TGFβ inhibits GM‐CSF‐induced phosphorylation of ERK and MEK in human myeloid leukaemia cell lines via inhibition of phosphatidylinositol 3‐kinase (PI3‐k)

Objectives:  Activation of SMAD‐independent p44/42 MAPK (ERK1/2) signalling by TGFβ has been recently reported in various cell types. However, the mechanisms for the linkage between the SMAD‐dependent and ‐independent pathways are poorly understood. In this study, we investigated whether TGF‐β activates the ERK pathway and how TGFβ communicates with the MAP kinase signals induced by a mitogen, in human myeloid leukaemia cells. Materials and methods and results:  TGFβ dramatically suppressed proliferation of MV4–11 and TF‐1 cells without detectable phosphorylation of ERK1/2 and MEK1/2 for the duration of 48 h, as detected by MTT assay and Western blot analysis, respectively. In contrast, GM‐CSF induced rapid and transient phosphorylation of MEK1/2 and ERK1/2 and up‐regulated cell proliferation. Both GM‐CSF‐induced ERK1/2 activation and cell proliferation were significantly inhibited by TGFβ. GM‐CSF also induced transient phosphorylation of the p85 subunit of PI3‐kinase. Corresponding to this change, phosphorylated p85 was found to bind to the GM‐CSF receptor‐α subunit, as detected by immunoprecipitation and Western blot analysis. PD98059, a selective inhibitor of MEK, blocked GM‐CSF‐induced phosphorylation of MEK and ERK but not p85. However, TGFβ and LY294002, a potent inhibitor of PI3‐kinase, significantly inhibited phosphorylation of both p85 and ERK1/2. Conclusions:  These studies thus indicate that TGFβ does not activate the ERK pathway but turns off the GM‐CSF‐induced ERK signal via inhibition of the PI3‐kinase‐Akt pathway, in these human laeukemia cells.