α_1B- and α_1D-adrenergic receptors exhibit different requirements for agonist and mitogen-activated protein kinase activation to regulate growth responses in Rat 1 fibroblasts

We compared DNA replication, protein biosynthesis, and mitogen-activated protein kinase (MAPK) activity in Rat 1 fibroblasts stably expressing either the α_1B-adrenergic receptor (AR) or α_1D-AR subtypes. Activation of both the α_1B-AR and α_1D-AR inhibited DNA synthesis (as assessed by [₃H]thymidine incorporation). In contrast, both receptors stimulated protein biosynthesis (as measured by [₃₅S]methionine incorporation) and activated extracellular signal-regulated kinase (ERK)1/2. Importantly, these responses were agonist-dependent for the α_1B-AR, but were agonist-independent for the α_1D-AR. Agonist activation of the α_1B-AR resulted in increased p38 kinase activity, but not c-Jun NH₂-terminal kinase (JNK) activity, whereas the α_1D-AR activated JNK but not p38 kinase. Unlike ERK1/2, JNK activity was increased by agonist treatment in the α_1D-AR cells. An ERK1/2-pathway inhibitor PD98059 had no effect on phenylephrine-mediated inhibition of DNA synthesis in either cell line but blocked protein biosynthesis mediated by both receptors. The p38 kinase inhibitor SB203580 blocked α_1B-AR effects on [³H]thymidine and [³⁵S]methionine incorporation in α_1B-AR-expressing ceils, but had no effect on α_1D-AR-mediated growth responses, consistent with the inability of the α_1D-AR to activate p38 kinase. Therefore, α_1B- and α_1D-ARs mediated similar growth responses but differ with respect to the MAPK family member involved and the requirement for agonist.