Azithromycin, a potent autophagy inhibitor for cancer therapy, perturbs cytoskeletal protein dynamics

BACKGROUND

Autophagy plays an important role in tumour cell growth and survival and also promotes resistance to chemotherapy. Hence, autophagy has been targeted for cancer therapy. We previously reported that macrolide antibiotics including azithromycin (AZM) inhibit autophagy in various types of cancer cells in vitro. However, the underlying molecular mechanism for autophagy inhibition remains unclear. Here, we aimed to identify the molecular target of AZM for inhibiting autophagy.

METHODS

We identified the AZM-binding proteins using AZM-conjugated magnetic nanobeads for high-throughput affinity purification. Autophagy inhibitory mechanism of AZM was analysed by confocal microscopic and transmission electron microscopic observation. The anti-tumour effect with autophagy inhibition by oral AZM administration was assessed in the xenografted mice model.

RESULTS

We elucidated that keratin-18 (KRT18) and α/β-tubulin specifically bind to AZM. Treatment of the cells with AZM disrupts intracellular KRT18 dynamics, and KRT18 knockdown resulted in autophagy inhibition. Additionally, AZM treatment suppresses intracellular lysosomal trafficking along the microtubules for blocking autophagic flux. Oral AZM administration suppressed tumour growth while inhibiting autophagy in tumour tissue.

CONCLUSIONS

As drug-repurposing, our results indicate that AZM is a potent autophagy inhibitor for cancer treatment, which acts by directly interacting with cytoskeletal proteins and perturbing their dynamics.

Targeted disruption of GAK stagnates autophagic flux by disturbing lysosomal dynamics

The autophagy-lysosome system allows cells to adapt to environmental changes by regulating the degradation and recycling of cellular components, and to maintain homeostasis by removing aggregated proteins and defective organelles. Cyclin G-associated kinase (GAK) is involved in the regulation of clathrin-dependent endocytosis and cell cycle progression. In addition, a single nucleotide polymorphism at the GAK locus has been reported as a risk factor for Parkinson's disease. However, the roles of GAK in the autophagy-lysosome system are not completely understood, thus the present study aimed to clarify this. In the present study, under genetic disruption or chemical inhibition of GAK, analyzing autophagic flux and observing morphological changes of autophagosomes and autolysosomes revealed that GAK controlled lysosomal dynamics via actomyosin regulation, resulting in a steady progression of autophagy. GAK knockout (KO) in A549 cells impaired autophagosome-lysosome fusion and autophagic lysosome reformation, which resulted in the accumulation of enlarged autophagosomes and autolysosomes during prolonged starvation. The stagnation of autophagic flux accompanied by these phenomena was also observed with the addition of a GAK inhibitor. Furthermore, the addition of Rho-associated protein kinase (ROCK) inhibitor or ROCK1 knockdown mitigated GAK KO-mediated effects. The results suggested a vital role of GAK in controlling lysosomal dynamics via maintaining lysosomal homeostasis during autophagy.

Abstract 296: BRCA1 degradation in response to mitochondrial damage in breast cancer cells

The tumor suppressor BRCA1 protein has been implicated in hereditary breast and ovarian cancer syndrome when its gene is mutated. Among the many functions of BRCA1 including DNA repair, transcriptional regulation, cell cycle checkpoint, apoptosis, chromatin remodeling, and centrosome replication, DNA double-strand breaks repair by homologous recombination (HR) is one of the most important. BRCA1-associated tumors increase DNA instability and become sensitive to the Poly (ADP-ribose) polymerase (PARP) inhibitor. It is well known that PTEN-induced kinase 1 (PINK1) and Parkin are involved in mitochondrial quality control and a variety of mutations in these genes causes early-onset Parkinson's disease. In the present study, we report a novel degradation mechanism for BRCA1 protein in response to mitochondrial damage.

While investigating the role of BRCA1 in mitophagy using a breast cancer cell line, we found that BRCA1 protein is rapidly degraded by the mitochondrial targeting reagents, which induce mitochondrial depolarization. The degradation was mediated by the ubiquitin-proteasome system through the direct interaction with the E3 ligase Parkin upon PINK1 upregulation in response to the mitochondrial damage. Moreover, BRCA1 knockdown repressed cancer cell growth. Immunostaining the specimens from breast cancer patients revealed higher BRCA1 and lower PINK1/Parkin expression in their mammary glands. This result correlates with the analysis using the mRNA expression data set from TCGA database. Additionally, BRCA1 expression inversely correlated with PINK1/Parkin expression in the case of relapse-free survival in breast cancer patients. Thus, these findings demonstrated the unanticipated physiological functions of BRCA1 for maintaining cancer cell growth.

