“miRNAs profile of bone marrow fibroblasts in multiple myeloma: relationship with disease progression and drug-resistance”

PURPOSE: microRNAs (miRs) regulate gene expression at post-transcriptional level modulating several biological processes. Bone marrow (BM) fibroblasts (or cancer associated fibroblasts, CAFs) from active multiple myeloma (MM) patients present an activated phenotype (FSP1+/FAP+/αSMA+), with higher proliferative rate compared to monoclonal gammopathy of undetermined significance (MGUS) CAFs1. BM CAFs from bortezomib (bort)-resistant patients are resistant in vitro to the drug and prevent bort-induced apoptosis of co-cultured MM cells2. Our purpose was to investigate whether a specific miR profile is associated to the phenotype and functional activities of BM CAFs in MGUS to MM transition and drug resistance. METHODS: miRs expression was analyzed by microarray and validated by qRT-PCR and flow cytometry on CAFs purified from BM aspirates of MGUS and MM patients. miRs target genes were identified by interrogating different tools commonly used to predict human miR gene targets and validated by western blot analysis. miRs functional effects were analyzed in CAFs transiently transfected with miRCURY LNA inhibitors and mimics. RESULTS AND DISCUSSION: MM and MGUS CAFs showed a different miRs profile, including 9 up-regulated and 17 down-regulated miRs. Among the over-expressed miRs, we focused on miRs showing a major significant p-value: miR-27b-3p and -214-3p. Target genes of miR-27b-3p and -214-3p were FBXW7 and PTEN, respectively, involved in cell apoptosis, proliferation and CAFs activation. Inhibition of miR-27b-3p induced the over-expression of FBXW7, an ubiquitin ligase, which negatively modulated the expression of MCL-1, NOTCH and Cyclin E1/2. miR-214-3p inhibition increased PTEN levels down-regulating the AKT/GSK3 pathway and Cyclin D1. Finally, co-cultures of MM cells with CAFs and bort treatment increased miRs expression. CONCLUSIONS: MGUS to MM transition and drug resistance is related to a specific miRs profile. Over-expression of miR-27b-3p and -214-3p induces cell proliferation and resistance to spontaneous and bort-induced apoptosis in MM CAFs. BIBLIOGRAPHY: 1. Frassanito MA et al. Leukemia 2014. 2. Frassanito MA et al. Leukemia 2016.