High-density lipoprotein (HDL) is known for its cardioprotective properties independent from its cholesterol transport activity. These properties are mediated by activation of kinases such as protein kinase C (PKC). Connexin43 (Cx43) is a gap junction protein present in ventricular cardiomyocytes. PKC-dependent phosphorylation modifies Cx43 gap junction channel properties and is involved in cardioprotection. We hypothesized that cardioprotective properties of HDL may be mediated in part by affecting Cx43 gap junction channels.
METHODS AND RESULTS:
Neonatal rat cardiomyocytes were treated with HDL and Cx43 phosphorylation was evaluated by western blotting and immunofluorescence. We found that HDL promoted phosphorylation of Cx43 with a maximal induction at 5 min, which was inhibited by pre-treatment with various PKC inhibitors. Sphingosine-1-phosphate (S1P), a component of HDL, induced effects that were similar to those of HDL. These compounds significantly reduced diffusion of fluorescent dye among cardiomyocytes (∼50%) which could be prevented by PKC inhibition. As observed during optical recordings of transmembrane voltage, HDL and S1P depressed impulse conduction only minimally (<5%). Moreover, 5 min of HDL and S1P treatment at the onset of reperfusion significantly reduced infarct size (∼50%) in response to 30 min ischaemia in ex vivo experiments.
Short-term treatment with HDL or S1P induces phosphorylation of Cx43 by a PKC-dependent pathway. HDL-induced phosphorylation of Cx43 reduced the diffusion of large tracer molecules between cells, whereas impulse conduction was maintained. Moreover, 5 min treatment with HDL confers cardioprotection against ischaemia/reperfusion injury. These results link Cx43 for the first time to the short-term cardioprotective effects of HDL.