Phosphorylation of SEK1/MKK4 was not induced by IL-3, Epo, or Tpo stimulation. FDC-P2 cells (A and D), FD-EPO cells (B), and FD-TPO cells (C) were stimulated with IL-3 (A), Epo (B), and Tpo (C), respectively, for the indicated time up to 60 minutes or stimulated with (+) or without (−) sorbitol (D) for 30 minutes. Total cell lysates were separated by SDS-PAGE and immunoblotted with anti-phospho–specific SEK1/MKK4 antibody. An arrow indicates the phosphorylated SEK1 (46 kD).