Received for publication February 26, 2007.
Revised August 22, 2007.
Accepted for publication August 22, 2007.

Ligand-directed signaling at the ₃-adrenoceptor produced by SR59230A relative to receptor agonists

Abstract

This study examines signaling pathways activated by the mouse ₃-adrenoceptor (AR) expressed in Chinese hamster ovary cells at high (CHO₃H) or low (CHO₃L) levels. Functional responses included extracellular acidification rate (ECAR), cyclic AMP accumulation, and p38 MAPK or Erk1/2 phosphorylation. (-)-Isoproterenol and the ₃-AR agonist CL316243 caused concentration-dependent increases in cAMP accumulation and ECAR in CHO₃H and CHO₃L cells. For cAMP accumulation, the ₃-AR ligand SR59230A was a partial agonist in CHO₃H and an antagonist in CHO₃L cells but for ECAR was an agonist at both expression levels. This suggested that SR59230A which is normally regarded as an antagonist can selectively activate pathways leading to ECAR. Examination of the pathways stimulated by (-)-isoproterenol, CL316243 and SR59230A for both ECAR and cAMP accumulation suggested that the cAMP pathway predominates in CHO₃H cells while p38 MAPK is a major contributor to ECAR in CHO₃L cells and was the sole contributor to responses to SR59230A. Western blots of p38 MAPK and Erk1/2 phosphorylation confirmed that MAPKs are activated in CHO₃H and CHO₃L cells by CL316243 and SR59230A but that SR59230A has much higher efficacy. In addition, p38 MAPK phosphorylation displayed differences in drug potency and efficacy between CHO₃H and CHO₃L cells related to inhibition of the response by cAMP. Thus CL316243 and SR59230A display reversed orders of efficacy for cAMP accumulation compared to Erk1/2 and p38 MAPK phosphorylation providing a strong indication of ligand-directed signaling.

Key words: Adrenergic, cAMP, MAP Kinase, P38 MAP Kinase

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