Negative Regulation of Erythroid Differentiation via the CBX8-TRIM28 Axis

Although the mechanism of chronic myeloid leukemia (CML) initiation through BCR/ABL oncogene has been well characterized, CML cell differentiation into erythroid lineage cells remains poorly understood. Using CRISPR-Cas9 screening, we identify Chromobox 8 (CBX8) as a negative regulator of K562 cell differentiation into erythrocytes. CBX8 is degraded via proteasomal pathway during K562 cell differentiation, which activates the expression of erythroid differentiation-related genes that are repressed by CBX8 in the complex of PRC1. During the differentiation process, the serine/threonine-protein kinase PIM1 phosphorylates serine 196 on CBX8, which contributes to CBX8 reduction. When CD235A expression levels are analyzed, the result reveals that the knockdown of PIM1 inhibits K562 cell differentiation. We also identify TRIM28 as another interaction partner of CBX8 by proteomic analysis. Intriguingly, TRIM28 maintains protein stability of CBX8 and TRIM28 loss significantly induces proteasomal degradation of CBX8, resulting in an acceleration of erythroid differentiation. Here, we demonstrate the involvement of the CBX8-TRIM28 axis during CML cell differentiation, suggesting that CBX8 and TRIM28 are promising novel targets for CML research.

TRIM28 facilitates type I interferon activation by targeting TBK1

Type I interferons play a fundamental role in innate host defense against viral infections by eliciting the induction of an antiviral gene program that serves to inhibit viral replication. Activation of type I interferon is regulated by the IRF3 transcription factor, which undergoes phosphorylation-dependent activation by the upstream kinase, TBK1, during viral infection. However, the mechanisms by which TBK1 achieves activation to support signaling to IRF3 remain incompletely understood. Here we identified the E3 ubiquitin ligase, tripartite motif containing 28 (TRIM28), as a positive regulator of type I interferon activation by facilitating TBK1 signaling. Genetic deletion of TRIM28 via CRISPR-Cas9 editing resulted in impaired type I interferon activation upon both RNA and DNA virus challenge, corresponding with increased susceptibility to virus infections in TRIM28 knockout cells. Mechanistically, TRIM28 interacted with TBK1 and mediated the assembly of K63-linked ubiquitin chains onto TBK1, a post-translational modification shown to augment TBK1 signal transmission events. TRIM28 knockout cells further displayed defective TBK1 phosphorylation and complex assembly with IRF3, resulting in impaired IRF3 phosphorylation. Altogether, our data demonstrate TBK1 to be a novel substrate for TRIM28 and identify TRIM28 as an essential regulatory factor in controlling innate antiviral immune responses.