Protein kinase C-independent inhibition of arterial smooth muscle K(+) channels by a diacylglycerol analogue

Cholesterol and PIP2 Modulation of BKCa Channels

Ca2+/voltage-gated, large conductance K+ channels (BKCa) are formed by homotetrameric association of α (slo1) subunits. Their activity, however, is suited to tissue-specific physiology largely due to their association with regulatory subunits (β and γ types), chaperone proteins, localized signaling, and the channel's lipid microenvironment. PIP2 and cholesterol can modulate BKCa activity independently of downstream signaling, yet activating Ca2+i levels and regulatory subunits control ligand action. At physiological Ca2+i and voltages, cholesterol and PIP2 reduce and increase slo1 channel activity, respectively. Moreover, slo1 proteins provide sites that seem to recognize cholesterol and PIP2: seven CRAC motifs in the slo1 cytosolic tail and a string of positively charged residues (Arg329, Lys330, Lys331) immediately after S6, respectively. A model that could explain the modulation of BKCa activity by cholesterol and/or PIP2 is hypothesized. The roles of additional sites, whether in slo1 or BKCa regulatory subunits, for PIP2 and/or cholesterol to modulate BKCa function are also discussed.

FGIN-1-27 Inhibits Melanogenesis by Regulating Protein Kinase A/cAMP-Responsive Element-Binding, Protein Kinase C-β, and Mitogen-Activated Protein Kinase Pathways

FGIN-1-27 is a synthetic mitochondrial diazepam binding inhibitor receptor (MDR) agonist that has demonstrated pro-apoptotic, anti-anxiety, and steroidogenic activity in various studies. Here we report, for the first time, the anti-melanogenic efficacy of FGIN-1-27 in vitro and in vivo. FGIN-1-27 significantly inhibited basal and α-melanocyte-stimulating hormone (α-MSH)-, 1-Oleoyl-2-acetyl-sn-glycerol (OAG)- and Endothelin-1 (ET-1)-induced melanogenesis without cellular toxicity. Mushroom tyrosinase activity assay showed that FGIN-1-27 did not directly inhibit tyrosinase activity, which suggested that FGIN-1-27 was not a direct inhibitor of tyrosinase. Although it was not capable of modulating the catalytic activity of mushroom tyrosinase in vitro, FGIN-1-27 downregulated the expression levels of key proteins that function in melanogenesis. FGIN-1-27 played these functions mainly by suppressing the PKA/CREB, PKC-β, and MAPK pathways. Once inactivated, it decreased the expression of MITF, tyrosinase, TRP-1, TRP-2, and inhibited the tyrosinase activity, finally inhibiting melanogenesis. During in vivo experiments, FGIN-1-27 inhibited the body pigmentation of zebrafish and reduced UVB-induced hyperpigmentation in guinea pig skin, but not a reduction of numbers of melanocytes. Our findings indicated that FGIN-1-27 exhibited no cytotoxicity and inhibited melanogenesis in both in vitro and in vivo models. It may prove quite useful as a safer skin-whitening agent.

Evolving mechanisms of vascular smooth muscle contraction highlight key targets in vascular disease

