p85α Degradation Mediated by Ubiquitin E3 Ligase FBXO21 Offers Therapeutic Potential in Leukemia

Acute myeloid leukemia (AML) is a complex and heterogeneous disease characterized by the clonal expansion of myeloid blasts in the bone marrow. Despite significant therapeutic advances over the years, the prognosis for AML patients remains dismal, with high relapse rates and poor overall survival. The ubiquitin-proteasome system (UPS) plays a critical role in maintaining cellular homeostasis by regulating the degradation of proteins involved in essential processes such as cell cycle control, DNA repair, apoptosis, and various signaling pathways. Given this vital function, targeting ubiquitin E3 ligases within the UPS presents a promising strategy for developing more effective and targeted therapies in hematological cancers. High expression levels of the F-box ubiquitin E3 ligase, FBXO21, correlate with poor patient survival in various hematopoietic malignancies. We identified that silencing FBXO21 in human-derived AML cell lines and primary patient samples induces differentiation, inhibits tumor progression, and sensitizes cells to chemotherapy agents. Moreover, FBXO21 knockdown upregulates cytokine signaling pathways, further implicating its role in AML pathogenesis. Interestingly, our lab has previously shown that Fbxo21 knockout in our novel mouse model has minimal impact on normal hematopoiesis, suggesting that FBXO21’s role in AML is tumor specific. Through a mass spectrometry-based proteomic analysis, we identified that FBXO21 ubiquitinates p85α, the regulatory subunit of the phosphoinositide 3-kinase (PI3K) pathway, targeting it for degradation. This degradation leads to decreased PI3K signaling, dimerization of free p85α, and activation of ERK. Further, we found that the YccV domain of FBXO21 is crucial for the interaction with p85α and pinpointed the specific phosphodegron within p85α necessary for its ubiquitination. This knowledge facilitated the creation of a novel compound targeting FBXO21, offering a highly specific approach to modulating PI3K activity in AML. In vivo studies further confirmed FBXO21’s oncogenic role, as Fbxo21 knockdown delayed AML initiation, disease progression, and extended survival in mouse models. These findings underscore FBXO21 as a critical regulator of AML pathogenesis, with its targeting representing a promising therapeutic strategy that may overcome the limitations of current PI3K inhibitors and provide a more selective approach to treating hematopoietic malignancies.