Beth Levine's Legacy: From the Discovery of BECN1 to Therapies. A Mentees' Perspective.

With great sadness, the scientific community received the news of the loss of Beth Levine on 15 June 2020. Dr. Levine was a pioneer in the autophagy field and work in her lab led not only to a better understanding of the molecular mechanisms regulating the pathway, but also its implications in multiple physiological and pathological conditions, including its role in development, host defense, tumorigenesis, aging or metabolism. This review does not aim to provide a comprehensive view of autophagy, but rather an outline of some of the discoveries made by the group of Beth Levine, from the perspective of some of her own mentees, hoping to honor her legacy in science.

New screening system using Twist1 promoter activity identifies dihydrorotenone as a potent drug targeting cancer-associated fibroblasts.

Cancer-associated fibroblasts (CAFs) are the most abundant stromal cells in tumor microenvironments. These cells strongly support tumor progression and are considered to be potent therapeutic targets. Therefore, drugs targeting CAFs have been developed, but most of them have failed in clinical trials. The discovery of additional drugs to inactivate or eliminate CAFs is thus essential. In this study, we developed a high-throughput screening system to find anti-CAF drugs using reporter cells that express Twist1 promoter-GFP. This screening system uses the activity of the Twist1 promoter as an indicator of CAF activation because Twist1 is known to be a central player in CAF activation. Using this screening system, we found that dihydrorotenone (DHR), an inhibitor of electron transfer chain complex 1 in mitochondria, can effectively deactivate CAFs. DHR-treated CAFs exhibited reduced expression of CAF-enriched markers, decreased capability of collagen gel contraction, and impaired ability to engage in tumor-promoting activities, such as facilitating the proliferation and colonization of cancer cells. Furthermore, conditioned media from DHR-treated CAFs attenuated tumor progression in mice grafted with MNK28 cells. In conclusion, DHR can be considered as a candidate drug targeting CAFs.

Drug repurposing screening identifies bortezomib and panobinostat as drugs targeting cancer associated fibroblasts (CAFs) by synergistic induction of apoptosis.

Cancer associated fibroblasts (CAFs) are the most abundant components of cancer-microenvironment. They play important roles in cancer initiation, progression, and metastasis. In addition, CAFs can confer drug-resistance to cancer cells. Considering their pro-tumorigenic roles, it is recommended to remove CAFs to prevent cancer recurrence after chemotherapy. Despite their clinical significance, few anti-CAF drugs have been developed. The objective of this study was to find a drug that could suppress the viability of patient-derived CAFs through repurposed screening of 51 drugs that were in clinical trials or received FDA approval. As a result, bortezomib (BTZ), carfilzomib (CFZ), and panobinostat (PST) were identified as anti-CAF drug candidates. It was confirmed that BTZ and PST could decrease the viability of various patients derived CAFs through inducing of caspase-3 mediated apoptosis. Interestingly, combination therapy with BTZ and PST showed better efficacy of inhibiting CAFs than single treatment. The synergistic effect between BTZ and PST on viability of CAFs was observed both in vitro CAF culture and in vivo mouse model. Furthermore, combination therapy with BTZ/PST and conventional anticancer compound docetaxel strongly inhibited tumor growth in xenografts of mouse breast cancer cells with mouse CAFs. In conclusion, our present study revealed that BTZ and PST could significantly reduce the viability of CAFs. Therefore, a combination therapy with BTZ/PST and anticancer drugs might be considered as a new rational for the development of anticancer therapy.

Genetic inhibition of autophagy promotes p53 loss-of-heterozygosity and tumorigenesis.

Autophagy is an evolutionarily conserved lysosomal degradation pathway that plays an essential role in enabling eukaryotic organisms to adapt to nutrient deprivation and other forms of environmental stress. In metazoan organisms, autophagy is essential for differentiation and normal development; however, whether the autophagy pathway promotes or inhibits tumorigenesis is controversial, and the possible mechanisms linking defective autophagy to cancer remain unclear. To determine if autophagy is important for tumor suppression, we inhibited autophagy in transgenic zebrafish via stable, tissue-specific expression of a dominant-negative autophagy protein Atg5K130R. In heterozygous tp53 mutants, expression of dominant-negative atg5K130R increased tumor incidence and decreased tumor latency compared to non-transgenic heterozygous tp53 mutant controls. In a tp53-deficient background, Tg(mitfa:atg5K130R) mutantsdeveloped malignant peripheral nerve sheath tumors (MPNSTs), neuroendocrine tumors and small-cell tumors. Expression of a Sox10-dependent GFP transgene in the tumors demonstrated their origin from neural crest cells, lending support to a model in which mitfa-expressing cells can arise from sox10+ Schwann cell precursors. Tumors from the transgenic animals exhibited increased DNA damage and loss-of-heterozygosity of tp53. Taken together, our data indicate that genetic inhibition of autophagy promotes tumorigenesis in tp53 mutant zebrafish, and suggest a possible role for autophagy in the regulation of genome stability during oncogenesis.

