ATRA improves megakaryopoiesis in immune thrombocytopenia through up-regulating the HIF-1α/SPHK2/S1P pathway

Our previous clinical studies show that all-trans retinoic acid (ATRA) induces a sustained response in patients with immune thrombocytopenia (ITP). However, its mechanisms of action remain unclear. In this study, we observed disorganized cytoskeleton and impaired proplatelet formation (PPF) in megakaryocytes from patients with ITP. Metabolite profiling revealed reduced sphingosine 1-phosphate (S1P) levels in ITP. Decreased sphingosine kinase 2 (SPHK2) expression was responsible for the low S1P levels in ITP. In addition, S1P was essential for activating S1P receptor 1 and Rac1, which regulate cytoskeletal reorganization and PPF. Furthermore, hypoxia-inducible factor-1α (HIF-1α) was shown to mediate SPHK2 and S1P production. Decreased HIF-1α expression in megakaryocytes from patients with ITP contributed to impaired PPF. We subsequently found that ATRA up-regulated HIF-1α and corrected impaired PPF in vitro and in vivo. These findings reveal that ATRA targets the HIF-1α/SPHK2/S1P pathway to improve PPF dysfunction, offering mechanistic insights into its clinical efficacy in ITP.