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Abstract: Caveolin-3, the major caveolin isoform in cardiomyocytes, plays an important role in the rapid signaling pathways initiated by stimulation of the membrane-associated molecules. To examine the role of caveolin-3 in regulating estrogen receptor α in cardiomyocytes, we investigate whether the membrane estrogen receptor α associates with caveolin-3 and whether this association is linked to the 17β-estradiol-mediated signals. In control cardiomyocytes, following discontinuous sucrose gradient centrifugation, caveolin-3 was found predominantly in the lipid raft buoyant fractions, whereas it was distributed to both the buoyant and non-lipid raft heavy fractions following metabolic inhibition treatment. Confocal microscopy showed that estrogen receptor α co-localized with caveolin-3 on the plasma membrane of neonatal and adult rat cardiomyocytes. This membrane labeling of estrogen receptor α was not seen following treatment with the cholesterol-depleting agent methyl-β-cyclodextrin (5mM), whereas metabolic inhibition had little effect on the membrane distribution of estrogen receptor α. Metabolic inhibition induced tyrosine phosphorylation of caveolin-3 and decreased its association with estrogen receptor α, both effects being mediated via a Src activation mechanism, since they were inhibited by the selective tyrosine kinase inhibitor PP2. Metabolic inhibition also induced tyrosine phosphorylation of connexin43 and increased its association with c-Src, both effects being prevented by 17β-estradiol (200nM). The effect of 17β-estradiol on metabolic inhibition-induced tyrosine phosphorylation of connexin43 was inhibited by the specific estrogen receptor antagonist ICI182780. These data identify cardiac caveolin-3 as juxtamembrane scaffolding for estrogen receptor α docking at caveolae, which provide a unique compartment for conveying 17β-estradiol-elicited, rapid signaling to regulate connexin43 phosphorylation during ischemia. [Copyright &y& Elsevier]

The effects of 17β-estradiol (E2) on gap junction intercellular communication (GJIC) were assessed by Lucifer yellow dye coupling in cultured neonatal rat cardiomyocytes after metabolic inhibition (MI) using potassium cyanide and sodium iodoacetate. MI significantly reduced dye coupling of cardiomyocytes to 8.5% ± 0.6% of control levels, and pretreatment with E2, but not its inactive isomer 17α-estradiol, dose-dependently (EC50 = 0.41 μM) increased the dye coupling up to 76% ± 15% of control levels. The effect of E2 on MI-induced dye uncoupling was abolished by tamoxifen, a potent estrogen receptor (ER) antagonist. The ligand, E2-BSA-FITC, labeled the cardiomyocyte surface, whereas BSA-FITC did not, suggesting the presence of membrane-associated E2 receptors. Double immunofluorescence microscopy showed that MI-induced the accumulation of non-phosphorylated Cx43 at the gap junction and that this was prevented by E2 pretreatment. Labeling of Lucifer yellow-microinjected cardiomyocytes with antibodies specific for Ser368-phosphorylated Cx43 (Ser368Cx43) or non-phosphorylated Cx43 confirmed that E2 reduced the MI-induced inhibition of dye coupling and accumulation of non-phosphorylated Cx43 concomitant with the reappearance of Ser368Cx43 at the gap junction. MI caused a decrease in Ser368Cx43 protein levels, and pretreatment with E2 significantly increased the levels of Ser368Cx43. Inhibition of protein kinase C (PKC) with chelerythrine blocked the E2-induced increase of Ser368Cx43 levels in MI-treated cardiomyocytes. These results suggest that E2 attenuates the inhibitory effect of MI on GJIC in cardiomyocytes by affecting the phosphorylation of Cx43, possibly mediated by activation of PKC via a membrane-associated signaling mechanism. [Copyright &y& Elsevier]

Abstract: We have demonstrated that activation of ATP-sensitive potassium (KATP) channels can attenuate sympathetic hyperinnervation. Sildenafil, a phosphodiesterase-5 inhibitor, has been shown to provide a preconditioning-like cardioprotective effect via opening of KATP channels. The aim of this study was to investigate whether chronic administration of sildenafil attenuates cardiac sympathetic hyperinnervation after myocardial infarction through activation of KATP channels and to compare it with the nitric oxide donor isosorbide dinitrate. Male Wistar infarcted rats induced by ligation of the anterior descending artery were randomized to either vehicle, nicorandil, sildenafil, isosorbide dinitrate, glibenclamide, or a combination of nicorandil and glibenclamide, or sildenafil and glibenclamide. Myocardial norepinephrine levels revealed a significant elevation in vehicle-treated rats compared with sham-operated rats, consistent with sympathetic hyperinnervation after infarction assessed by immunohistochemical analysis for tyrosine hydroxylase, growth associated factor 43 and neurofilament and by protein expression and mRNA of nerve growth factor. Sympathetic hyperinnervation was reduced after administering either nicorandil or sildenafil. Arrhythmic scores during programmed stimulation in the sildenafil-treated rats were significantly lower than those treated with the vehicle. Furthermore, the beneficial effects of sildenafil-induced were reversed by the addition of either glibenclamide or 5-hydroxydecanoate, implicating mitochondrial KATP channels as the relevant target. Isosorbide dinitrate failed to confer similar antiarrhythmia. 1H-[1,2,4]oxadiazolo[4,3-α]quinoxalin-1-one, a soluble guanylyl cyclase inhibitor, did not influence the effect of sildenafil on the nerve growth factor. These data indicate that sildenafil after infarction attenuated sympathetic hyperinnervation and arrhythmias by activation of mitochondrial KATP channels through a guanylyl cyclase-cGMP-independent pathway. [Copyright &y& Elsevier]

Abstract: 18β-Glycyrrhetinic acid (18β-GA) regulates serine/threonine dephosphorylation of connexin43 (Cx43). Phospho-specific antibodies were used here to determine the effect of 18β-GA on serine 368-phosphorylated Cx43 (pSer368Cx43) in cultured rat neonatal cardiomyocytes by immunofluorescence microscopy and immunoblot analyses. 18β-GA caused a time-dependent increase in pSer368Cx43 levels and induced gap junction disassembly, shown by a change in pSer368Cx43 immunostaining from large aggregates to dispersed punctates at cell–cell contact areas. 18β-GA also induced a time-dependent increase in the levels of serine 729-phosphorylated PKCɛ, the active form of PKCɛ. The 18β-GA-induced increase in pSer368Cx43 levels and changes in pSer368Cx43 staining pattern were abolished by the PKC inhibitor, chelerythrine. Furthermore, 18β-GA increased the co-immunoprecipitation of Cx43 with PKCɛ. However, the 18β-GA-induced increase in pSer368Cx43 levels and increased association of Cx43 with PKCɛ were inhibited by co-treatment with the protein phosphatase type 1 and type 2A inhibitor, calyculin A. We conclude that 18β-GA induces Ser368 phosphorylation of Cx43 via PKCɛ. [Copyright &y& Elsevier]

Highlights: [•] HYS-32 is a new analogue of combretastatin A-4 containing a cis-stilbene moiety. [•] We investigate the effect of HYS-32 on Cx43 gap junctions in rat primary astrocytes. [•] HYS-32 enhances Cx43 expression and GJIC via a PKC–ERK signaling cascade. [•] HYS-32 prevents LPS-induced downregulation of Cx43 and inhibition of GJIC. [•] The effects of HYS-32 on astrocytes potentiate its application in neuroprotection. [Copyright &y& Elsevier]