Activation of beta(2)-adrenoceptor prevents Shiga toxin 2-induced TNF-alpha gene transcription

Exposure of renal tubular epithelial cells to shiga toxin 2 (Stx-2) causes cytotoxicity, and the potency of this toxin is enhanced in the presence of tumor necrosis factor-alpha (TNF-alpha). It has been shown that Stx-2 induc es TNF-alpha production and that activation of beta (2)-adrenoceptors downr egulates TNF-alpha. However, little is known about the signaling pathway by which beta (2)-adrenoceptor agonists suppress the Stx-2-induced TNF-alpha gene transcription. The possible signaling components involved in this path way were investigated. Human adenocarcinoma-derived renal tubular epithelia l cells (ACHN) were exposed to Stx-2 in the presence or absence of a beta ( 2)-adrenoceptor agonist. Mitogen-activated protein kinase (MAPK), activatin g protein-1 (AP-1), and nuclear factor-kappaB (NF-kappaB) were measured to evaluate the regulatory mechanisms involved in TNF-alpha gene transcription . Stx-2 (4 pg/ml) stimulated MAPK (p42/p44, p38) and AP-1 and increased TNF -alpha promoter activity by 2.4-fold. The increase in TNF-alpha was attenua ted by both a p42/p44 inhibitor, PD098059 (10(-6) M), and a p38 inhibitor, SB203580 (10(-6) M), and AP-1-binding activity was inhibited by PD098059. T erbutaline ( 10(-6) M to 10(-8) M) suppressed MAPK (p42/p44, p38), NF-kappa B (p50. p65), and TNF-alpha promoter activity in a dose-dependent way that was prevented by the beta (2)-adrenoceptor antagonist, ICI118.551. However, inhibition of MAPK (p42/p44) and TNF-alpha promoter activity was partially prevented by the cAMP-protein kinase (PKA) inhibitors, H-89 (5 x 10(-6) M) and KT5720 (10(-5) M), whereas the suppression of p38 MAPK or NF-kappaB (p 50) was not blocked by these inhibitors. The suppression of NF-kappaB (p65) was completely overcome by H-89 or KT5720. In summary, the downregulation of TNF-alpha transcription by terbutaline was mediated by in inhibitory eff ect of beta (2)-adrenoceptor activation on MAPK (p42/p44, p38) and NF-kappa B (p50/p65), which were exerted through a cAMP-PKA pathway and a cAMP-indep endent mechanism. It is likely that cAMP-PKA and MAPK (p42/p44, p38) may pl ay a critical role in the regulation of the Stx-2-induced TNF-alpha transcr iption via beta (2)-adrenoceptor activation.