specific for the plasmalemma NHE1, but not the endosome-associated NHE6, inhibited actin remodeling induced by anti-ICAM/NC and internalization. Anti-ICAM/NC binding to EC stimulated formation of a transient ICAM-1/NHE1 complex. One hour after uptake, ICAM-1 dissociated from NHE1, and anti-ICAM/NC were transported to NHE6-positive vesicles en route to lysosomes. Inhibition of PKC (an activator of intracellular NHE) accelerated nanocarrier lysosomal trafficking. In contrast, monensin, which enhances the endosomal sodium influx and proton efflux maintained by NHE6, inhibited delivery of anti-ICAM/NC to lysosomes by switching their trafficking to a plasma membrane recycling pathway. This markedly prolonged the protective effect of catalase-coated anti-ICAM/NC. Therefore, 1) NHE1 and NHE6 regulate distinct phases of anti-ICAM/NC uptake and trafficking; 2) pharmacological agents affecting these regulatory elements alter the itinerary of anti-ICAM/NC intracellular trafficking; and 3) these agents modulate duration of the therapeutic effects of targeted drugs.

Coordinate regulation of membrane cAMP by Ca2+-inhibited adenylyl cyclase and phosphodiesterase activities

Activation of store-operated Ca(2+) entry inhibits type 6 adenylyl cyclase (EC; AC(6); Yoshimura M and Cooper DM. Proc Natl Acad Sci USA 89: 6712-6720, 1992) activity in pulmonary artery endothelial cells. However, in lung microvascular endothelial cells (PMVEC), which express AC(6) and turn over cAMP at a rapid rate, inhibition of global (whole cell) cAMP is not resolved after direct activation of store-operated Ca(2+) entry using thapsigargin. Present studies sought to determine whether the high constitutive phosphodiesterase activity in PMVECs rapidly hydrolyzes cAMP so that Ca(2+) inhibition of AC(6) is difficult to resolve. Direct stimulation of adenylyl cyclase using forskolin and inhibition of type 4 phosphodiesterases using rolipram increased cAMP and revealed Ca(2+) inhibition of AC(6). Enzyme activity was assessed using PMVEC membranes, where Ca(2+) and cAMP concentrations were independently controlled. Endogenous AC(6) activity exhibited high- and low-affinity Ca(2+) inhibition, similar to that observed in C6-2B cells, which predominantly express AC(6). Ca(2+) inhibition of AC(6) in PMVEC membranes was observed after enzyme activation and inhibition of phosphodiesterase activity and was independent of the free cAMP concentration. Thus, under basal conditions, the constitutive type 4 phosphodiesterase activity rapidly hydrolyzes cAMP so that Ca(2+) inhibition of AC(6) is difficult to resolve, indicating that high phosphodiesterase activity works coordinately with AC(6) to regulate membrane-delimited cAMP concentrations, which is important for control of cell-cell apposition.