α₂-Adrenergic agonists stimulate DNA synthesis in Chinese hamster lung fibroblasts transfected with a human α₂-adrenergic receptor gene

To test the hypothesis that agents activating receptors negatively coupled to adenylyl cyclase (AC) can stimulate cell proliferation, we have expressed a human α₂-adrenergic receptor (α₂-C10) in CCL39 cells and studied the effects of α₂-agonists on reinitiation of DNA synthesis in quiescent cells. We report that the α₂-agonists epinephrine and clonidine stimulate [³H]-thymidine incorporation in synergy with fibroblast growth factor and that the α₂-antagonist yohimbine efficiently inhibits this response. Epinephrine- and clonidine-stimulated DNA synthesis is completely blocked by pertussis toxin and correlates well with the inhibition of prostaglandin E₁-stimulated AC. Thus, their action closely resembles the action of serotonin in the same cell system, which is mediated through 5-HT_1b receptors. In fact, serotonin- and epinephrine-stimulated DNA synthesis reinitiation is not additive, suggesting that both agents act through a common pathway. Interestingly, α₂-agonists also induced a moderate release of inositol phosphates, indicating that α₂-adrenergic receptors can interact both with the AC and phospholipase C messenger system. Activation of phosphoinositide (PI) turnover by epinephrine leads to a significant stimulation of Na⁺/ H⁺ exchange but is insufficient to trigger a mitogenic response in CCL39 cells, as will be discussed. We found no evidence for epinephrine-induced activation of Na⁺/H⁺ exchange by a mechanism independent of PI breakdown. Our data show that α₂-adrenergic receptors can play a role in the regulation of cell proliferation in an appropriate context; also, the data support the hypothesis that receptors negatively coupled to AC must be taken into account as mediators of growth factor action in fibroblasts, in particular when activated in parallel with receptor tyrosine kinases.