Taken together, these findings suggest that NGF and LPA exert cross-talk regulation both at the level of p42/p44 MAPK signaling and in the nuclear translocation of LPA(1) and Trk A receptors..     ///    "Fourth, the over-expression of C-terminal GRK2 peptide (which sequesters G-protein betagamma subunits) or beta-arrestin I clathrin binding domain (amino acids: 319-418) or pre-treatment of cells with pertussis toxin reduced the LPA- and NGF-dependent stimulation of p42/p44 MAPK. "    ///    "First, the treatment of PC12 cells, which express LPA(1) receptors, with a sub-maximal concentration of LPA and NGF induced synergistic activation of p42/p44 MAPK. "    ///    These findings support a model in which the Trk A receptor uses a G-protein-mediated mechanism to regulate the p42/p44 MAPK pathway.     ///     We report here that the nerve growth factor (NGF) and lysophosphatidate (LPA) receptor signaling systems interact to regulate the p42/p44 MAPK pathway in PC12 cells.     ///    "Second, the transfection of PC12 cells with LPA(1) receptor anti-sense construct, which reduced the expression of LPA(1), abrogated both LPA- and NGF-stimulated activation of p42/p44 MAPK. "    ///    "Third, the over-expression of recombinant LPA(1) receptor potentiated LPA- and NGF-dependent activation of p42/p44 MAPK. "