Abstract 3772: The amplified cancer gene LAPTM4B plays a critical role in autophagosome maturation and promotes cancer cell survival during metabolic stress
Authors: Li Yang , Tian Ruiyang , Iglehart J. Dirk , Wang Zhigang C. , Richardson Andrea L. 
Published: June 2012

Abstract
Research has shown that lysosome-mediated autophagy is up regulated in response to various stimuli such as hypoxia, nutrient limitation, and drug-induced cytotoxic stress. In cancer progression, autophagy plays a cytoprotective role by generating needed nutrients or by eliminating damaged cellular components. Although autophagy has been recognized as a critical tumor cell survival mechanism and much is known about the induction and early steps of autophagy, the molecule(s) which control the later stages of autophagic progression are less clear. Previously, we have identified lysosomal-associated protein transmembrane 4b (LAPTM4B) as an amplified and over-expressed gene in breast cancer that is associated with de novo resistance to anthracycline chemotherapy and increased risk of distant metastatic recurrence. We reported that LAPTM4B overexpression alters intracellular trafficking of anthracyclines by retaining the drug in the cytoplasmic compartment thereby limiting drug-induced DNA damage in the nucleus. However, how this protein contributes to tumor cell survival in the setting of other types of stress requires further investigation. In the present studies, we used fluorescence microscopy to confirm LAPTM4B protein distribution in the lysosomal compartment of cells. Depletion of this gene results in increased lysosome membrane permeability as demonstrated by release from lysosomes of low to intermediate molecular weight dextran. Furthermore, we find that over expression of LAPTM4B renders cells resistant to apoptosis when placed in nutrient-free media suggesting a possible role of LAPTM4B in promoting lysosome-mediated autophagy. We induced autophagy by serum starvation in BT549 breast cancer cells with or without depletion of LAPTM4B. Autophagic progression was monitored by immunofluorescence for the autophagosome marker, LC3, and for degradation of P62 as evidence of autolysosome maturation. We find that depletion of LAPTM4B interrupted autophagosome maturation to autolysosomes as indicated by appearance of LC3 but absence of P62 degradation. In addition, we observed aggregation of autophagosomes in LAPTM4B-depleted cells and caspase activation, indicative of apoptosis. These events are a recognized sequela of blocked autophagic maturation. In summary, we find that ablation of LAPTM4B results in increased lysosome membrane permeability, inhibition of autophagosome-lysosome fusion, and toxic accumulation of autophagasomes triggering cell death. These results suggest a novel function for the amplified cancer gene, LAPTM4B, in promoting the later stages of autophagic maturation and tumor resistance to apoptosis-inducing stress. Targeting LAPTM4B may provide a new strategy to prevent metabolic adaptation of cancer cells by abolishing the cytoprotective effects of autophagy and increasing drug therapy efficacy.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3772. doi:10.1158/1538-7445.AM2011-3772