Homologous regulation of the α2C-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 work, we examined the homologous regulation of the human alpha 2C-adrenoceptor in the hepatocarcinoma cell-line, HepG2; a model which expresses this subtype spontaneously. 2 Short-period treatment of the cells with UK14304 provoked neither a diminution of the potency of the alpha 2-agonist to inhibit forskolin-induced cyclic 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]MK912 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% decrease, respectively) and was blocked by the addition of alpha 2-antagonists. 4 Conversely to that observed with alpha 2-agonists, treatment of the cells with RX821002 or yohimbine alone, but not with phentolamine, promoted a significant increase of the receptor expression. 5 The observed alterations of receptor density are not the reflection of changes at the alpha 2C4 mRNA level. Estimation of the receptor protein turnover and measurement of its half-life demonstrated that down-regulation by alpha 2-agonists and up-regulation by alpha 2-antagonists, with inverse-agonist efficacy, are respectively the consequence of increased and decreased rate 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, downregulation fits with results obtained in OK but not in transfected cells. The reasons for these discrepancies are discussed. Our results also demonstrated that certain alpha 2-antagonists behave as inverse agonist on the HepG2 model and thus provide for the first time evidence of inverse efficacy of antagonists on a cellular model expressing physiological level of a wild-type alpha 2-adrenoceptor.