Not surprisingly, autophagy inhibition early in the development of a cancer promotes tumor development. 41Indeed, deletion of genes important for autophagy induction, including ATG6/Beclin-1, UVRAG, 32ATG5, 42or Bax interacting factor 143promote tumor development by accelerating Crotonoside inflammatory responses, 44enhancing survival bioenergetics45and propagating oxidative stress. 46Thus, defects in autophagy have been linked to early tumorigenesis in a number of mouse models. 4749 Similar to the development of solid tumors in mice with impaired autophagy, defective autophagy in hematopoietic stem cells promotes the development of myelodysplastic syndrome and leukemia. autophagy inhibitors are in development and promise to provide new therapeutic opportunities for patients with leukemia. == INTRODUCTION == Acute leukemias are clonal malignancies of the hematopoietic system characterized by accumulation of immature cell populations in the bone marrow or peripheral blood. Acute myeloid leukemia (AML) Crotonoside is the most common Crotonoside type of acute leukemia in adults but has the lowest survival rate of all leukemias. 1In spite of intensive treatment, more than 10% of AML Crotonoside patients fail to respond to initial therapy and over 60% relapse with resistant disease. 2Achieving complete response depends on multiple factors including age, the presence of cytogenetic and molecular abnormalities, and performance status and organ function at diagnosis. However , these factors alone do not fully explain the observed patterns of resistance. 3Various mechanisms have been proposed to explain chemotherapy resistance in patients with AML, including multidrug resistance and inability of current therapies to eradicate leukemia stem cells (LSCs). Recent studies have also suggested that autophagy represents an important mechanism of resistance. An understanding of the multiple mechanisms contributing to treatment failure is necessary to develop more effective therapeutic approaches for patients with acute leukemia. Several groups of proteins that promote the efflux of cytostatic drugs have been investigated in drug resistant acute leukemia. 4One group is the adenosine triphosphate (ATP)-binding cassette transporters (ABC transporters). P-gp, encoded by the multidrug resistance gene 1 (MDR1 or ABCB1), is involved in resistance to several drugs that are commonly used in AML, including anthracyclines. Expression of P-gp is an adverse prognostic factor for attaining a complete response and survival in adult leukemia, particularly in elderly patients. 5The breast cancer resistance protein, encoded by the ABCG2 gene, and the multidrug-resistant protein MRP3 also correlate with diminished survival in AML. 4However, studies of involving pharmacologic inhibition of P-gp have not demonstrated a substantial impact on chemotherapy outcomes. 4 Recent studies have determined that LSCs have a central role in leukemia pathogenesis, and that failure to eradicate these cells is an important factor in patient outcomes. 6A number of genetic and cellular adaptations have been found to confer chemotherapy resistance in LSCs. Standard chemotherapy kills leukemia cells that are progressing through the cell cycle, but spares LCSs that are in a quiescent state in the bone marrow. This dormancy is also associated with an extremely low rate of oxidative phosphorylation that further renders LSCs insensitive to toxins affecting bioenergetics or inhibitors targeting key signaling pathways. Inducing LSCs to enter the cell cycle before chemotherapy treatment enhances their susceptibility to chemotherapeutic drugs, facilitating their eradication and sustained disease remission. 6Interestingly, autophagy is enhanced when both normal and malignant cells are in a resting G0/G1 state, 7, 8suggesting that LSCs may be resistant to chemotherapy in part through enhanced autophagy. Autophagy is a lysosomal-degradation process that serves as a vehicle for clearing dysfunctional organelles and for maintaining genomic stability by clearing excised genomic fragments. As such, it regulates the responses of eukaryotic cells to cellular stress including those arising from hypoxia, genomic instability, endoplasmic reticulum stress, nutrient stress and facultative intracellular infections. In the setting of chemotherapy, radiation therapy and immunotherapy, autophagy serves as a major Crotonoside means for resistance from cytotoxic stress. 9The precise mechanism for how autophagy inhibits cell death is unclear but may involve limiting DNA-damage response-mediated apoptosis1012or, alternatively, activation of the high-mobility group box 1 (HMGB1)/RAGE (receptor for advanced glycation end products) signaling axis. 13In addition, autophagy helps promote cell survival through nutrient recycling and the clearance of damaged organelles or oxidized and aggregated proteins. 14Recently, increasing evidence suggests that the role of autophagy changes rather dramatically during the period over which tumors develop. This complexity should be kept in mind when analyzing data from preclinical and clinical studies. Specifically, during the early phase of tumor initiation, autophagy appears to be suppressed, allowing the emergence of genomic instability. 1520Later in tumor development, autophagy is more active, leading to enhanced cancer cell survival, as described in this review. Therefore , efforts to inhibit autophagy are particularly warranted in established tumors. Notably, emerging data indicate that autophagy is a major contributor to chemotherapy resistance in AML, and its inhibition may represent an important therapeutic strategy. Rabbit Polyclonal to CDC42BPA In this review, we will first describe the autophagic process and its regulation and then review evidence of.
Not surprisingly, autophagy inhibition early in the development of a cancer promotes tumor development
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