We searched for genetic alterations in human B cell lymphoma that affect the ubiquitin-proteasome system. This approach identified FBXO25 within a minimal common region of frequent deletion in mantle cell lymphoma (MCL). FBXO25 encodes an orphan F-box protein that determines the substrate specificity of the SCF (SKP1-CUL1-F-box)(FBXO25) ubiquitin ligase complex. An unbiased screen uncovered the prosurvival protein HCLS1-associated protein X-1 (HAX-1) as the bona fide substrate of FBXO25 that is targeted after apoptotic stresses. Protein kinase Cδ (PRKCD) initiates this process by phosphorylating FBXO25 and HAX-1, thereby spatially directing nuclear FBXO25 to mitochondrial HAX-1. Our analyses in primary human MCL identify monoallelic loss of FBXO25 and stabilizing HAX1 phosphodegron mutations. Accordingly, FBXO25 re-expression in FBXO25-deleted MCL cells promotes cell death, whereas expression of the HAX-1 phosphodegron mutant inhibits apoptosis. In addition, knockdown of FBXO25 significantly accelerated lymphoma development in Eμ-Myc mice and in a human MCL xenotransplant model. Together we identify a PRKCD-dependent proapoptotic mechanism controlling HAX-1 stability, and we propose that FBXO25 functions as a haploinsufficient tumor suppressor and that HAX1 is a proto-oncogene in MCL.
Cyclin-dependent kinase subunit 1 (Cks1) is a critical rate-limiting component of the Skp1-Cullin1-Skp2 (SCF(Skp2)) ubiquitin ligase that controls cell cycle inhibitor abundance. Cyclin-dependent kinase (Cdk) inhibitors (CKIs) regulate hematopoietic stem cell (HSC) self-renewal, regeneration after cytotoxic stress and tumor cell proliferation. We thus studied the role of Cks1 in HSC and in a prototypic stem cell disorder, chronic myeloid leukemia (CML). Cks1 transcript was highly expressed in Lin-Sca-1+Kit+ (LSK) HSC, and the loss resulted in accumulation of the SCF(Skp2)/Cks1 substrates p21, p27, p57 and p130 particularly in CD150+ LSK cells. This accumulation correlated with decreased proliferation and accumulation of Cks1(-/-) HSC, slower regeneration after stress and prolonged HSC quiescence. At the hematopoietic progenitor (HPC) level, loss of Cks1 sensitized towards apoptosis. In CML, Cks1 expression was increased, and treatment with the Abl kinase inhibitor, imatinib, reduced Cks1 expression. Also, we found that Cks1 is critical for Bcr-Abl-induced cytokine-independent clonogenic activity. In conclusion, our study presents a novel function of Cks1 in maintaining HSC/HPC homeostasis and shows that Cks1 is a possible target in therapies aimed at the SCF(Skp2)/Cks1 complex that controls CKI abundance and cancer cell proliferation.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.