Bone Marrow Microenvironment Niche Regulates miR-221/222 in Acute Lymphoblastic Leukemia.

Acute lymphoblastic leukemia (ALL) has many features in common with normal B-cell progenitors, including their ability to respond to diverse signals from the bone marrow microenvironment (BMM) resulting in regulation of cell-cycle progression and survival. Bone marrow-derived cues influence many elements of both steady state hematopoiesis and hematopoietic tumor cell phenotypes through modulation of gene expression. miRNAs are one regulatory class of small noncoding RNAs that have been shown to be increasingly important in diverse settings of malignancy. In the current study, miRNA profiles were globally altered in ALL cells following exposure to primary human bone marrow niche cells, including bone marrow stromal cells (BMSC) and primary human osteoblasts (HOB). Specifically, mature miR-221 and miR-222 transcripts were decreased in ALL cells cocultured with BMSC or HOB, coincident with increased p27 (CDKN1B), a previously validated target. Increased p27 protein in ALL cells exposed to BMSC or HOB is consistent with accumulation of tumor cells in the G0 phase of the cell cycle and resistance to chemotherapy-induced death. Overexpression of miR-221 in ALL cells during BMSC or HOB coculture prompted cell-cycle progression and sensitization of ALL cells to cytotoxic agents, blunting the protective influence of the BMM. These novel observations indicate that BMM regulation of miR-221/222 contributes to marrow niche-supported tumor cell quiescence and survival of residual cells. IMPLICATIONS: Niche-influenced miR-221/222 may define a novel therapeutic target in ALL to be combined with existing cytotoxic agents to more effectively eradicate refractory disease that contributes to relapse. Mol Cancer Res; 14(10); 909-19. ©2016 AACR.

BCL6 modulation of acute lymphoblastic leukemia response to chemotherapy.

The bone marrow niche has a significant impact on acute lymphoblastic leukemia (ALL) cell phenotype. Of clinical relevance is the frequency with which quiescent leukemic cells, in this niche, survive treatment and contribute to relapse. This study suggests that marrow microenvironment regulation of BCL6 in ALL is one factor that may be involved in the transition between proliferative and quiescent states of ALL cells. Utilizing ALL cell lines, and primary patient tumor cells we observed that tumor cell BCL6 protein abundance is decreased in the presence of primary human bone marrow stromal cells (BMSC) and osteoblasts (HOB). Chemical inhibition, or shRNA knockdown, of BCL6 in ALL cells resulted in diminished ALL proliferation. As many chemotherapy regimens require tumor cell proliferation for optimal efficacy, we investigated the consequences of constitutive BCL6 expression in leukemic cells during co-culture with BMSC or HOB. Forced chronic expression of BCL6 during co-culture with BMSC or HOB sensitized the tumor to chemotherapy induced cell death. Combination treatment of caffeine, which increases BCL6 expression in ALL cells, with chemotherapy extended the event free survival of mice. These data suggest that BCL6 is one factor, modulated by microenvironment derived cues that may contribute to regulation of ALL therapeutic response.

Modeling Chemotherapy Resistant Leukemia In Vitro.

It is well established that the bone marrow microenvironment provides a unique site of sanctuary for hematopoietic diseases that both initiate and progress in this site. The model presented in the current report utilizes human primary bone marrow stromal cells and osteoblasts as two representative cell types from the marrow niche that influence tumor cell phenotype. The in vitro co-culture conditions described for human leukemic cells with these primary niche components support the generation of a chemoresistant subpopulation of tumor cells that can be efficiently recovered from culture for analysis by diverse techniques. A strict feeding schedule to prevent nutrient fluxes followed by gel type 10 cross-linked dextran (G10) particles recovery of the population of tumor cells that have migrated beneath the adherent bone marrow stromal cells (BMSC) or osteoblasts (OB) generating a "phase dim" (PD) population of tumor cells, provides a consistent source of purified therapy resistant leukemic cells. This clinically relevant population of tumor cells can be evaluated by standard methods to investigate apoptotic, metabolic, and cell cycle regulatory pathways as well as providing a more rigorous target in which to test novel therapeutic strategies prior to pre-clinical investigations targeted at minimal residual disease.

Bone marrow microenvironment modulation of acute lymphoblastic leukemia phenotype.

Acute lymphoblastic leukemia (ALL) treatment regimens have dramatically improved the survival of ALL patients. However, chemoresistant minimal residual disease that persists following cessation of therapy contributes to aggressive relapse. The bone marrow microenvironment (BMM) is an established "site of sanctuary" for ALL, as well as myeloid-lineage hematopoietic disease, with signals in this unique anatomic location contributing to drug resistance. Several models have been developed to recapitulate the interactions between the BMM and ALL cells. However, many in vitro models fail to accurately reflect the level of protection afforded to the most resistant subset of leukemic cells during coculture with BMM elements. Preclinical in vivo models have advantages, but can be costly, and are often not fully informed by optimal in vitro studies. We describe an innovative extension of 2-D coculture wherein ALL cells uniquely interact with bone marrow-derived stromal cells. Tumor cells in this model bury beneath primary human bone marrow-derived stromal cells or osteoblasts, termed "phase dim" ALL, and exhibit a unique phenotype characterized by altered metabolism, distinct protein expression profiles, increased quiescence, and pronounced chemotherapy resistance. Investigation focused on the phase dim subpopulation may more efficiently inform preclinical design and investigation of the minimal residual disease and relapse that arise from BMM-supported leukemic tumor cells.