FTY720-P promotes Jak2 and suppresses PP2A activities. (A) Schematic representation of the FTY720-P–induced and SIPR1/Jak2 (WT and V617F)–mediated PP2A inhibition. (B) Western blots show levels of active Jak2 (pY972) and Grb2 in parental Ba/F3 and Ba/F3-Jak2^V617F cells untreated and treated with the S1PR1 agonists FTY720-P (2.5 µM), SEW 2871 (10 µM), and S1P (1 µM), and with the PP2A activators FTY720 (2.5 µM), OSU-2S (2.5 µM), and (S)-FTY720-regioisomer (2.5 µM), respectively. (C) PP2A activity in Ba/F3 cells untreated and treated with FTY720 (2.5 µM), SEW 2871 (10 µM), S1P (1 µM), FTY720-P (2.5 µM), and FTY720-P in combination with S1PR1 shRNA, TG101348 (1 µM), AS-604850 (1 µM), or PKC-412 (2 µM). (D) Effect of Jak2^V617F expression on the messenger RNA levels (quantitative reverse-transcription polymerase chain reaction) of specific SPHK1/2 isoforms in TF-1 cells. Fold change are relative to levels of SPHK1 and SPHK2 isoforms in parental TF-1 cells (left). Effect of Jak2^V617F expression on sphingosine kinase 1 (SPHK1) expression in TF-1 cells (middle top). SPHK1 kinase activity measured as fold change in the amount of sphingosine converted to sphingosine-1-phosphate (S1P) in Ba/F3-Jak2^V617F cells untreated and treated with FTY720 (2.5 µM) alone or in combination with okadaic acid (0.25 nM) (middle bottom). PP2A activity in Ba/F3 cells and in parental and S1PR1 shRNA-expressing Ba/F3-Jak2^V617F cells left untreated or treated with the transduced with the S1PR1 antagonist VPC 23019 (right).