Rapamycin-resistant phosphorylation of the initiation factor-4E-binding protein (4e-BPI) in v-src-transformed hamster fibroblasts

Increased phosphorylation of the translational repressor protein 4E-BPI was found in the cell line derived from the tumor induced in Syrian hamster by Rous sarcoma virus (RSV), This was accompanied by its dissociation from th e complex with initiation factor eIF4E, The ribosomal S6 protein kinase p70 (S6k) is supposed to be regulated by the same or a closely related rapamyci n-sensitive signalling pathway to that which modulates 4E-BPI. Phosphorylat ion and activity of p70(S6k) were found to be also increased in RSV-transfo rmed H19 cells that express significantly higher amounts of the Src protein (p60(src)) relative to the non-transformed hamster fibroblasts NIL-2. The increased activity and phosphorylation of p70(S6k) were blocked by rapamyci n, indicating that the rapamycin-sensitive pathway is involved in its regul ation in v-src-transformed hamster fibroblasts. In agreement with this, rap amycin reduced the expression of elongation factor eEF1 alpha (whose transl ation is regulated by a rapamycin-sensitive mechanism thought to involve p7 0(S6k)) and did not affect the production of a housekeeping protein, alpha- tubulin, in these cells. Synthesis of Src protein was also inhibited in cel ls treated with rapamycin, However, treatment of cells with a concentration of rapamycin sufficient to completely inhibit the activity and phosphoryla tion of p70(S6k) resulted in only partial de-phosphorylation of 4E-BPI and its re-association with eIF4E in the transformed cells, indicating that add itional rapamycin-insensitive mechanisms/pathways are implicated in the con trol of 4E-BPI phosphorylation in RSV-transformed hamster fibroblasts, Over -expression of eIF4E favours cell proliferation and can lead to a transform ed phenotype, while over expression of 4E-BPI has the opposite effect. The altered signalling to the phosphorylation of 4E-BPI in RSV-transformed cell s, which leads to its dissociation from eIF4E and thus relief of inhibition of eIF4E function, may therefore represent an important regulatory mechani sm in malignant cell growth, (C) 1999 Wiley-Liss, Inc.