Mature porcine peripheral blood mononuclear cells (PPBMCs) exist in a resting state both in vivo and when maintained in culture, with low translation rates consistent with their non-proliferative state. When cultured in the presence of the appropriate mitogen, there is a 2–4-fold increase in the rate of protein synthesis per ribosome within 4 h of stimulation [Kay, J. E., Ahern, T. and Atkins, M. (1971) Biochim. Biophys. Acta 247, 322–334]. Studies on extracts prepared from unstimulated cells have suggested lesions in initiation factor activity, primarily affecting the binding of mRNA to ribosomes [Ahern, T., Sampson, J. and Kay, J. E. (1974) Nature 248, 519–521].
In these studies, we have demonstrated that activation of quiescent PPBMCs with the phorbol ester phorbol 12-myristate 13-acetate or concanavalin A leads to a rapid 2–4-fold increase in the rate of protein synthesis within 1 h or 4 h, respectively, which is insensitive to the transcriptional inhibitor, 5,6-dichlorobenzimidazole riboside. Relative to control cells, both phorbol ester and concanavalin A induce a 2–4-fold increase in labelling of the eukaryotic initiation factor eIF-4α with phosphate in vivo, which primarily reflects a small net increase in phosphorylation rather than phosphate turnover on eIF-4α. Similarly, with the human leukaemic T cell line JURKAT, stimulation of the T cell receptor with the monoclonal antibody, OKT-3, or treatment with phorbol ester induces a 2–3-fold increase in eIF-4α phosphorylation within 30 min. Analysis of phosphorylation by two-dimensional gel electrophoresis and measurement of kinase activity towards synthetic peptides, indicate that this increased labelling also reflects increased eIF-4α kinase activity rather than phosphate turnover on eIF-4α.
Of central importance is the finding that, concomitant with increased rates of protein synthesis following stimulation of PPBMCs with either phorbol ester or concanavalin A, there is a significant increase in the level of eIF-4α recovered in high-molecular-mass complexes. These data suggest that, in quiescent PPBMCs, eIF-4F may be limiting and that the association of eIF-4α and eIF-4γ into high-molecular-mass complexes is regulated by phosphorylation and may play a pivotal role in translational control.