Overall, our study shows that: 1) Degradation of BRCA1 due to PINK1-Parkin activity through the ubiquitin-proteasome system occurs in response to mitochondrial damage. 2) BRCA1 promotes the growth of breast cancer cells. 3) Immunostaining of patient specimens and cancer genome data set analysis revealed higher BRCA1 expression with lower PINK1/Parkin expression was observed in the cancerous mammary glands. Moreover, recent reports have suggested that BRCA1 is involved in the pathogenesis of Alzheimer's disease. Thus, transmission of mitochondrial damage to the nucleus causing nuclear DNA double-strand breaks via the PINK1-Parkin-BRCA1 axis may not only be seen in the field of oncology, but also in various other fields including neurodegenerative diseases.

Citation Format: Kana Miyahara, Naoharu Takano, Yumiko Yamada, Hiromi Kazama, Mayumi Tokuhisa, Hirotsugu Hino, Koji Fujita, Edward Barroga, Masaki Hiramoto, Hiroshi Handa, Masahiko Kuroda, Takashi Ishikawa, Keisuke Miyazawa. BRCA1 degradation in response to mitochondrial damage in breast cancer cells [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 296.

Abstract 1227: Identification of the molecular target of azithromycin as an autophagy inhibitor and its potential application in cancer therapeutics

Autophagy is a self-digestive cellular mechanism that enables survival under nutrient depleted conditions. In normal cells, autophagy has been known to prevent tumorigenesis by removal of genotoxicity-causing mitochondria, as well as transformed cells. In contrast, once cancer has been established, autophagy appears to play a cytoprotective role to adapt to the insufficiently vascularized hypoxic and low nutrient environment; and also works for metastasis and therapeutic resistance. Therefore, autophagic inhibitors could be suitable candidates for cancer therapeutics. However, the only two autophagic inhibitors clinically available at present are chloroquine (CQ) and hydroxychloroquine (HCQ). We previously reported that azithromycin (AZM), a macrolide antibiotic, has an inhibitory effect on autophagy and that combined administration of AZM with various anti-cancer drugs, such as a tyrosine kinase inhibitor and a proteasomal inhibitor, appears to enhance the anti-cancer effect. With an aim to explore the clinical application of AZM for cancer therapy, we attempted to identify the molecular target of AZM for autophagy inhibition. High throughput affinity purification using AZM-conjugated magnetic nano-beads was used to identify several candidates for the AZM-binding protein involved in autophagy inhibition from human NSCLC-derived A549 cell lysates. Knockdown of one of the candidate genes resulted in a prominent accumulation of autophagosomes and lysosomes, leading to autophagy inhibition. Fluorescence microscopic observation of A549 cells expressing GFP-LC3 and LAMP1-mCherry markers suggested that AZM suppressed the fusion between autophagosomes and lysosomes. Comparison of autophagy inhibitory capacity using the GFP-LC3/RFP-LC3ΔG fluorescent reporter system revealed an almost equivalent autophagic inhibition between AZM and HCQ but less cytotoxicity by itself at the same concentrations. Suppression of lysosomal acidification was not observed upon AZM administration as same as in the case of HCQ administration. However, unlike HCQ, AZM did not increase the levels of HIF-1α and phospho-ERK1/2. This indicates the existence of different autophagy inhibitory mechanisms for AZM and HCQ. In the A549 murine xenograft model, daily oral administration of AZM (100 μg/g/day) significantly suppressed tumor cell growth. These results from the present study encourage the use of AZM as an autophagy inhibitor for cancer therapy.

Citation Format: Naoharu Takano, Yumiko Yamada, Mayumi Tokuhisa, Hirotsugu Hino, Masaki Hiramoto, Keisuke Miyazawa. Identification of the molecular target of azithromycin as an autophagy inhibitor and its potential application in cancer therapeutics [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1227.

Sequestosome 1 (p62) accumulation in breast cancer cells suppresses progesterone receptor expression via argonaute 2

Sequestosome 1 (p62) is a multifunctional adapter protein involved in various physiological functions, such as selective autophagy and oxidative stress response. Hence, aberrant expression and defective regulation of p62 are thought to lead to the onset of various diseases, including cancer. The expression of p62 has been shown to be increased in breast cancer tissues, and is correlated with a poor prognosis. However, the role of p62 in the breast cancer pathophysiology is still unclear. Here, we aimed to analyze the effect of changes in p62 expression on breast cancer cell lines. DNA microarray analysis revealed that the expression of progesterone receptor (PR), which is one of the indices for the classification of breast cancer subtypes, was markedly suppressed by forced expression of p62. The protein expression of PR was also decreased by forced expression of p62, but increased by knockdown of p62. Moreover, we found that p62 knockdown induced the protein expression of argonaute 2 (AGO2). Luciferase reporter assay results showed that the gene expression of PR was promoted by AGO2. Furthermore, results revealed that overexpression of AGO2 partially rescued the decrease in PR expression induced by forced expression of p62. Collectively, our findings indicated that p62 accumulation suppressed the expression of AGO2, which in turn decreased the expression of PR, suggesting that p62 may serve as a marker of aggressive breast cancer and poor prognosis. Moreover, the p62-AGO2-PR axis was identified as a crucial signaling cascade in breast cancer progression.