Vascular smooth muscle (VSM) plays an important role in the regulation of vascular function. Identifying the mechanisms of VSM contraction has been a major research goal in order to determine the causes of vascular dysfunction and exaggerated vasoconstriction in vascular disease. Major discoveries over several decades have helped to better understand the mechanisms of VSM contraction. Ca2+ has been established as a major regulator of VSM contraction, and its sources, cytosolic levels, homeostatic mechanisms and subcellular distribution have been defined. Biochemical studies have also suggested that stimulation of Gq protein-coupled membrane receptors activates phospholipase C and promotes the hydrolysis of membrane phospholipids into inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 stimulates initial Ca2+ release from the sarcoplasmic reticulum, and is buttressed by Ca2+ influx through voltage-dependent, receptor-operated, transient receptor potential and store-operated channels. In order to prevent large increases in cytosolic Ca2+ concentration ([Ca2+]c), Ca2+ removal mechanisms promote Ca2+ extrusion via the plasmalemmal Ca2+ pump and Na+/Ca2+ exchanger, and Ca2+ uptake by the sarcoplasmic reticulum and mitochondria, and the coordinated activities of these Ca2+ handling mechanisms help to create subplasmalemmal Ca2+ domains. Threshold increases in [Ca2+]c form a Ca2+-calmodulin complex, which activates myosin light chain (MLC) kinase, and causes MLC phosphorylation, actin-myosin interaction, and VSM contraction. Dissociations in the relationships between [Ca2+]c, MLC phosphorylation, and force have suggested additional Ca2+ sensitization mechanisms. DAG activates protein kinase C (PKC) isoforms, which directly or indirectly via mitogen-activated protein kinase phosphorylate the actin-binding proteins calponin and caldesmon and thereby enhance the myofilaments force sensitivity to Ca2+. PKC-mediated phosphorylation of PKC-potentiated phosphatase inhibitor protein-17 (CPI-17), and RhoA-mediated activation of Rho-kinase (ROCK) inhibit MLC phosphatase and in turn increase MLC phosphorylation and VSM contraction. Abnormalities in the Ca2+ handling mechanisms and PKC and ROCK activity have been associated with vascular dysfunction in multiple vascular disorders. Modulators of [Ca2+]c, PKC and ROCK activity could be useful in mitigating the increased vasoconstriction associated with vascular disease.

Protein Kinase C as Regulator of Vascular Smooth Muscle Function and Potential Target in Vascular Disorders

Vascular smooth muscle (VSM) plays an important role in maintaining vascular tone. In addition to Ca2+-dependent myosin light chain (MLC) phosphorylation, protein kinase C (PKC) is a major regulator of VSM function. PKC is a family of conventional Ca2+-dependent α, β, and γ, novel Ca2+-independent δ, ɛ, θ, and η, and atypical ξ, and ι/λ isoforms. Inactive PKC is mainly cytosolic, and upon activation it undergoes phosphorylation, maturation, and translocation to the surface membrane, the nucleus, endoplasmic reticulum, and other cell organelles; a process facilitated by scaffold proteins such as RACKs. Activated PKC phosphorylates different substrates including ion channels, pumps, and nuclear proteins. PKC also phosphorylates CPI-17 leading to inhibition of MLC phosphatase, increased MLC phosphorylation, and enhanced VSM contraction. PKC could also initiate a cascade of protein kinases leading to phosphorylation of the actin-binding proteins calponin and caldesmon, increased actin-myosin interaction, and VSM contraction. Increased PKC activity has been associated with vascular disorders including ischemia-reperfusion injury, coronary artery disease, hypertension, and diabetic vasculopathy. PKC inhibitors could test the role of PKC in different systems and could reduce PKC hyperactivity in vascular disorders. First-generation PKC inhibitors such as staurosporine and chelerythrine are not very specific. Isoform-specific PKC inhibitors such as ruboxistaurin have been tested in clinical trials. Target delivery of PKC pseudosubstrate inhibitory peptides and PKC siRNA may be useful in localized vascular disease. Further studies of PKC and its role in VSM should help design isoform-specific PKC modulators that are experimentally potent and clinically safe to target PKC in vascular disease.

