Endoplasmic Reticulum stress, Unfolded Protein Response and autophagy contribute to resistance to glucocorticoid treatment in human Acute Lymphoblastic Leukemia cells

Acute lymphoblastic leukemia (ALL) is the most frequent childhood cancer and although highly treatable, resistance to therapy, toxicity and side effects remain challenging. The synthetic glucocorticoid (GC) – dexamethasone (Dex) –is commonly used to treat ALL. The leukemia cell lines CEM-C7-14, CEM-C1-15 and Molt 4 treated with chloroquine (CLQ), thapsigargin (TG) and rotenone (ROT) were used to explore the role of autophagy, endoplasmic reticulum stress/unfolded protein response (ER stress/UPR) and reactive oxygen species (ROS) generation in the response to GC treatment. ROS levels positively correlated with increased cell death and mitochondria membrane potential in rotenone treated CEM cells. Autophagy inhibition by CLQ exhibited the strongest cytotoxic effect in GC resistant leukemia. Autophagy may act as a pro-survival mechanism in GC resistant leukemia since increasing trends in beclin1 and LC3 levels were detected in CEM-C1-15 and Molt 4 cells treated with Dex whereas decreasing trends in these autophagy markers were observed in CEM-C7-14 cells. The intracellular protein levels of the ER stress markers GRP78 and GRP94 were stimulated by Dex only in the GC-sensitive cells suggesting an important role of these chaperones in the GCs mediated cell death in ALL cells. Increased GRP94 cell surface levels were recorded in CEM-C7-14 cells treated with combination of Dex with TG compared to those measured in these cells treated with TG alone whereas decreasing trends were observed in CEM-C1-15 cells under these conditions. Taken together results presented here demonstrate that autophagy is a potentially pro-survival mechanism in GC resistant leukemia and by modulating intracellular and surface GRP94 protein levels Dex is involved in the regulation of ER stress/UPR, Ca2+-dependent cell death and immune surveillance. These observations could be of significant clinical importance if confirmed in patients.