Cloning and characterization of d-threonine aldolase from the green alga Chlamydomonas reinhardtii

d-Threonine aldolase (DTA) catalyzes the pyridoxal 5'-phosphate (PLP)-dependent interconversion of d-threonine and glycine plus acetaldehyde. The enzyme is a powerful tool for the stereospecific synthesis of various β-hydroxy amino acids in synthetic organic chemistry. In this study, DTA from the green alga Chlamydomonas reinhardtii was discovered and characterized, representing the first report to describe the existence of eukaryotic DTA. DTA was overexpressed in recombinant Escherichia coli BL21 (DE3) cells; the specific activity of the enzyme in the cell-free extract was 0.8 U/mg. The recombinant enzyme was purified to homogeneity by ammonium sulfate fractionation, DEAE-Sepharose, and Mono Q column chromatographies (purified enzyme 7.0 U/mg). For the cleavage reaction, the optimal temperature and pH were 70 °C and pH 8.4, respectively. The enzyme demonstrated 90% of residual activity at 50 °C for 1 h. The enzyme catalyzed the synthesis of d- and d-allo threonine from a mixture of glycine and acetaldehyde (the diastereomer excess of d-threonine was 18%). DTA was activated by several divalent metal ions, including manganese, and was inhibited by PLP enzyme inhibitors and metalloenzyme inhibitors.

Crystallization and X-ray analysis of D-threonine aldolase from Chlamydomonas reinhardtii

D-Threonine aldolase from the green alga Chlamydomonas reinhardtii (CrDTA) catalyzes the interconversion of several β-hydroxy-D-amino acids (e.g. D-threonine) and glycine plus the corresponding aldehydes. Recombinant CrDTA was overexpressed in Escherichia coli and purified to homogeneity; it was subsequently crystallized using the hanging-drop vapour-diffusion method at 295 K. Data were collected and processed at 1.85 Å resolution. Analysis of the diffraction pattern showed that the crystal belonged to space group P1, with unit-cell parameters a = 64.79, b = 74.10, c = 89.94 Å, α = 77.07, β = 69.34, γ = 71.93°. The asymmetric unit contained four molecules of CrDTA. The Matthews coefficient was calculated to be 2.12 Å3 Da-1 and the solvent content was 41.9%.

Antibodies against mycobacterial antigens in the synovial fluid of patients with temporomandibular disorders

In the absence of active pulmonary disease, mycobacterial infection frequently causes arthritis and can be considered to initiate autoimmune diseases such as rheumatoid arthritis. Temporomandibular disorder (TMD) is a disease in which pain and impaired mandibular movement appear to arise directly from degenerative or inflammatory changes within the temporomandibular joint, but its precise pathogeny has not been elucidated. Here we examined whether mycobacterial infection is related to the pathology of TMD. The antibody levels against mycobacterial antigen in the synovial fluid (SF) of patients with TMD were assessed by enzyme-linked immunosorbent assay (ELISA). Six of 17 TMD patients (35%) were found to possess mycobacterial antigen-specific immunoglobulin (Ig) G but not IgM, while the six healthy volunteers possessed neither. Western-blot analysis was used to isolate the reacted antigen, and the IgG reacted strongly to 44-kDa antigen. The first 14 N-terminal amino acid sequences were determined, and computer analysis revealed that it was homologous to translational elongation factor Tu (EF-Tu) of Mycobacterium tuberculosis, which was a major target antigen for these antibodies. The 44-kDa protein of Mycobacterium bovis BCG (BCG) was identical with the EF-Tu of M. tuberculosis. We cloned the gene encoding the EF-Tu of BCG by using the synthesized oligonucleotide primers by means of polymerase chain-reaction. The gene was expressed in Escherichia coli. The protein was purified, and the antibody levels against this recombinant protein in the SF of TMD patients were assessed by ELISA. Our findings suggest that some cases of TMD are concerned with the synovial IgG against the EF-Tu of M. tuberculosis, and that the existence of the antibody is a clinical indication of TMD.