Ca2+ dependent but PKC independent signalling mediates UTP induced contraction of rat mesenteric arteries

Uridine triphosphate (UTP) can be released from damaged cells to cause vasoconstriction. Although UTP is known to act through P2Y receptors and PLC activation in vascular smooth muscle, the role of PKC in generating the response is somewhat unclear. Here we have used Tat-linked membrane permeable peptide inhibitors of PKC to assess the general role of PKC and also of specific isoforms of PKC in the UTP induced contraction of rat mesenteric artery. We examined the effect of PKC inhibition on UTP induced contraction, increased cytoplasmic Ca²⁺ and reduction of K⁺ currents and found that PKC inhibition caused a relatively small attenuation of contraction but had little effect on changes in cytoplasmic Ca²⁺. UTP attenuation of both voltage-gated (K_v) and ATP-dependent (K_ATP) K⁺ currents was abolished when intracellular Ca²⁺ was decreased from 100 to 20 nM. PKC inhibition reduced slightly the ability of UTP to attenuate K_v currents but had no effect on K_ATP current inhibition. In conclusion, both UTP induced contraction of mesenteric artery and the inhibition of K_v and K_ATP currents of mesenteric artery smooth muscle cells by UTP are relatively independent of PKC activation; furthermore, the inhibition of both K_v and K_ATP currents requires intracellular Ca²⁺.

Maternal high-salt diets affected pressor responses and microvasoconstriction via PKC/BK channel signaling pathways in rat offspring

SCOPE

High-salt (HS) intake is linked to hypertension, and prenatal exposure to maternal HS diets may have long-term impact on cardiovascular systems. The relationship between HS diets and cardiovascular disease has received extensive attention. This study determined pressor responses and microvessel functions in the adult offspring rats exposed to prenatal HS.

METHODS AND RESULTS

The offspring of 5-month old as young adults in rats were used. Blood pressure, vascular tone, intracellular Ca(2+), and BK channels in mesenteric arteries were measured in the offspring. Phenylephrine (Phe)-induced pressor responses were significantly higher in the prenatal HS offspring. Vessel tension and intracellular Ca(2+) concentrations associated with Phe-induced pressor responses were increased in the mesenteric arteries of the HS offspring. PKC α- and δ-isoforms were upregulated in mesenteric arteries of the HS offspring. The enhanced Phe-mediated vascular activity was linked to the altered PKC-modulated BK channel functions.

CONCLUSION

The results suggested that prenatal exposure to HS altered microvascular activity probably via changes in PKC/BK signaling pathways, which may lead to increased risks of hypertension in the offspring.

Steady-state modulation of voltage-gated K+ channels in rat arterial smooth muscle by cyclic AMP-dependent protein kinase and protein phosphatase 2B

Voltage-gated potassium channels (K_v) are important regulators of membrane potential in vascular smooth muscle cells, which is integral to controlling intracellular Ca2+ concentration and regulating vascular tone. Previous work indicates that K_v channels can be modulated by receptor-driven alterations of cyclic AMP-dependent protein kinase (PKA) activity. Here, we demonstrate that K_v channel activity is maintained by tonic activity of PKA. Whole-cell recording was used to assess the effect of manipulating PKA signalling on K_v and ATP-dependent K+ channels of rat mesenteric artery smooth muscle cells. Application of PKA inhibitors, KT5720 or H89, caused a significant inhibition of K_v currents. Tonic PKA-mediated activation of K_v appears maximal as application of isoprenaline (a β-adrenoceptor agonist) or dibutyryl-cAMP failed to enhance K_v currents. We also show that this modulation of K_v by PKA can be reversed by protein phosphatase 2B/calcineurin (PP2B). PKA-dependent inhibition of K_v by KT5720 can be abrogated by pre-treatment with the PP2B inhibitor cyclosporin A, or inclusion of a PP2B auto-inhibitory peptide in the pipette solution. Finally, we demonstrate that tonic PKA-mediated modulation of K_v requires intact caveolae. Pre-treatment of the cells with methyl-β-cyclodextrin to deplete cellular cholesterol, or adding caveolin-scaffolding domain peptide to the pipette solution to disrupt caveolae-dependent signalling each attenuated PKA-mediated modulation of the K_v current. These findings highlight a novel, caveolae-dependent, tonic modulatory role of PKA on K_v channels providing new insight into mechanisms and the potential for pharmacological manipulation of vascular tone.