Thus we presumed that ALDH A might play an important
Thus, we presumed that ALDH1A3 might play an important role in TMZ-chemoresistance in glioblastoma patients. As we expected, the glioblastoma cell lines and primary glioma Z-WEHD-FMK were more sensitive to TMZ treatment when ALDH1A3 was inhibited or depleted. Consistently, it has been shown that downregulation of ALDH1A3 reduced the survival of the ALDHhigh mesothelioma cells after chemotherapy , and in head and neck squamous cell carcinoma ALDH1A3 interference decreased radioresistance .
Interestingly, we observed that TMZ treatment decreased ALDH1A3 protein and ALDH enzyme activity, ALDH1A3 mRNA levels showed no significant change after TMZ treatment, indicating that the protein reduction is post-transcriptionally regulated. Thus, we hypothesized that ALDH1A3 might be regulated in a proteolytic pathway. Autophagic degradation could be one explanation, since TMZ lead to a significant up-regulation of autophagy, and both ALDH1A3 protein and enzyme activity can be recovered with bafilomycin co-treatment. Bafilomycin inhibits autophagy by preventing endosomal acidification and disturbing the fusion of autophagosomes and lysosomes . On the other hand, Rapamycin is a direct inhibitor of mTOR pathway which has been shown as the major negative regulatory axis of autophagy . We found that autophagy inhibition led to ALDH1A3 accumulation and activation of autophagy resulted in a decrease of ALDH1A3. We next confirmed a direct interaction between ALDH1A3 and p62 by proximity ligation assay and co-immunoprecipitation, which is in line with our hypothesis that ALDH1A3 is recruited to autophagosomes and degrades in the final stage of autophagy. As the change of ALDH1A3 protein levels are not totally consistent with p62 expression (Fig. 3, Fig. 8), we presume that a proportion of ALDH1A3 molecules might recruit to the autophagosome membrane without connection to p62 and dissociates the autolysosome in the last step of autophagy.
TMZ treatment induces not only DNA damage but also metabolic stress to the cells. Lin et al. confirmed TMZ induces ROS/ERK-mediated or mitochondrial damage- and ER stress-dependent autophagy to protect glioma cells [37,38]. ALDH1A3 also exert its role in the chemoresistance phenotype of malignant gliomas by a repairing or detoxifying effect [39,40]. Therefore, Autophagy seems also to be induced as an alternative source of metabolites and/or for disposal of damaged molecules or organelles. Since ALDH1A3 depleted cells exhibit stronger autophagic flux than wild type cells, we presume that autophagy might act as a complementary mechanism to the lost detoxifying effects when ALDH1A3 has been knocked out.
Funding This study was funded by the Deutsche Forschungsgemeinschaft under Collaborative Research Grant SFB 824 (Project B6).
Conflicts of interest
Introduction Renal Cell Carcinoma (RCC) is one of the most common malignancies of the genitourinary tract, accounting for about 3% of all adult cancers . Despite advances in diagnosis, about 20–30% of all patients are diagnosed already with metastatic disease and extremely poor prognosis. Moreover one-third of RCC patients develops distant metastases after resection of the primary tumor . Tumor status has been associated with a number of metabolic and biochemical alterations. The subject of a new studies are certain molecules which could have a meaning in carcinogenesis and tumor progression. Several data have shown a role of tetraspanins, CD 133, calbindin, glutathione S-transferase P, retinal dehydrogenase 1 and aldehyde dehydrogenase in pathogenesis of renal cell carcinoma [3–6]. Studies of Jelski and Szmitkowski show that differences of alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) activity between cells of many cancers and healthy tissue may be one of the factors intensifying carcinogenesis . Moreover, changes of ADH and ALDH activity in cancer cells are reflected in serum of patients because of releasing these enzymes from malignant tissue . Human alcohol dehydrogenase and aldehyde dehydrogenase exist in multiple molecular forms that have been grouped into several classes. The best characterized function of these enzymes is a metabolism of ethanol and the other alcohols, also role in protection against products of lipid peroxidation and some exogenous xenobiotics. It was also found that ADH and ALDH take part in biosynthesis of retinoic acid, an important factor for cell differentiation and regeneration [8,9].