Activation of Ras signaling pathways by pyrroloquinoline quinone in NIH3T3 mouse fibroblasts

Pyrroloquinoline quinone (PQQ) has been implicated in certain physiological activities in mammals such as functioning as a potent growth factor in mice, and promoting DNA synthesis in human fibroblasts. These are clearly important physiological functions, however, the molecular mechanisms involved in PQQ activity are not yet fully understood. In order to address this, in this study we analyzed the effects of PQQ on the proliferation of NIH3T3 mouse fibroblasts and on their intracellular signal transduction mechanism. When activated c-Ha-ras-transformed NIH3T3 cells were treated with PQQ in the presence of 0.5% calf serum in DMEM, the cells showed significantly increased viability. After PQQ addition, flow cytometric analysis revealed a decrease in the population of cells in the G0/G1 phase and a concomitant increase in cells in the S and G2/M phases. Although treatment with SNAP, an NO donor, reduced cell viability, this effect was abolished by the addition of PQQ. Activation of ERK and PKC-epsilon was detected immediately after the addition of PQQ, and subsequent increases in the phosphorylation of Rb and c-Jun were observed. On the other hand, protein expression levels of growth-inhibitory molecules such as IkappaB and p27 decreased after PQQ treatment. These results suggest that PQQ stimulates cell proliferation through NO-sensitive Ras-mediated signaling pathways.