Contrasting signaling pathways of α1A- and α1B-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 alpha(1)-adrenergic receptors, The present study was designed to investigate the capacity of the three cloned subtypes of human alpha(1)-receptors, namely, alpha(1A), alpha(1B), and alpha(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 alpha(1A) and alpha(1B) via pertussis toxin-insensitive G proteins; alpha(1D)-receptors did not detectably activate this kinase, Transient transfection of NIH 3T3 cells with the alpha-subunit of the G protein transducin (alpha(t)) a scavenger of beta gamma-subunits released from activated G proteins, inhibited alpha(1B)-receptor but not alpha(1A)-receptor-stimulated PI 3-kinase activity. Stimulation of both alpha(1A)- and alpha(1B)-receptors activated p21(ras) and stimulated guanine nucleotide exchange on Ras protein. Overexpression of a dominant negative mutant of p21(ras) attenuated alpha(1B)-receptor but not alpha(1A)-receptor activation of PI 3-kinase, Overexpression of a dominant negative mutant of PI 3-kinase attenuated alpha(1A)- but not alpha(1B)-receptor-stimulated mitogen-activated protein kinase activity. These results demonstrate the capacity for heterologous signaling of the a,adrenergic receptor subtypes in promoting cellular responses in NIH3T3 cells.