Differential Regulation of the Cell Cycle by α1-Adrenergic Receptor Subtypes

α1-Adrenergic receptors have been implicated in growth-promoting pathways. A microarray study of individual α1-adrenergic receptor subtypes (α1A, α1B, and α1D) expressed in Rat-1 fibroblasts revealed that epinephrine altered the transcription of several cell cycle regulatory genes in a direction consistent with the α1A- and α1D-adrenergic receptors mediating G1-S cell cycle arrest and the α1B-mediating cell-cycle progression. A time course indicated that in α1A cells, epinephrine stimulated a G1-S arrest, which began after 8 h of stimulation and maximized at 16 h, at which point was completely blocked with cycloheximide. The α1B-adrenergic receptor profile also showed unchecked cell cycle progression, even under low serum conditions and induced foci formation. The G1-S arrest induced by α1A- and α1D-adrenergic receptors was associated with decreased cyclin-dependent kinase-6 and cyclin E-associated kinase activities and increased expression of the cyclin-dependent kinase inhibitor p27Kip1, all of which were blocked by prazosin. There were no differences in kinase activities and/or expression of p27Kip1 in epinephrine α1B-AR fibroblasts, although the microarray did indicate differences in p27Kip1 RNA levels. Cell counts proved the antimitotic effect of epinephrine in α1A and α1D cells and indicated that α1B-adrenergic receptor subtype expression was sufficient to cause proliferation of Rat-1 fibroblasts independent of agonist stimulation. Analysis in transfected PC12 cells also confirmed the α1A- and α1B-adrenergic receptor effect. The α1B-subtype native to DDT1-MF2 cells, a smooth muscle cell line, caused progression of the cell cycle. These results indicate that the α1A- and α1D-adrenergic receptors mediate G1-S cell-cycle arrest, whereas α1B-adrenergic receptor expression causes a cell cycle progression and may induce transformation in sensitive cell lines.