Experimental results sometimes differ despite seemingly identical inputs. Such a situation has sparked a debate in the November issue of Molecular Pharmacology over the nature of G-protein partners for β2-adrenoceptors (β2ARs). This should be essential reading not just for the dedicated adrenoceptor community, but also for all those using the experimental setup in question: cultured human embryonic kidney 293 (HEK293) cells.

For some years, evidence has been accumulating which indicates that β2ARs can change their G-protein partner depending on their phosphorylation state. Protein kinase A (PKA) phosphorylation of the receptor was found to lead to an increased activation of Gi and a concommittant reduction in the activation of the β2AR's usual partner, Gs. This Gi-mediated signalling was proposed to be important in transducing some actions of β2ARs, such as the activation of extracellular signal-related kinase (ERK) (see further reading). However, other studies have questioned whether β2ARs are truly able to switch partners upon phosphorylation, and whether such switching would be of functional relevance.

The new papers from the Clark and Lefkowitz groups bring this controversy into the limelight. Clark and colleagues present evidence that β2AR-mediated activation of ERK in HEK293 cells does not require Gi. In an accompanying Perspective article, Lefkowitz et al. review the large body of evidence in the literature that implies that PKA-mediated phosphorylation of β2ARs regulates their coupling to Gi, and hence their control of various signalling systems, including ERK.

Intriguingly, the key to the observed differences may lie in the non-uniformity of apparently identical experimental systems. Lefkowitz and colleagues present data showing that signalling through endogenous β2ARs in nine different isolates of HEK293 cells exhibits vastly different sensitivity to pertussis toxin, which inhibits Gi. Clearly, all HEK293 cell lines are far from equal.