Requirement of Lipid Raft Integrity for Signaling by JAK2-V617F

Abstract 4191

JAK2 is a cytoplasmic tyrosine kinase that plays an important role in signaling following activation of various cytokine receptors. JAK2 activation promotes growth, survival, and differentiation of various cell types. Mutation of JAK2 is seen in numerous hematopoietic diseases, most notably in myeloproliferative neoplasms (MPNs). JAK2-V617F is a frequent mutation found in the classical MPNs: polycythemia vera, essential thrombocythemia, and primary myelofibrosis. The single amino acid change of valine to phenylalanine occurs in the pseudokinase domain of JAK2, relieving auto-inhibition of the kinase domain and allowing constitutive kinase activity. Numerous mouse models have demonstrated that JAK2-V617F can induce MPN-like disorders in mice. Thus, this point mutation, as well as other less common JAK2 mutations, is believed to play an important etiologic role in the development of MPNs in humans. The development and use of JAK2 inhibitors in clinical trials has shown promising results, again demonstrating the important role JAK2 plays in MPNs. While the JAK2-V617F mutation, as well as other JAK2 mutations, decreases auto-inhibition of JAK2 kinase activity, it is clear that mutated JAK2 still requires the expression of cytokine receptors to induce activation of transforming signals in hematopoietic cells. Normally, JAK2 binds to homodimeric and heterodimeric cytokine receptors through specific receptor motifs and is activated by various structural changes induced by cytokine stimulation. Following activation it utilizes receptor tyrosines as substrates for phosphorylation, leading to recruitment of downstream signaling molecules, such as STAT5, among others. JAK2 then activates STAT5 via phosphorylation and STAT5 then translocates to the nucleus to regulate transcription of target genes. JAK2-V617F does not require ligand for activation, but still requires the scaffolding function of cytokine receptors to facilitate its full activation and activation of downstream signaling via phosphorylation. Lipid rafts are microdomains of the plasma membrane that are enriched in cholesterol and sphingolipids. They have gained appreciation in signal transduction as sites of localization of signaling mediators, including membrane-bound receptors. Congregation of signaling proteins in lipid rafts within the plasma membrane promotes complex formation and signaling cascade activation. We have recently demonstrated that JAK2 is present in lipid rafts during erythropoietin signaling and that lipid raft integrity is required for erythropoietin-mediated signal transduction (Blood 2009, 114: 292). In our current study, we demonstrate that constitutive JAK-STAT signaling driven by JAK2-V617F is sensitive to lipid raft disruption. Human erythroleukemia (HEL) cells express constitutive activation of JAK-STAT signaling due to the presence of JAK2-V617F. Treatment of these cells with methyl-beta-cyclodextrin to disrupt lipid rafts abolished JAK2, STAT5, and STAT3 activation. Similar results are obtained in other cell lines harboring JAK2-V617F and that exhibit JAK-STAT activation that is dependent on this activated form of JAK2. We also demonstrate that JAK2-V617F co-localizes with lipid rafts, as shown by immunofluorescence, and that this co-localization is abolished by lipid raft disruption. This suggests the loss of JAK2-V617F-mediated JAK-STAT activation we observe following lipid raft disruption may be due to an inhibition of properly localized protein complex formation in the plasma membrane that is necessary for JAK2-V617F signaling. Lipid rafts may provide a site for an accumulation of JAK2-V617F-containing signaling complexes and may be necessary for the cellular signals initiated by JAK2-V617F. Our data show JAK2-V617F-driven JAK-STAT pathway activation is vulnerable to lipid raft disrupting agents and suggest lipid raft integrity as a potential therapeutic target for JAK2-V617F positive neoplasms. Targeting lipid rafts in combination with JAK2 kinase inhibitors may allow for more effective kinase inhibition at lower doses, potentially decreasing undesirable side effects associated with kinase inhibitor treatment.