For the widely distributed P2Y receptors for nucleotides, the transductional and functional responses downstream of their coupling to G proteins are poorly characterized. Here we describe apoptotic induction and the associated differential stimulation of mitogenactivated protein (MAP) kinase family members by the human P2Y 1 receptor. The potent P2Y 1 receptor agonist, 2-methylthio-ADP (2-MeSADP), stimulated the extracellular-signal regulated kinases (ERK1/2) (EC 50 ϳ5 nM) as well as several, but not all isoforms detected, of the stress-activated protein kinase (SAPK) family. Phosphoisoforms of p38 were unaffected. The induced kinase activity was blocked by the P2Y 1 receptor-selective antagonist, adenosine-2-phosphate-5-phosphate, but unaffected by pertussis toxin. In addition, the endogenous ligand ADP, and significantly also 2-MeSATP, induced concentration-dependent phosphorylation changes in the same MAP kinase family members. The sustained activation of ERK1/2 was associated with Elk-1 phosphorylation that was abolished by the MEK1 inhibitor, PD 98059. However, the concomitant transient activation of the SAPKs was not sufficient to induce c-Jun or ATF-2 phosphorylation. The transient phase of the ERK activity was partially inhibited either by the phosphatidylinositol 3-kinase inhibitor, LY 294002, or the PKC inhibitor, Gö 6976. In addition, the Src inhibitor, PP1, or expression of dominant negative Ras also attenuated the transient phase of ERK phosphorylation. In contrast, inhibition of Ras or Src had no effect on the sustained ERK activity, which was critically dependent on phosphatidylinositol 3-kinase. The transient SAPK activity was suppressed by expression of a dominant negative form of MKK4. Furthermore, this kinase-deficient mutant inhibited 2-MeSADP-induced caspase-3 stimulation and the associated decrease in cell number. In conclusion, adenosine di-and triphosphate stimulation of the human P2Y 1 receptor can transiently activate the Ras-ERK cascade via the cooperative effects of phosphatidylinositol 3-kinase, Src and PKC. The sustained ERK stimulation, via a Ras-insensitive pathway, culminates in Elk-1 activation without inducing a proliferation effect. The transient SAPK activity did not evoke transcription factor phosphorylation but was required for the P2Y 1 receptor-mediated apoptotic function.Extracellular nucleotides can interact with cell surface P2 receptors both in the central nervous system and in peripheral tissues to produce a broad range of physiological effects. The P2 family is divided into two main types as follows: the P2X receptors are ligand-gated ion channels, and the P2Y receptors are G protein-coupled (1, 2). Part of the present study describes the signaling pathways of the P2Y 1 receptor, the first member of the P2Y family to be identified (3). The P2Y 1 receptor is widely distributed and has been described in mammalian heart, vascular, liver, kidney, prostate, gastrointestinal, pulmonary, connective, and immune tissues (4, 5). It has also been identified in skeletal muscle ...