Homologous regulation of the alpha 2C-adrenoceptor subtype in human hepatocarcinoma, HepG2

1 Previous studies of the regulation of the alpha 2C-adrenoceptor in OK and in transfected cells have led to discrepant conclusions. In the present wo rk, we examined the homologous regulation of the human alpha 2C-adrenocepto r in the hepatocarcinoma cell-line, HepG2; a model which expresses this sub type spontaneously. 2 Short-period treatment of the cells with UK14304 provoked neither a dimin ution of the potency of the alpha 2-agonist to inhibit forskolin-induced cy clic AMP-accumulation nor a change in the degree of receptor coupling to G- proteins. 3 Long-period exposure to UK14304 resulted in a large reduction of [H-3]MK9 12 binding sites (55% decrease). The action of UK14304 was dose-dependent ( EC50 = 190 +/- 45 nM), rapid (t(1/2) = 4.2 h) and reversible. Receptor down -regulation was also observed with clonidine or (-)adrenaline (38 and 36% d ecrease, respectively) and was blocked by the addition of alpha 2-antagonis ts. 4 Conversely to that observed with alpha 2-agonists, treatment of the cells with RX821002 or yohimbine alone, but not with phentolamine, promoted a si gnificant increase of the receptor expression. 5 The observed alterations of receptor density are not the reflection of ch anges at the alpha 2C4 mRNA level. Estimation of the receptor protein turno ver and measurement of its half-life demonstrated that down-regulation by a lpha 2-agonists and up-regulation by alpha 2-antagonists, with inverse-agon ist efficacy, are respectively the consequence of increased and decreased r ate of receptor degradation. 6 In conclusion, our data show that alpha 2C-adrenoceptor does not undergo desensitization but is down-regulated in HepG2. The lack of desensitization agrees with previous results obtained in cells transfected with the alpha 2C4 gene, but not with observations made in OK cells. Inversely, downregula tion fits with results obtained in OK but not in transfected cells. The rea sons for these discrepancies are discussed. Our results also demonstrated t hat certain alpha 2-antagonists behave as inverse agonist on the HepG2 mode l and thus provide for the first time evidence of inverse efficacy of antag onists on a cellular model expressing physiological level of a wild-type al pha 2-adrenoceptor.