Contrasting signaling pathways of α(1A)- and α(1B)-adrenergic receptor subtype activation of phosphatidylinositol 3-kinase and ras in transfected NIH3T3 cells

Activation of protein kinases is an important intermediate step in signaling pathways of many G protein-coupled receptors including α₁- adrenergic receptors. The present study was designed to investigate the capacity of the three cloned subtypes of human α₁-receptors, namely, α(1A), α(1B) and α(1D), to activate phosphatidylinositol 3-kinase (PI 3- kinase) and p21(ras) in transfected NIH3T3 cells. Norepinephrine activated PI 3-kinase in cells expressing human α(1A) and α(1B) via pertussis toxin- insensitive G proteins; α(1D)-receptors did not detectably activate this kinase. Transient transfection of NIH 3T3 cells with the α-subunit of the G protein transducin (α(t)) a scavenger of βγ-subunits released from activated G proteins, inhibited α(1B)-receptor but not α(1A)-receptor- stimulated PI 3-kinase activity. Stimulation of both α(1a)- and α(1b)- receptors activated p21(ras) and stimulated guanine nucleotide exchange on Ras protein. Overexpression of a dominant negative mutant of p21(ras) attenuated α(1B)-receptor but not α(1A)-receptor activation of PI 3- kinase. Overexpression of a dominant negative mutant of PI 3-kinase attenuated α(1A)- but not α(1B)-receptor-stimulated mitogen-activated protein kinase activity. These results demonstrate the capacity for heterologous signaling of the α₁-adrenergic receptor subtypes in promoting cellular responses in NIH3T3 cells.