Mechanism of extracellular signal-regulated kinase activation by the CB1 cannabinoid receptor

Cannabinoids, the active components of marijuana and their endogenous counterparts, exert many of their actions in brain through the seven-transmembrane receptor CB1. This receptor is coupled to the activation of the extracellular signal-regulated kinase (ERK) cascade. However, the precise molecular mechanism for CB1-mediated ERK activation is still unknown. Here, we show that in U373 MG human astrocytoma cells, CB1 receptor activation with the cannabinoid agonist Delta(8)-tetrahydrocannabinol dimethyl heptyl (HU-210) was coupled to ERK activation and protection from ceramide-induced apoptosis. HU-210-induced ERK activation was inhibited by tyrphostin AG1478 and PP2, widely employed inhibitors of the epidermal growth factor receptor (EGF(R)) and the Src family of cytosolic tyrosine kinases, respectively. However, HU-210 stimulation resulted in neither EGF(R) phosphorylation, Src tyrosine phosphorylation, nor increased Src activity. In addition, dominant-negative forms of both proteins were unable to prevent cannabinoid-induced ERK activation, thus excluding the existence of CB1-mediated EGF(R) transactivation or Src activation. Wortmannin and 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY294,002), inhibitors of the phosphatidylinositol 3-kinase (PI3K) signaling pathway, blocked cannabinoid-induced ERK activation. Likewise, HU-210 stimulated the PI3K downstream targets protein kinase B (PKB), as shown by its phosphorylation in Thr 308 and Ser 473 residues, and Raf-1. Moreover, betagamma subunit release mimicked ERK and PI3K/PKB activation, suggesting that activation of class IB PI3K mediates cannabinoid action. Pro-survival HU-210 action also required activation of both PI3K and ERK signaling pathways. In conclusion, CB1-induced ERK activation was mediated by PI3K(IB) and this effect may have important consequences in the control of cell death/ survival decision.