Chronic regulation of the expression of gap junction proteins connexin40, connexin43, and connexin45 in neonatal rat cardiomyocytes

Abstract

Gap junction channels form the basis of intercellular communication in the heart. In the working myocardium, the connexin43 (Cx43) is most abundantly found, whereas connexin40 (Cx40) is expressed in the atria and in the conduction system [together with low levels of connexin45 (Cx45)]. However, little is known about the differential regulation of the connexins by pathophysiologically stimuli such as tumor necrosis factor α (TNFα). Inasmuch as TNFα may play a contributory role in the concert of factors involved in the pathophysiology of heart failure and because this cardiac disease often leads to ventricular reentrant arrhythmia, the goal of our study was to find out whether TNFα may influence the expression of the cardiac connexins connexin43, connexin40, and connexin45. Neonatal rat cardiomyocytes were exposed to TNFα (10, 40, 100, 400, and 1000 pg/ml) for 24 h with or without additional treatment with the mitogenic-activated protein kinase (MAP-kinase) inhibitors SB203580 [4-(4-fluorophenyl)-2-(4-methyl-sulfinylphenyl)-5-(4-pyridyl)-1 H-imidazole; 10 −5 M, protein38 mitogenic-activated protein kinase (p38 MAP kinase) inhibitor] or the MEK1 (mitogenic-activated protein kinase/extracellular signal-regulated kinase kinase) inhibitor PD98059 [2-(2-amino-3-methoxyphenyl)-4 H-1-benzopyran-4-one; 10 −5 M]. Connexin43, connexin40, and connexin45 expressions were analysed using Western blot analysis, immunohistology, and polymerase chain reaction (PCR) studies (connexin43 and connexin40). TNFα induced a concentration-dependent increase in connexin43 (by 2.9±0.6, P<0.05, n=5) but not in connexin40 or connexin45 expressions. Both connexins (40 and 45) showed a very low expression near the detection limit. The increases in connexin43 expression could be completely suppressed by SB203580 (0.9±0.4, P<0.05, n=5) but not by PD98059. In absence of a stimulating drug, these inhibitors (SB203580 or PD98059) did not affect connexin43 content. Additional PCR experiments revealed increases in connexin43 mRNA under the influence of 100 pg/ml TNFα (211±38%, P<0.05, n=5), which could be completely suppressed by SB203580. In contrast, the connexin40 expression remained unchanged. From these results, we conclude that TNFα can differentially regulate cardiac connexin expression via p38 MAP kinase pathway and thus may alter intercellular communication. This may contribute to the changes observed in heart failure with regard to the formation of an arrhythmogenic substrate.