The molecular interplay between USP32 and ERP44 in driving multiple myeloma progression and tumor growth.

Multiple myeloma (MM) is a malignancy of plasma cells with complex pathogenesis and unmet clinical needs. ERP44 is an Endoplasmic Reticulum (ER)-resident protein involved in protein folding, but its role in MM remains unclear. Kaplan-Meier survival analysis showed that high ERP44 expression correlated with poor prognosis in MM patients, prompting us to investigate its biological function and regulatory mechanisms. Using lentiviral-mediated overexpression and knockdown strategies in MM cell lines, we demonstrated that ERP44 significantly promoted cell proliferation in vitro, and ERP44 knockdown suppressed tumor growth in a xenograft mouse model. Mechanistically, we identified USP32 as an upstream regulator that interacted with and stabilized ERP44 through deubiquitination. Functional rescue experiments showed that ERP44 overexpression partially reversed USP32 knockdown-induced proliferation inhibition, confirming that USP32 promoted MM cell proliferation through ERP44. RNA sequencing combined with Western blotting revealed that ERP44 activated the PI3K-AKT signaling pathway, leading to upregulation of CDK2 and Cyclin D1 and driving cell cycle progression. Notably, the PI3K-AKT inhibitor LY294002 significantly attenuated the proliferative advantage conferred by ERP44 overexpression, suggesting this pathway as a critical downstream effector. Furthermore, ERP44 overexpression upregulated UPR markers (GRP78, XBP1, and CHOP) as well as osteolytic factors (DKK1, RANKL, and MIP-1α), indicating its broader role in UPR and bone destruction. Collectively, these findings identify ERP44 as a pro-proliferative factor in MM and establish a USP32-ERP44-PI3K-AKT regulatory axis.