Enforced, selective activation of the ERK/MAPK pathway overrides stromal inhibition of megakaryocytic differentiation (indicated by glycophorin A down-regulation).

As in Figure 2, the cells analyzed consisted of either K562 parent cells (B) or K5ΔRafER.5 cells (A) engineered to express the estrogen-responsive Raf-1 kinase mutant ΔRafER. Cells were treated for 48 hours as indicated with either 25 nM TPA or 1 μM estradiol in the presence or absence of HESS-5 stromal cell monolayers. Cells treated as indicated were stained with PE–anti-GPA (GA-R 2-PE) or isotype-matched control antibodies and analyzed by single-color flow cytometry. Log fluorescence intensity is indicated on the x-axis.