Abstract
BackgroundIn spite of major advances in treatment, multiple myeloma (MM) is currently an incurable malignancy due to the emergence of drug-resistant clones. We previously showed that MM cells upregulate the transcriptional repressor, growth factor independence 1 (Gfi1), in bone marrow stromal cells (BMSCs) that induces prolonged inhibition of osteoblast differentiation. However, the role of Gfi1 in MM cells is unknown.MethodsHuman primary CD138+ and BMSC were purified from normal donors and MM patients’ bone marrow aspirates. Gfi1 knockdown and overexpressing cells were generated by lentiviral-mediated shRNA. Proliferation/apoptosis studies were done by flow cytometry, and protein levels were determined by Western blot and/or immunohistochemistry. An experimental MM mouse model was generated to investigate the effects of MM cells overexpressing Gfi1 on tumor burden and osteolysis in vivo.ResultsWe found that Gfi1 expression is increased in patient’s MM cells and MM cell lines and was further increased by co-culture with BMSC, IL-6, and sphingosine-1-phosphate. Modulation of Gfi1 in MM cells had major effects on their survival and growth. Knockdown of Gfi1 induced apoptosis in p53-wt, p53-mutant, and p53-deficient MM cells, while Gfi1 overexpression enhanced MM cell growth and protected MM cells from bortezomib-induced cell death. Gfi1 enhanced cell survival of p53-wt MM cells by binding to p53, thereby blocking binding to the promoters of the pro-apoptotic BAX and NOXA genes. Further, Gfi1-p53 binding could be blocked by HDAC inhibitors. Importantly, inoculation of MM cells overexpressing Gfi1 in mice induced increased bone destruction, increased osteoclast number and size, and enhanced tumor growth.ConclusionsThese results support that Gfi1 plays a key role in MM tumor growth, survival, and bone destruction and contributes to bortezomib resistance, suggesting that Gfi1 may be a novel therapeutic target for MM.
Highlights
In spite of major advances in treatment, multiple myeloma (MM) is currently an incurable malignancy due to the emergence of drug-resistant clones
We recently reported that MM cells upregulate the transcriptional repressor growth factor independence 1 (Gfi1) in bone marrow stromal cell (BMSC), which induces epigenetic changes in the Runx2 gene to inhibit osteoblast (OB) differentiation [5] thereby increasing MM cell growth and chemoresistance [5]
Gfi1 expression is upregulated in human MM cells Gfi1 mRNA levels were significantly increased in human CD138+ cells from MM patients compared with normal donors (Additional file 1: Table S1) (Fig. 1a)
Summary
In spite of major advances in treatment, multiple myeloma (MM) is currently an incurable malignancy due to the emergence of drug-resistant clones. We previously showed that MM cells upregulate the transcriptional repressor, growth factor independence 1 (Gfi1), in bone marrow stromal cells (BMSCs) that induces prolonged inhibition of osteoblast differentiation. We recently reported that MM cells upregulate the transcriptional repressor growth factor independence 1 (Gfi1) in bone marrow stromal cell (BMSC), which induces epigenetic changes in the Runx gene to inhibit osteoblast (OB) differentiation [5] thereby increasing MM cell growth and chemoresistance [5]. Gfi encodes a nuclear zinc finger DNA-binding protein that acts as a transcriptional repressor of genes involved in hematopoiesis and hematopoietic stem cell self-renewal and quiescence [6]. Gfi protein levels are differentially regulated by the ubiquitin-proteasome system during myeloid differentiation with rapid proteasomal degradation in granulocytes and stabilization in immature myeloid cells [14]
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