Endotoxin induces desensitization of cardiac endothelin-1 receptor signaling by increased expression of RGS4 and RGS16

OBJECTIVE: Endotoxin (LPS)-induced acute cardiac failure during sepsis is associated with alterations in G protein mediated signal transduction. We therefore examined the expression of the G proteins G(i), G(q), and G(s) and of four 'regulators of G protein signaling' (RGS) proteins, RGS1, RGS4, RGS5, and RGS16 in rat hearts.

METHODS: For in vivo experiments, Wistar rats were treated with 600 microg/day E. coli LPS, intravenously) and hearts were excised after 6, 24 and 72 h. Cultured neonatal rat cardiomyocytes were treated with 4 microg/ml LPS for 24 and 72 h. Isolated membrane proteins were used for Western blot analysis and for evaluation of phospholipase C (PLC) activity. RGS16 mRNA was detected by RNAse protection.

RESULTS: LPS induced G(i) protein 1.4-fold 72 h after in vivo administration of LPS, whereas expression of G(s) and G(q) was unaltered. After 6 h of LPS treatment, RGS16 mRNA was transiently up-regulated 3.7-fold, followed by transient protein induction (24 h: 2.5-fold; 72 h: 1.5-fold). Similarly, RGS4 protein was transiently induced (24 h: 3.1-fold; 72 h: 1.5-fold), whereas expression of RGS1 and RGS5 was not altered. Similar to the LPS-treated rat hearts, RGS16 expression was transiently induced by LPS in cultured neonatal rat cardiomyocytes (24 h: 1.6-fold, 72 h: 0.9-fold). To determine the functional consequences of the RGS protein induction phospholipase C (PLC) activity was analyzed in membranes obtained from solvent and LPS-treated hearts. Basal and endothelin-1-stimulated PLC activity was transiently repressed by LPS with a maximum after 24 h although no apparent changes in PLCbeta1 or endothelin receptor expression could be detected.

CONCLUSION: These data suggest that the rapid up-regulation of cardiac RGS4 and RGS16 is associated with a desensitization of endothelin-1 receptor signaling. Up-regulation of these RGS proteins may thus be involved in the early onset of cardiac failure during sepsis.