Adenylyl cyclase P-site ligands accelerate differentiation in Ob1771 preadipocytes

Differentiation of Ob1771 preadipocytes to adipocytes was characterized by morphological changes and elevated expression of the specific marker enzyme, glycerol-3-phosphate dehydrogenase. A differentiation response substantially more complete and rapid than that obtained with insulin and 3,5,3'-triiodothyronine was observed with established inhibitors of adenylyl cyclases: 2',5'-dideoxyadenosine (2',5'-dd-Ado), 9-(cyclopentyl)adenine (9- CP-Ade), and 9-(arabinofuranosyl)adenine (9-Ara-Ade), coincident with decreased cellular cAMP levels. These ligands inhibit adenylyl cyclases noncompetitively, via a domain referred to as the P-site because of its requirement for an intact purine moiety. Differentiation was not induced by inosine, a nucleoside known not to act at the P-site, or by N⁶-(2- phenylisopropyl)adenosine or 1,3-diethyl-8-phenylxanthine, agonist and antagonist, respectively, for adenosine A₁ receptors. Also ineffective were IBMX or forskolin, agents that can raise intracellular cAMP levels. Potency of the differentiation response followed the order 2',5'-dd-Ado (1-20 μM) > 9-CP-Ade (10-100 μM) = 9-Ara-Ade (10-100 μM) >> inosine, consistent with their potencies to inhibit adenylyl cyclases. The data suggest that inhibition of adenylyl cyclase via the P-site and the consequent reduction in cell cAMP levels facilitate the induction of differentiation in Ob1771 cells. The findings raise the question whether the known endogenous P-site ligands participate in the differentiation response induced by hormones.