Nonstochastic reprogramming from a privileged somatic cell state.

Reprogramming somatic cells to induced pluripotency by Yamanaka factors is usually slow and inefficient and is thought to be a stochastic process. We identified a privileged somatic cell state, from which acquisition of pluripotency could occur in a nonstochastic manner. Subsets of murine hematopoietic progenitors are privileged whose progeny cells predominantly adopt the pluripotent fate with activation of endogenous Oct4 locus after four to five divisions in reprogramming conditions. Privileged cells display an ultrafast cell cycle of ∼8 hr. In fibroblasts, a subpopulation cycling at a similar ultrafast speed is observed after 6 days of factor expression and is increased by p53 knockdown. This ultrafast cycling population accounts for >99% of the bulk reprogramming activity in wild-type or p53 knockdown fibroblasts. Our data demonstrate that the stochastic nature of reprogramming can be overcome in a privileged somatic cell state and suggest that cell-cycle acceleration toward a critical threshold is an important bottleneck for reprogramming. PAPERCLIP:

A STUB1 ubiquitin ligase/CHIC2 protein complex negatively regulates the IL-3, IL-5, and GM-CSF cytokine receptor common ß chain (CSF2RB) protein stability.

The IL-3, IL-5, and GM-CSF family of cytokines play an essential role in the growth, differentiation, and effector functions of multiple hematopoietic cell types. Receptors in this family are composed of cytokine-specific α chains and a common ß chain (CSF2RB), responsible for the majority of downstream signaling. CSF2RB abundance and stability influence the magnitude of the cellular response to cytokine stimulation, but the exact mechanisms of regulation are not well understood. Here, we use genetic screens in multiple cellular contexts and cytokine conditions to identify STUB1, an E3 ubiquitin ligase, and CHIC2 as regulators of CSF2RB ubiquitination and protein stability. We demonstrate that Stub1 and Chic2 form a complex that binds Csf2rb and that genetic inactivation of either Stub1 or Chic2 leads to reduced ubiquitination of Csf2rb. The effects of Stub1 and Chic2 on Csf2rb were greatest at reduced cytokine concentrations, suggesting that Stub1/Chic2-mediated regulation of Csf2rb is a mechanism of reducing cell surface accumulation when cytokine levels are low. Our study uncovers a mechanism of CSF2RB regulation through ubiquitination and lysosomal degradation and describes a role for CHIC2 in the regulation of a cytokine receptor.

CBL mutations drive PI3K/AKT signaling via increased interaction with LYN and PIK3R1.

Casitas B-lineage lymphoma (CBL) encodes an E3 ubiquitin ligase and signaling adaptor that regulates receptor and nonreceptor tyrosine kinases. Recurrent CBL mutations occur in myeloid neoplasms, including 10% to 20% of chronic myelomonocytic leukemia (CMML) cases, and selectively disrupt the protein's E3 ubiquitin ligase activity. CBL mutations have been associated with poor prognosis, but the oncogenic mechanisms and therapeutic implications of CBL mutations remain incompletely understood. We combined functional assays and global mass spectrometry to define the phosphoproteome, CBL interactome, and mechanism of signaling activation in a panel of cell lines expressing an allelic series of CBL mutations. Our analyses revealed that increased LYN activation and interaction with mutant CBL are key drivers of enhanced CBL phosphorylation, phosphoinositide-3-kinase regulatory subunit 1 (PIK3R1) recruitment, and downstream phosphatidylinositol 3-kinase (PI3K)/AKT signaling in CBL-mutant cells. Signaling adaptor domains of CBL, including the tyrosine kinase-binding domain, proline-rich region, and C-terminal phosphotyrosine sites, were all required for the oncogenic function of CBL mutants. Genetic ablation or dasatinib-mediated inhibition of LYN reduced CBL phosphorylation, CBL-PIK3R1 interaction, and PI3K/AKT signaling. Furthermore, we demonstrated in vitro and in vivo antiproliferative efficacy of dasatinib in CBL-mutant cell lines and primary CMML. Overall, these mechanistic insights into the molecular function of CBL mutations provide rationale to explore the therapeutic potential of LYN inhibition in CBL-mutant myeloid malignancies.