Autophagy is essential for cardiac morphogenesis during vertebrate development.

Genetic analyses indicate that autophagy, an evolutionarily conserved lysosomal degradation pathway, is essential for eukaryotic differentiation and development. However, little is known about whether autophagy contributes to morphogenesis during embryogenesis. To address this question, we examined the role of autophagy in the early development of zebrafish, a model organism for studying vertebrate tissue and organ morphogenesis. Using zebrafish that transgenically express the fluorescent autophagy reporter protein, GFP-LC3, we found that autophagy is active in multiple tissues, including the heart, during the embryonic period. Inhibition of autophagy by morpholino knockdown of essential autophagy genes (including atg5, atg7, and becn1) resulted in defects in morphogenesis, increased numbers of dead cells, abnormal heart structure, and reduced organismal survival. Further analyses of cardiac development in autophagy-deficient zebrafish revealed defects in cardiac looping, abnormal chamber morphology, aberrant valve development, and ectopic expression of critical transcription factors including foxn4, tbx5, and tbx2. Consistent with these results, Atg5-deficient mice displayed abnormal Tbx2 expression and defects in valve development and chamber septation. Thus, autophagy plays an essential, conserved role in cardiac morphogenesis during vertebrate development.

Docosahexaenoic acid sensitizes colon cancer cells to sulindac sulfide-induced apoptosis.

Sulindac analogs represent one of the most efficacious groups of NSAIDs reducing the risk of colon cancer. Recent studies have shown that sulindac sulfide, a sulindac analog effective at lower doses compared to its parent compound, triggers the death receptor (DR)5-dependent extrinsic apoptotic pathway. Induction of apoptosis via activation of the DR-mediated pathway would be an ideal therapeutic strategy to eliminate cancer cells. In this study, we investigated the possibility that colon cancer cells are sensitized to sulindac sulfide-induced apoptosis by docosahexaenoic acid (DHA), via activation of the DR/extrinsic apoptotic pathway. Our data demonstrated that DHA combination sensitized colon cancer cells to sulindac sulfide-induced apoptosis, leading to enhanced growth suppression of human colon cancer xenografts. The combination effect was primarily attributed to increased cleavage of poly(ADP-ribose) polymerase (PARP) and caspase-8 activation. Moreover, pretreatment with z-IETD-FMK (caspase-8 inhibitor) or stable expression of dominant negative caspase-8 genes blocked DHA/sulindac sulfide cotreatment-induced apoptosis. In view of the finding that DR5 silencing abrogated the combination-stimulated apoptosis, we propose that apoptotic synergy induced by sulindac sulfide plus DHA is mediated via DR5. Our findings collectively support the utility of a combination of sulindac sulfide and DHA in the effective prevention and treatment of colon cancer.

Preparation and evaluation of tributyrin emulsion as a potent anti-cancer agent against melanoma.

Histone deacetylase inhibitors such as butyrate are known to exhibit anti-cancer activities in a wide range of cancer including melanoma. In spite of these potencies, butyrate is not practically used for cancer treatment due to its rapid metabolism and very short plasma half-life. Tributyrin, a triglyceride analog of butyrate, can act as a pro-drug of butyrate after being cleaved by intracellular enzymes. The present study sought to investigate a possibility to develop tributyrin emulsion as a potent anti-cancer agent against melanoma. Mixture of Tween80 and 1, 2-dimyristoyl-sn-glycero-3-phosphocholine as a surfactant to disperse tributyrin produced homogeneous emulsions with nanometer sizes, even without a harsh homogenization procedure. Tributyrin emulsion was more potent than butyrate in inhibiting the growth of B16-F10 melanoma cells. Accumulation of cells at sub G(0)/G(1) phase and the DNA fragmentation induced by tributyrin emulsion treatment revealed that tributyrin emulsion inhibited the growth of B16-F10 cells by inducing apoptosis. Treatment with tributyrin emulsion suppressed the colony formation of melanoma cells in a dose-dependent manner. Furthermore, after intraperitoneal administration into mice, tributyrin emulsion inhibited the formation of tumor colonies in the lung following intravenous injection of melanoma cells. Taken together, our data suggests that tributyrin emulsion may be developed as a potent anti-cancer agent against melanoma.

