The PI3 kinase inhibitor caffeine inhibits Epo-induced Akt, GSK3α/β and Erk1/2 activation. Starved PEPs were mock-stimulated (m) or treated with 0.3 U/ml Epo or 25 ng/ml SCF as indicated. Some of the samples were pretreated with caffeine as detailed below. 100 μg cell protein were immunoblotted with P-STAT5, P-Akt, P-GSK3 α/β, P-Erk1/2 or Akt antibodies. Ø-lanes represent non-starved and non-treated PEPs. (A) Cells were pretreated with 0.1, 1 or 10 mM caffeine for 1 h before Epo stimulation and with 10 mM before SCF stimulation where indicated or (B) with 0.1, 0.3, 1, 3 or 10 mM caffeine for 1 h.