Phosphatidic acid: a novel mechanical mechanism for how resistance exercise activates mTORC1 signalling

Resistance exercise and other models that promote skeletal muscle hypertrophy have clearly established an essential role for the mammalian target of rapamycin (mTORC1) pathway in regulating muscle protein synthesis and cell size (Baar & Esser 1999; Bodine et al. 2001). The rate of muscle protein synthesis is elevated following a single bout of resistance exercise and use of a specific mTOR inhibitor (rapamycin) can block the contraction-induced increase in protein synthesis in both rodents (Kubica et al. 2005) and humans (Drummond et al. 2009). However, the cellular mechanism(s) responsible for stimulating mTORC1 signalling following muscle contraction have remained elusive. A widely accepted notion has been that resistance exercise activates mTORC1 via phosphoinositide 3-kinase (PI3K) and its downstream target PKB/Akt since resistance exercise increases the expression of IGF-1 and the overexpression of PKB/Akt in mice results in a large amount of muscle hypertrophy (Bodine et al. 2001). On the other hand, last year The Journal of Physiology published an interesting paper in which it was suggested that the IGF-1 receptor in skeletal muscle is not required for overload-induced muscle hypertrophy (Spangenburg et al. 2008). The findings by Spangenburg et al. were surprising and suggested that IGF-1 activation of the PI3K pathway may not be necessary for the contraction-induced activation of mTORC1.

In this issue of The Journal of Physiology, O'Neil and colleagues provide compelling evidence that activation of mTORC1 by resistance exercise requires the synthesis of phosphatidic acid by phospholipase D and is independent of PI3K–PKB signalling (O'Neil et al. 2009). Previous work by Hornberger and colleagues has shown that intermittent passive stretch in skeletal muscle activates mTORC1 by direct binding of phosphatidic acid to mTOR (Hornberger et al. 2006) and is independent of PI3K–PKB signalling (Hornberger & Chien 2006). In the current study the authors extended their findings into an in vivo rodent resistance exercise model. First, they measured the signalling response following an acute bout of eccentric exercise and detected a different temporal response in mTORC1 and PI3K–PKB signalling. The phosphorylation of PKB was immediate and very transient lasting only a few minutes as compared to the prolonged (>12 h) phosphorylation of S6K1 (a downstream target of mTORC1). To verify that PI3K–PKB signalling is not necessary for mTORC1 activation, the authors then developed an ex vivo eccentric contraction model which produced a similar signalling response as compared to their in vivo model. Using Wortmannin, an inhibitor of PI3K, they were able to show that the insulin-induced increase in S6K1 phosphorylation was completely blocked but Wortmannin did not prevent the eccentric exercise increase in S6K1 phosphorylation. Furthermore, in the ex vivo model, the authors demonstrated that muscle phosphatidic acid concentration and S6K1 phosphorylation increased following eccentric exercise and could be blocked by l-butanol (an inhibitor of phospholipase D). Next, the authors used C2C12 muscle cells to determine whether phosphatidic acid could activate mTOR independent of PI3K signalling. In these experiments they incubated the muscle cells with varying concentrations of phosphatidic acid which led to a progressive increase in S6K1 phosphorylation. When rapamycin was added to the cells the phosphatidic acid-induced increase in S6K1 phosphorylation was completely blocked but Wortmannin had no effect. Finally, the authors went back to their in vivo resistance exercise model in which it has been shown that eccentric exercise induced growth in the tibialis anterior muscle but not in the soleus muscle undergoing concentric contractions (Baar & Esser, 1999). They examined the tibialis anterior and found that mTORC1 was significantly activated following exercise and this correlated with an increase in muscle phosphatidic acid concentrations. Phosphatidic acid concentration and mTORC1 activation were unaffected in the soleus. Overall, these series of well-designed experiments clearly show that an increase in phosphatidic acid can activate mTORC1 (independent of the PI3K–PKB pathway) following resistance exercise.

At this point in time, the data from this study (O'Neil et al. 2009) provide the strongest evidence for how mechanical events, such as muscle contraction during resistance exercise, are transduced to growth-promoting pathways within the muscle cell. However, we should keep in mind that very little is known about how mechanical stimuli induce muscle hypertrophy and that other mechanisms are probably involved. For example, an earlier publication this year in The Journal demonstrated that the mammalian Vps34 (a class 3 PI3K which is also an amino acid sensor) is activated following a similar in vivo eccentric exercise model in rodents (MacKenzie et al. 2009). The increase in mVps34 activity activates mTORC1 and this may be due to increased leucine availability within the muscle cell (see Fig. 1). This is consistent with human studies which have shown an increase in leucine transport into muscle 3 h following resistance exercise (Biolo et al. 1995) and increased muscle intracellular leucine concentration immediately following resistance exercise (Dreyer et al. 2006). Unfortunately, the temporal response of muscle leucine, mVps34 and S6K1 activity does not always correlate and additional work is required to determine whether mVps34 activation following resistance exercise is directly responsible for mTORC1 activation following resistance exercise. In any event, the finding by O'Neil and colleagues (O'Neil et al. 2009) that mechanical stimulation of mTORC1 is dependent on the synthesis of phosphatidic acid is exciting and an excellent addition in our effort to elucidate the complex pathways through which resistance exercise regulates mTORC1 signalling and promotes muscle growth.

Figure 1.

Mechanical mechanisms responsible for mTORC1 activation.