Aberrantly Expressed Mitochondrial Lipid Kinase, AGK, Activates JAK2–Histone H3 Axis and BCR Signal: A Mechanistic Study with Implication in CLL Therapy

Although the B-cell receptor (BCR) signal plays a critical role in chronic lymphocytic leukemia (CLL) cell survival and a target of current therapies (ibrutinib targets Bruton’s tyrosine kinase; idelalisib targets PI3Kδ), contribution of the cytokine-driven JAK2 pathway to the “CLL cell-survival signaling network” is largely undefined.

Patients with CLL were enrolled to investigate expression/activation of JAK2 and acylglycerol kinase (AGK), and their functional implication in primary CLL cell survival. A series of biochemical and molecular biology assays were employed to uncover the underlying mechanism.

We detected that compared with normal B cells, CLL cells aberrantly express constitutively active JAK2. Mechanistically, HSP90 forms a chaperoning complex with JAK2, resulting in its aberrant accumulation in CLL cells. We also discovered aberrant upregulation of a novel mitochondrial lipid kinase, AGK, which remains complexed with HSP90 in CLL cells activating JAK2. Although AGK is typically mitochondrial, we detected its nuclear localization in association with JAK2 in some CLL cells. Functionally, JAK2 phosphorylates its noncanonical substrate, histone H3(Y41), but not STAT3, activating transcription of diverse sets of genes in a patient-specific manner. Additionally, JAK2 activates the BCR signal in CLL cells via LYN/Bruton’s tyrosine kinase axis. Targeted inhibition of JAK2 as monotherapy, or in combination with the BCR inhibitors or venetoclax (a BCL2 inhibitor), induced apoptosis synergistically in CLL cells.

These findings suggest that aberrantly expressed AGK activates JAK2, independent of cytokine, leading to activation of diverse sets of gene transcription in CLL cells. Combined targeting of JAK2 and BCR signals or BCL2 may be effective in some patients with CLL.