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Depletion of Programmed Cell Death Protein 4 (PDCD4), an mTORC1/S6K1 Substrate, Attenuates Proteolysis in L6 Myotubes

The mammalian (mechanistic) target of rapamycin complex 1/ribosomal S6 Protein kinase 1 (mTORC1/S6K1) signaling pathway is a critical regulator of skeletal muscle protein synthesis and mass. Because mTORC1 signaling is also implicated in autophagy control, regulation of muscle protein mass by mTORC1 may be mediated by regulation of protein synthesis and/or degradation. We recently characterized the expression of programmed cell death protein 4 (PDCD4), a substrate of mTORC1/S6K1, in skeletal muscle. This pro apoptotic protein normally inhibits mRNA translation by binding to eukaryotic translation initiation factor 4A (eIF4A), but upon phosphorylation by S6K1, it is targeted for proteolysis by the ubiquitin dependent proteolytic system. Depletion of PDCD4 in myoblasts attenuated the suppression of protein synthesis that is normally observed in response to amino acid starvation. Surprisingly in myotubes, PDCD4 depletion led to a reduction in protein synthesis. This led us to hypothesize that PDCD4 likely regulates both protein synthesis and degradation, and that its depletion would lead to a greater suppression of proteolysis (compared to suppression of protein synthesis), thus leading to positive protein balance. L6 myotubes were depleted of PDCD4 by RNA interference. Thirty‐six hours later, cells were pulsed with S‐35 methionine/cysteine for 12 hours. To activate proteolysis, cells were then incubated either in amino acid‐free medium (starvation), or in 1 μM dexamethasone for varying lengths of time. Samples were then harvested and subjected to autoradiography to measure residual radioactivity. At 24–36 hours, starvation increased the disappearance of radiolabeled protein by 50–80% compared with non‐starved cells (P<0.05), consistent with increased proteolysis. In PDCD4 depleted cells, starvation‐induced proteolysis was suppressed by 40–60% (P<0.05). Ubiquitinated proteins accumulated to a greater extent (40–100%) in starved PDCD4‐depleted cells compared to cells treated with control siRNA (P<0.05), consistent with inhibition of degradation of ubiquitinated proteins. In dexamethasone treated cells, proteolysis was inhibited by ~20% in PDCD4 depleted cells compared to cells treated with control siRNA (P<0.001). These data are consistent with a role for PDCD4 in regulating not only protein synthesis but also proteolysis. Therefore, interventions that target PDCD4 may help in limiting muscle proteolysis seen in catabolic conditions like cancer and insulin resistance states.