The Ubiquitin Ligase RNF5 in Acute Myeloid Leukemia Growth and Response to Therapy

Introduction: Acute myeloid leukemia (AML) is a hematologic cancer of heterogeneous nature characterized by accumulation of somatic genetic alterations in hematopoietic progenitor cells that alter mechanisms of self-renewal, proliferation and differentiation. Despite the development of new investigational therapies, the majority of patients who achieve complete remission (CR) with initial chemotherapy will eventually relapse.

The ER stress (ERS) response plays a pivotal role in normal cellular homeostasis by governing cell commitment to survival or death in response to internal and external cues. Among ubiquitin ligases that play a key role in the ERS and unfolded protein response (UPR), is Ring Finger Protein 5 (RNF5), an ER-associated E3 ubiquitin ligase. RNF5 has been implicated in the regulation of stability and clearance of misfolded proteins. RNF5 controls basal autophagy with implication to bacterial clearance, and glutamine metabolism with implications to breast cancer response to ER stress based therapies.

Materials and Methods: The expression of RNF5 in cancer cells was studied by RT-PCR and Western blotting. RNF5 loss of function studies were performed using different shRNA with subsequent biological assessments (viability, proliferation, colony formation, and FACS analysis). MLL-AF9 mediated leukemogenesis was done using retroviral bone marrow transduction (from WT and RNF5 KO mice) and transplantation assay.

Results and Dicsussion: We observed that RNF5 expression is elevated in AML cell lines and patient samples. Using a panel of AML cell lines, we found that RNF5 is required for AML cell proliferation and clonogenicity. Inhibition of RNF5 expression in human and murine AML cells effectively attenuated their growth and induced cell death. By using MLL-AF9 transformed cells, we revealed that RNF5 loss decreased blast colony formation in vitro and significantly prolonged the survival of leukemic mice in vivo. Finally, loss of RNF5 sensitized AML cells, while its overexpression protected them from ER stress inducing therapy (i.e. bortezomib treatment).

Conclusion: Our data suggest that RNF5 constitutes a novel regulatory axis in the development of AML and its resistance to ER stress inducing therapies. The mechanism underlying RNF5 contribution to leukemogenesis will be discussed.