Ca²⁺- and phospholipase D-dependent and -independent pathways activate mTOR signaling

The mammalian target of rapamycin (mTOR) promotes increased protein synthesis required for cell growth. It has been suggested that phosphatidic acid, produced upon activation of phospholipase D (PLD), is a common mediator of growth factor activation of mTOR signaling. We used Rat-1 fibroblasts expressing the α_1A adrenergic receptor to study if this G _q-coupled receptor uses PLD to regulate mTOR signaling. Phenylephrine (PE) stimulation of the α_1A adrenergic receptor induced mTOR autophosphorylation at Ser2481 and phosphorylation of two mTOR effectors, 4E-BP1 and p70 S6 kinase. These PE-induced phosphorylations were greatly reduced in cells depleted of intracellular Ca²⁺. PE activation of PLD was also inhibited in Ca²⁺-depleted cells. Incubation of cells with 1-butanol to inhibit PLD signaling attenuated PE-induced phosphorylation of mTOR, 4E-BP1 and p70 S6 kinase. By contrast, platelet-derived growth factor (PDGF)-induced phosphorylation of these proteins was not blocked by Ca²⁺ depletion or 1-butanol treatment. These results suggest that the α_1A adrenergic receptor promotes mTOR signaling via a pathway that requires an increase in intracellular Ca ²⁺ and activation of PLD. The PDGF receptor, by contrast, appears to activate mTOR by a distinct pathway that does not require Ca²⁺ or PLD.