with invasion, progression or increased aggres sion of numerous, but not all, cancers, including ovar ian, lung, breast, liver and colon cancers. method As a substrate of mTORC1 S6K1, PDCD4 may me diate the effect of this kinase pathway on protein synthesis in skeletal muscle. However, not much is known about the role or regulation of PDCD4 in muscle, the tissue that is quantitatively the most important in whole body protein metabolism. It was recently shown that the abundance of PDCD4 in rat skeletal muscle is sensitive to feeding and food deprivation cycle, its abundance increased in skeletal targeted by S6K1 phosphorylation. Fur thermore, serum and amino acid deprivation had no effect on phosphorylation on Ser457, although phos phorylation on this residue was increased by refeeding.
However, PDCD4 abundance in creased more than four fold in starved cells and decreased progressively with time during refeeding such that by 3 h of refeeding, values in re fed cells were not different from control. Incubation with rapa mycin, an mTORC1 inhibitor, abolished the effect of re feeding on PDCD4 abundance. Because the ubiquitin system is implicated in the phosphorylation dependent degradation of PDCD4, we incubated the cells with MG132, a proteasome inhibitor. muscle of food deprived rats, but in fed or refed rats, its abundance decreased along with increase in muscle fractional protein synthesis. These data suggest that interventions that regulate PDCD4 abundance may be explored in the treatment of muscle wasting, a feature of diseases like cancer, AIDS, and trauma.
However this study was mainly correlative and did not examine whether or not mTORC1 S6K1 is required for PDCD4 regulation in muscle. In the present work, using L6 Cilengitide myotubes, our specific ob jectives were to, 1 examine the requirement for mTORC1 S6K1 and the ubiquitin proteolytic system in regulating PDCD4, 2 examine the contribution of amino acids vs. growth factors in mediating the effect of nutrition on PDCD4, and 3 determine whether nutritional status af fects the interaction of PDCD4 with components of eIF4F. Because others have suggested that signalling pathways that regulate protein metabolism may be regulated differ ently in myotubes versus myoblasts and because the regulation of PDCD4 may depend on cell type, we also assessed the effect of PDCD4 depletion by RNA inter ference on myotube total and myofibrillar protein synthesis.
Results Abundance of PDCD4 in L6 myotubes selleck kinase inhibitor is sensitive to medium composition and requires mTORC1 and the proteasome Given the identification of PDCD4 as a substrate of mTORC1 S6K1 signalling, and the fact this kinase pathway is regulated by nutrients, we examined the ef fect of nutrient deprivation on the regulation PDCD4 in L6 myotubes. Neither 12 h of serum and amino acid deprivation nor refeeding in a complete medium had any significant effect on PDCD4 Ser67. Growth factors, but not amino acids, regulate PDCD4 abundance The experiments above did not indicate whethe