Researcher Università Vita-Salute San Raffaele milano, Italy
Introduction: TENT5C locus on chromosome 1p12 is mutated or deleted in up to 20% of myeloma (MM) patients, indicating a plasma cell (PC)-specific tumor suppressor activity. TENT5C is a non-canonical poly(A)polymerase that selectively polyadenylates and stabilizes mRNA encoding immunoglobulins (Igs) and endoplasmic reticulum (ER)-targeted proteins, thus promoting the proteomic reshaping occurring during B to PC differentiation and sustaining humoral immune responses. We have recently demonstrated that MM cells have an advantage in losing TENT5C to restrict Ig production saving energy for proliferation. In line, its re-expression in mutated MM lines boosts the secretory activity beyond sustainability, inducing intracellular ATP shortage, ROS accumulation and reduced cell growth in vitro. Our goal is to challenge the effects of TENT5C manipulation in in vivo models of MM, and to dissect the precise molecular circuits coordinating intensive Ig synthesis in PCs to unveil new MM vulnerabilities.
Methods: To dissect the molecular mechanisms involved in TENT5C oncosuppressive role in MM, we combined cutting-edge molecular biology, protein biochemistry and unbiased proteomics, and adopted in vivo models.
Results: To assess the impact of TENT5C manipulation in vivo, we silenced or overexpressed the protein in AMLC-2 cells that bear a hemizygous loss in chromosome 1p. Following intravenous and subcutaneous injections of TENT5C-manipulated cells into Rag2–/– IL2rg–/– mice, we found remarkably reduced tumor growth and longer survival in recipients of TENT5C-overexpressing cells, despite higher initial circulating LC levels. This in vivo association of higher proliferation with drastically decreased secretory activity clearly reveals that the presence of TENT5C mutations uncouples monoclonal component levels from disease burden. In parallel, we disclosed that TENT5C promotes the presence of calreticulin and CD38 in the cell surface, with translational implications for immunotherapies. Finally, to clarify how TENT5C manages to orchestrate a global cellular reshaping towards secretion, we focused on the few TENT5C-modulated proteins not belonging to the secretory apparatus. Upon TENT5C overexpression, we found a specific induction of nucleolar methyltransferases of rRNAs and tRNAs, whose functions are to promote ribosome assembly and to improve the translation of rare codons. This suggests that TENT5C mRNA stabilizing activity is coordinated with mechanisms that optimize protein translation in order to harmonize intensive antibody synthesis with cellular homeostasis, thus revealing a potential novel Achille’s heel of MM.
Conclusions: Altogether, our data disclose an unexpected molecular network, centered on TENT5C, regulating the trade-off between Ig synthesis and cell growth that may be exploited to design new therapeutic strategies aiming at altering PC protein homeostasis, eliminating the pathogenic clone, and improving the efficacy of current therapeutic options.
