Bone Marrow Stromal Cells Modulate Mouse ENT1 Activity and Protect Leukemia Cells from Cytarabine Induced Apoptosis

BACKGROUND: Despite a high response rate to chemotherapy, the majority of patients with acute myeloid leukemia (AML) are destined to relapse due to residual disease in the bone marrow (BM). The tumor microenvironment is increasingly being recognized as a critical factor in mediating cancer cell survival and drug resistance. In this study, we propose to identify mechanisms involved in the chemoprotection conferred by the BM stroma to leukemia cells. METHODS: Using a leukemia mouse model and a human leukemia cell line, we studied the interaction of leukemia cells with the BM microenvironment. We evaluated in vivo and in vitro leukemia cell chemoprotection to different cytotoxic agents mediated by the BM stroma. Leukemia cell apoptosis was assessed by flow cytometry and western blotting. The activity of the equilibrative nucleoside transporter 1 (ENT1), responsible for cytarabine cell incorporation, was investigated by measuring transport and intracellular accumulation of (3)H-adenosine. RESULTS: Leukemia cell mobilization from the bone marrow into peripheral blood in vivo using a CXCR4 inhibitor induced chemo-sensitization of leukemia cells to cytarabine, which translated into a prolonged survival advantage in our mouse leukemia model. In vitro, the BM stromal cells secreted a soluble factor that mediated significant chemoprotection to leukemia cells from cytarabine induced apoptosis. Furthermore, the BM stromal cell supernatant induced a 50% reduction of the ENT1 activity in leukemia cells, reducing the incorporation of cytarabine. No protection was observed when radiation or other cytotoxic agents such as etoposide, cisplatin and 5-fluorouracil were used. CONCLUSION: The BM stroma secretes a soluble factor that significantly protects leukemia cells from cytarabine-induced apoptosis and blocks ENT1 activity. Strategies that modify the chemo-protective effects mediated by the BM microenvironment may enhance the benefit of conventional chemotherapy for patients with AML

PPAR alpha: an emerging therapeutic target in diabetic microvascular damage

The global pandemic of diabetes mellitus portends an alarming rise in the prevalence of microvascular complications, despite advanced therapies for hyperglycemia, hypertension and dyslipidemia. Peroxisome proliferator-activated receptor alpha (PPAR alpha) is expressed in organs affected by diabetic microvascular disease (retina, kidney and nerves), and its expression is regulated specifically in these tissues. Experimental evidence suggests that PPAR alpha activation attenuates or inhibits several mediators of vascular damage, including lipotoxicity, inflammation, reactive oxygen species generation, endothelial dysfunction, angiogenesis and thrombosis, and thus might influence intracellular signaling pathways that lead to microvascular complications. PPAR alpha has emerged as a novel target to prevent microvascular disease, via both its lipid-related and lipid-unrelated actions. Despite strong experimental evidence of the potential benefits of PPAR alpha agonists in the prevention of vascular damage, the evidence from clinical studies in patients with diabetes mellitus remains limited. Promising findings from the Fenofibrate Intervention and Event Lowering in Diabetes (FIELD) study on microvascular outcomes are countered by elevations in participants' homocysteine and creatinine levels that might potentially attenuate the benefits of PPAR alpha activation. This Review focuses on the role of PPAR alpha activation in diabetic microvascular disease and highlights the available experimental and clinical evidence from studies of PPAR alpha agonists