cAMP- and rapamycin-sensitive regulation of the association of eukaryotic initiation factor 4E and the translational regulator PHAS-I in aortic smooth muscle cells.

Abstract

Incubating rat aortic smooth muscle cells with either platelet-derived growth factor BB (PDGF) or insulin-like growth factor I (IGF-I) increased the phosphorylation of PHAS-I, an inhibitor of the mRNA cap binding protein, eukaryotic initiation factor (eIF) 4E. Phosphorylation of PHAS-I promoted dissociation of the PHAS-I-eIF-4E complex, an effect that could partly explain the stimulation of protein synthesis by the two growth factors. Increasing cAMP with forskolin decreased PHAS-I phosphorylation and markedly increased the amount of eIF-4E bound to PHAS-I, effects consistent with an action of cAMP to inhibit protein synthesis. Both PDGF and IGF-I activated p70S6K, but only PDGF increased mitogen-activated protein kinase activity. Forskolin decreased by 50% the effect ofPDGF on increasing p7OS6K, and forskolin abolished the effect of IGF-I on the kinase. The effects of PDGF and IGF-I on increasing PHAS-I phosphorylation, on dissociating the PHAS-I-eIF-4E complex, and on increasing p7OS6K were abolished by rapamycin. The results indicate that IGF-I and PDGF increase PHAS-I phosphorylation in smooth muscle cells by the same rapamycin-sensitive pathway that leads to activation of p7OS6K.

Effects of protein phosphorylation on ubiquitination and stability of the translational inhibitor protein 4E-BP1

“…In agreement with numerous previous studies (see for example, Lin et al, 1994;Graves et al, 1995;Gingras et al, 1996;Mendez et al, 1996), mouse and human 4E-BP1 were detected as multiple bands on gels in the vicinity of 20 kDa. These correspond to differentially phosphorylated forms, with hyperphosphorylated (g) 4E-BP1 migrating more slowly than the less phosphorylated (b and a) forms.…”

Degradation of eukaryotic polypeptide chain initiation factor (eIF) 4G in response to induction of apoptosis in human lymphoma cell lines

“…Another potential mechanism by which cycloheximide might destabilize eIF4G is by promoting the phosphorylation of the 4E-BPs via activation of the mTOR/p70 S6 kinase signalling pathway (Krieg et al, 1988;Brunn et al, 1997;Fadden et al, 1997;Lawrence and Abraham, 1997). This would have the e ect of dissociating eIF4E from the 4E-BPs and favouring its association with eIF4G (Graves et al, 1995;Von Manteu el et al, 1996;Brunn et al, 1997), thus potentially altering the conformation of the latter (Ohlmann et al, 1997). However, to date, there is no evidence that the other inducers of apoptosis used in our studies impinge upon the mTOR signalling pathway and no reason to believe that the activation of this pathway would be a ected by caspase inhibitors.…”

“…However this interaction is inhibited by a small family of 4E binding proteins (4E-BPs) which can compete with eIF4G for binding to eIF4E Haghighat et al, 1995;Altmann et al, 1997;Matsuo et al, 1997). The extent of complex formation between the best studied of these proteins, 4E-BP1 (also known as PHAS-I) (Lawrence and , and eIF4E is regulated by the phosphorylation of 4E-BP1 (Graves et al, 1995;Von Manteu el et al, 1996;Brunn et al, 1997). This phosphorylation is activated by a pathway which can be inhibited by the macrolide immunosuppressant, rapamycin Von Manteu el et al, 1996;Beretta et al, 1996a;Fadden et al, 1997;Brunn et al, 1997).…”

Mannan and peptidoglycan induce COX‐2 protein in human PMN via the mammalian target of rapamycin

“…Whereas the secondary structure energy of the 5 0 -UTR of COX-2 is more consistent with transcriptional control, the presence of polypyrimidine tracts and/or as yet unidentified binding proteins may make COX-2 mRNA responsive to translational control via 5 0 -UTR in PMN. In addition, the detection of COX-2 mRNA in resting PMN, but not the protein, the effect of rapamycin at the concentrations tested [45][46][47][48], and the phosphorylation of 4E-BP1, which allows eIF4E to recognize the 7-methyl cap of mRNA and the initiation of translation, support post-transcriptional regulation.…”

“…Whereas the secondary structure energy of the 5 0 -UTR of COX-2 is more consistent with transcriptional control, the presence of polypyrimidine tracts and/or as yet unidentified binding proteins may make COX-2 mRNA responsive to translational control via 5 0 -UTR in PMN. In addition, the detection of COX-2 mRNA in resting PMN, but not the protein, the effect of rapamycin at the concentrations tested [45][46][47][48], and the phosphorylation of 4E-BP1, which allows eIF4E to recognize the 7-methyl cap of mRNA and the initiation of translation, support post-transcriptional regulation.Regulation of COX-2 protein by the 3 0 -UTR has been described at least in monocytes, macrophages, and neoplastic cells since COX-2 mRNA contains 23 copies of the AUUUA RNA instability element, which makes this mRNA short-lived and suitable for post-transcriptional regulation by mechanisms dependent on at least the p38-MAP kinase [49] and the PI3K routes [50]. Interestingly, a recent report using a preparation of platelets and monocytes has shown that efficient induction of COX-2 protein requires combination of signals involving activation of jB-driven transcriptional activation and stabilization of COX-2 mRNA by silencing the AU-rich mRNA element [51].…”