The skeletal muscle is an integral part of our system. It not only acts as the storage reservoir of amino acids, but also serves as the site for protein synthesis and protein breakdown36. The rate of protein synthesis needs to exceed protein degradation to achieve muscle hypertrophy [18, 24]. The timing of protein intake, type and quantity play a significant role in achieving optimal outcomes when applied to resistance exercise36. Research has been going on since the past decade demonstrating the role of nutritional supplements like whey protein, soy, branched-chain amino acids (BCAAs, especially leucine) and creatine on protein synthesis before, during and after a bout of resistance exercise through careful investigations into intracellular signalling pathways like the mammalian target of rapamycin (mTOR) and its downstream targets-ribosomal protein S6 (kinase-1) and 4E binding protein (4E-BP1) [7, 18]. Intracellular signalling, amongst other variables, involves three essential components- abundant ATP in muscle for providing energy, insulin signalling and leucine (figure 1) [36, 37]. mTOR, regarded as the “key regulator” of translation comprises mTORC1 and mTORC2 36. mTORC1 plays a significant role in promoting muscular hypertrophy via phosphorylation of S6K1 and 4E-BP1 which prevent binding of the eukaryotic initiation factor (eIF) 4E to 4E-BP1 allowing a complex formation with eIF4G (eIF4E-eIF4G) thus enhancing protein synthesis [5, 34].
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Figure 1.The three essential components for signalling: Leucine, insulin and ATP affect intracellular signalling of mTOR, thereby stimulating the rate of protein synthesis. Leucine directly and indirectly (via insulin synthesis) activates mTOR pathway while reducing ATP levels within the cell reduces the rate of protein synthesis 36.
Another important signalling pathway affected by nutrition and resistance exercise is the extracellular signal-regulated kinase ½ (ERK1/2) which stimulates translation independent of mTOR via mitogen activated protein kinase (MAPK) signalling to eIF4E (figure 2) [9, 35]. Insulin signalling indirectly phosphorylates the phosphatidiylinositol-3-kinase (PI-3K) pathway facilitating glucose transport into the cell via a glucose transporter (GLUT4)23. Akt (component of mTOR pathway) activates mTOR and inactivates glycogen synthase kinase (GSK-3) stimulating eIF2B and mTOR thereby enhancing protein synthesis19. Despite this, high insulin levels fail to achieve significant protein synthesis in the absence of high levels of amino acids [3, 36]. Lastly, leucine by far has proved to be the most effective of amino acids in stimulating protein synthesis either directly through mTOR phosphorylation or indirectly by activating mTOR via the insulin pathway [26, 36]. This report highlights how whey proteins and BCAA (leucine) affect muscle protein synthesis via intracellular signalling pathways thereby contributing significant effects on muscular hypertrophy.
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Figure 2.A diagrammatic representation of the interaction between mTOR and ERK1/2 in stimulating protein synthesis following muscular contractions and/or insulin synthesis: Contractions activate ERK1/2 via the Ras signalling cascade which eventually stimulates the formation of the translation initiation complex leading to protein synthesis. The figure also shows the components inhibited as a result of the signalling (arrows with rounded ends). The dashed arrow represents the possible conversion to rpS6. mTOR-mammalian target of rapamycin, MEK1/2-Mitogen-activated protein kinase1/2, ERK1/2-extracellular signalregulated kinase1/2, MNK1-MAP kinase-interacting kinase1, PI-3K-phosphatidylinositol 3- kinase, PKB-protein kinase B, TSC1/TSC2-tuberous sclerosis complex, rpS6-ribosomal protein S6, Rheb-ras homolog enriched in brain [7, 16, 25].
The effects of whey protein supplementation
Whey protein, particularly whey protein concentrates (WPC 80) and isolates (WPI) have proved useful in individuals who are regularly training [8, 12]. Whey provides a quick and efficient recovery thereby enhancing performance, anaerobic fitness, muscle mass and body composition12. When applied to resistance exercise, whey protein supplementation has been found to increase muscle bulk by stimulating protein synthesis [8, 12, 18]. In a study demonstrating the effects of whey on subjects undergoing one session of resistance exercise, a rise in the phosphorylation of p70S6 K and 4E-BP1 was observed12. mTOR phosphorylation remained high not only from 1 hour to 48 hours post exercise, but also after 21 weeks of resistance training, showing that whey plays a critical role in mTOR signalling. This aspect was further investigated in a double-blind, placebo controlled trial which looked into the effect of 20 grams of WPI ingestion on mTOR signalling in young men8. The results of this study were in conjunction with the former trial above demonstrating an increase in mTOR phosphorylation 2 hours post exercise following WPI intake. Activation of mTOR was brought about by the phosphorylation of Ser2448 via leucine and Akt [14, 15]. p70S6 K was phosphorylated early on whey administration8. The increase in p70S6 K phosphorylation by whey protein activated the TORC1 complex along with mTOR and its regulatory proteins [13, 35]. A probable reason for this mechanism might lie in the rich source of BCAA which make up the whey protein. The BCAA’s evoke a similar response with respect to p70S6 K after a bout of resistance exercise [4, 13]. Moreover, activation of p70S6 K relies on the ability of these amino acids to stimulate the downstream signalling of rpS614. Higher doses of WPI would be required to activate rpS6 post exercise (i.e. during recovery). 4E-BP1 phosphorylation at Thr37 /46 (site-specific phosphorylation) increased by the ingestion of whey alone after 2 hours of resistance training [8, 12]. This prevention further dissociated 4E-BP1 from eIF4E, thus increasing protein synthesis12. These results suggested that 4E-BP1 phosphorylation could occur without depending on TORC1 complex unlike p70S6 K, reflecting that whey supplementation demonstrated TORC1 dependent and independent effects on mTOR signalling [13, 34, 35].
Signal transduction pathways such as PI3K/Akt activated by resistance training are responsible for eliciting skeletal hypertrophy (13,16). Akt is highly phosphorylated at Ser473 post 1 hour of resistance exercise for a short period of time (19,20). However, the two studies showed contradictory results- Farnfield et al. showed that Akt phosphorylation remained unchanged post resistance exercise on ingesting 26.6 grams WPI while Hulmi et al. reported a decline in Akt phosphorylation after 21 weeks of resistance training [8, 12]. This might be attributed to the differences in the duration of exercise, type of training and nutritional status 12. Whey protein intake was successful in maintaining sustained mTOR signalling in response to resistance exercise and training. Future studies should be aimed at defining the appropriate timings and dose regimens of WPI to stimulate protein synthesis, in relation to resistance exercise8.