Abstract
Side population (SP) cells are an enriched source of cancer-initiating cells with stemness characteristics, generated by increased ABC transporter activity, which has served as a unique hallmark for multiple myeloma (MM) stem cell studies. Here we isolated and identified MM SP cells via Hoechst 33342 staining. Furthermore, we demonstrate that SP cells possess abnormal cell cycle, clonogenicity, and high drug efflux characteristics-all of which are features commonly seen in stem cells. Interestingly, we found that bortezomib, As2O3, and melphalan all affected apoptosis and clonogenicity in SP cells. We followed by characterizing the miRNA signature of MM SP cells and validated the specific miR-451 target tuberous sclerosis 1 (TSC1) gene to reveal that it activates the PI3K/Akt/mTOR signaling in MM SP cells. Inhibition of miR-451 enhanced anti-myeloma novel agents' effectiveness, through increasing cells apoptosis, decreasing clonogenicity, and reducing MDR1 mRNA expression. Moreover, the novel specific PI3K/Akt/mTOR signaling inhibitor S14161 displayed its prowess as a potential therapeutic agent by targeting MM SP cells. Our findings offer insights into the mechanisms regulating MM SP cells and provide a novel strategy to overcome resistance to existing therapies against myeloma.
Highlights
Multiple myeloma (MM) is a clonal B-cell malignancy with terminally differentiated plasma cells
To isolate and provide evidence that the Side population (SP) cells exist in myeloma cells, we analyzed a panel of cell lines, including NCI-H929, RPMI 8226, KMS-11, LP-1, U266, and SKO, as well as seven primary myeloma cells with Hoechst 33342 staining
To determine if MM stem-like cells exist in a relatively quiescent state, SP and main population (MP) cells isolated from the NCI-H929 and KMS-11 cell lines were examined for their cell cycle status
Summary
Multiple myeloma (MM) is a clonal B-cell malignancy with terminally differentiated plasma cells. It has long been postulated that a small population of cancer stem cells (CSCs) persist in bone marrow niches resulting in the development of refractory clones and disease relapse [3]. Previous groups have successively described various CSC phenotypes in MM, including CD138-/CD34- with memory B-cells (CD19+/CD27+) [4, 5], CD138-/CD19+ cells with cytoplasmic light chain-restricted (LCR) cells [6], CD19-/CD45low-/CD38high/CD138+ [7], and CD138+ cell [8]. The distinct myeloma CSC marker is still one of the most controversial issues. Side population (SP) cells, first described by Goodell et al.[9], are a subset of enriched progenitor cells exhibiting stemlike phenotypes with a distinct low Hoechst 33342, which has been widely used as a unique source for studying CSC when specific markers are unavailable. Even though a few studies have previously explored the stem-like properties and tumorigenicity of SP cells in comparison to the main population (MP) cells, an understanding of MM SP cells remain elusive [5, 10,11,12,13]
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