Changes in adenosine A₁- and A₂-receptor expression during adipose cell differentiation

Two adenosine receptors A₁ and A₂ are associated with either stimulation (A₂) or inhibition (A₁) of adenylate cyclase. Using the clonal cell line Ob1771, we have studied the expression of the two receptors during the process of adipose conversion accelerated by exposure to dexamethasone and 3-isobutyl-1-methylxanthine (IBMX) during the first 3 days post-confluence. The effects mediated by the two receptors on preadipocyte differentiation and adipocyte metabolism were also investigated. The two adenosine agonists NECA and PIA were used as preferential agonists of the A₂- and A₁-receptor, respectively. In preadipose cells (just confluent), both of the mouse clonal line and human primary culture, NECA dose-dependently stimulated cAMP production with a significant higher potency (P < 0.01) than did PIA. In adipose cells (16-day post-confluent) NECA was found to exert a biphasic effect on forskolin-stimulated cAMP production; i.e., NECA was clearly inhibitory in the femto- to picomolar concentration range whereas this effect gradually diminished at higher concentrations. The effect of PIA in 16-day post-confluent adipose cells however, was purely inhibitory on both cAMP production (IC₅₀: 33.52 ± 0.44 fM) and lipolysis (64% ± 7%; P < 0.01). These findings were corroboratedby Northern blot analysis which revealed A₁-receptor mRNA to be exclusively expressed in the mature adipocytes, whereas A₂-receptor mRNA gradually declined during the differentiation process except in 16-day post-confluent cells. In addition, NECA significantly enhanced the effect of corticosterone-induced differentiation by 46.8% (P < 0.05) but failed to have any adipogenic potency acting either alone or in concert with carbaprostacyclin (cPGI₂). Thus, endogenous adenosine may have a bimodal action on adipose tissue metabolism mediated through stimulatory A₂- and inhibitory A₁-receptors, respectively, as a function of adipose conversion.