Studies of cAMP metabolism in cultured hepatoma cells: Presence of functional adenylate cyclase despite low camp content and lack of hormonal responsiveness

Abstract

The ability of isoproterenol, glucagon, PGE₁ and cholera toxin to stimulate the synthesis of cAMP and protein kinase activity in a line of liver cells (BRL) and a line of rat hepatoma cells (H35) has been determined. The concentration of cAMP in BRL cells (∼ 10 pmoles/mg protein) is in the range reported for other cultured cell lines but H35 cells contain extraordinarily low amounts of this cyclic nucleotide (∼ 0.05 pmoles/mg protein). Isoproterenol and PGE₁ caused an increase in cAMP content, and protein kinase activation in BRL cells, although glucagon was ineffective. H35 cells, in contrast, were completely insensitive to all hormonal agonists. Despite this fact, cholera toxin was able to produce a marked increase in cAMP content, adenylate cyclase activity and protein kinase activation in H35 cells. Binding studies with [¹²⁵I]‐iodohydroxybenzylpindolol, a specific β‐adrenergic receptor antagonist, revealed that each H35 cell possesses fewer than 10 β‐adrenergic receptors whereas BRL cells contain 2–5,000 receptors per cell. The low level of cAMP in H35 cells appears to result from a combination of totally unstimulated adenylate cyclase and apparently elevated phoshodiesterase activities.