(A) Western blot analysis of PSer¹³³-CREB and total CREB protein performed in HEL cells before (lane 1) and after stimulation with 100 ng/mL of TPO (lanes 2 through 5) or FK (lanes 6 and 7) in the absence (positive control, lane 2) or presence of 10⁻⁵ mol/L (lanes 3 and 7) or 5 × 10⁻⁵mol/L (lanes 4 and 5) of the specific MAPK inhibitor, PD 98059. Results from one representative experiment of three separate experiments are shown. The densitometric analysis is expressed in arbitrary units (a.u.). (B) Flow cytometric analysis of integrin α_IIbβ3 receptors at the plasma membrane of HEL cells left untreated or treated for 48 hours with 100 ng/mL TPO in the absence or presence of 5 × 10⁻⁵ mol/L PD98059, which was readded in culture every 8 hours. Results from one representative experiment of three separate experiments are shown. X axis: relative α_IIbβ3 (CD41a) fluorescence intensity. Y axis: relative cell number. (C) Assay of MAPK catalytic activity performed in HEL cells treated with TPO (100 ng/mL) or Epo (2 U/mL) for various time points. Data are expressed in cpm as the means ± SD of four separate experiments performed in duplicate.