Adenosine A 2 receptor modulation of erythropoietin secretion in hepatocellular carcinoma cells

Abstract

The present studies were undertaken using a cloned erythropoietin (Ep) producing hepatocellular carcinoma cell line (Hep3B) to attempt to correlate the receptor binding properties and biological activities of 5′-N-ethylcarboxamideadenosine (NECA), N 6-cyclohexyladenosine (CHA) and N 6-cyclopentyldenosine (CPA). Ep and cAMP levels in cultures of Hep3B cells in response to these adenosine analogues were measured by radioimmunoassay. Receptor binding affinities of the adenosine analogues were determined by measuring inhibition of binding of [ 3H] NECA to hep3B cell membranes. Scatchard analysis of [ 3H] NECA to Hep3B cell membranes. Scatchard analysis of [ 3H] NECA binding to Hep3B cell membranes indicates a single class of binding sites with a dissociation constant of 431 nM and a binding capacity of 573 fmol/mg protein. The adenosine analogues tested produced a significant increase in Ep secretion and cAMP accumulation (ED 50 for cAMP accumulation, NECA = 3.3×10 −7M, CHA = 4 4.2×10 −6M, CPA = 2.2×10 −6M). In addition, NECA showed a higher binding affinity (Ki, 3.8×10 −7M) for Hep3B cell membranes in comparison with CHA (Ki, 6.3×10 −6M) and CPA (Ki, 3.9×10 −6M). These results indicate that the rank order for potency for NECA, CHA and CPA in their binding to Hep3B cell membrane receptors correlates very well with their biological effects on Ep secretion and cAMP accumulation in Hep3B cells.