Alpha-tocopheryl succinate sensitizes human colon cancer cells to exisulind-induced apoptosis.

Sulindac sulfone (also known as exisulind) and its chemical derivatives are promising anticancer agents capable of inducing apoptosis in a variety of malignant cell types with minimal toxicity to normal cells. Here, we tested the ability of alpha-tocopheryl succinate (TOS), another promising anticancer agent, to sensitize colon cancer cells to exisulind-induced apoptosis. We found that sub-apoptotic doses of TOS greatly enhanced exisulind-induced growth suppression and apoptosis in the HCT116, LoVo and SNU-C4 human colon cancer cell lines. Our results revealed that this was accounted for primarily by an augmented cleavage of poly(ADP-ribose) polymerase (PARP) and enhanced activation of caspase-8, -9 and -3. Pretreatment with z-VAD-FMK (a pan-caspase inhibitor), z-IETD-FMK (a caspase-8 inhibitor) or z-LEHD-FMK (a caspase-9 inhibitor) blocked TOS and exisulind cotreatment-induced PARP cleavage and apoptosis. Furthermore, TOS/exisulind cotreatment induced JNK phosphorylation, while pretreatment with SP600151 (a JNK inhibitor) partially blocked cotreatment-induced caspase-dependent PARP cleavage and apoptosis. Taken together, these findings indicate that TOS sensitizes human colon cancer cells to exisulind-induced apoptosis. Apoptotic synergy induced by exisulind plus TOS seems likely to be mediated through a mechanism involving activation of caspases and JNK.

The association of increased lung resistance protein expression with acquired etoposide resistance in human H460 lung cancer cell lines.

Chemoresistance remains the major obstacle to successful therapy of cancer. In order to understand the mechanism of multidrug resistance (MDR) that is frequently observed in lung cancer patients, here we studied the contribution of MDR-related proteins by establishing lung cancer cell lines with acquired resistance against etoposide. We found that human H460 lung cancer cells responded to etoposide more sensitively than A549 cells. Among MDR-related proteins, the expression of p-glycoprotein (Pgp) and lung resistance protein (LRP) were much higher in A549 cells compared with that in H460 cells. When we established H460-R1 and -R2 cell lines by progressive exposure of H460 cells to increasing doses of etoposide, the response against etoposide as well as doxorubicin was greatly reduced in R1 and R2 cells, suggesting MDR induction. Induction of MDR was not accompanied by a decrease in the intracellular accumulation of etoposide and the expression of MDR-related proteins that function as drug efflux pumps such as Pgp and MRP1 was not changed. We found that the acquired resistance paralleled an increased expression of LRP in H460 cells. Taken together, our data suggest the implicative role of LRP in mediating MDR in lung cancer.

p38MAPK inhibitor SB203580 sensitizes human SNU-C4 colon cancer cells to exisulind-induced apoptosis.

Sulindac sulfone (exisulind), is a promising anticancer agent because of its ability to induce apoptosis in a variety of malignant cell types and its minimal toxicity to normal cells. The induction of apoptosis is thought to account for the growth inhibitory effect of exisulind. The mitogen-activated protein kinase (MAPK) cascade has been implicated in the regulation of apoptosis in response to exisulind. With human SNU-C4 colon cancer cells that were much more resistant to exisulind than other colon cancer cells, in this study, we investigated whether the modulation of MAPK activity by using selective MAPK inhibitors can contribute to sensitizing SNU-C4 cells to exisulind. Exisulind (400 and 600 microM) slightly increased the phosphorylation of pERK1/2 but pretreatment with the pERK1/2 inhibitor PD98059 did not significantly change the apoptotic response of SNU-C4 cells. The same doses of exisulind increased the phosphorylation of p38MAPK, and pretreatment with the p38MAPK inhibitor SB203580 significantly potentiated growth inhibition and apoptosis induced by exisulind in SNU-C4 cells. We further found that apoptosis induced by a combination of exisulind and SB203580 was mediated through caspase activation. Collectively, our findings indicate that selective p38MAPK inhibitors potentiate apoptosis induction by exisulind in SNU-C4 cells. Such combinations may provide a more effective and less toxic strategy for the prevention or treatment of colon cancer.

Role of p21CIP1 as a determinant of SC-560 response in human HCT116 colon carcinoma cells.

SC-560, a structural analogue of celecoxib, induces growth inhibition in a wide range of human cancer cells in a cyclooxygenase (COX)-independent manner. Since SC-560 suppresses the growth of cancer cells mainly by inducing cell cycle arrest, we sought to examine the role of p21CIP1, a cell cycle regulator protein, in the cellular response against SC-560 by using p21(+/+) and p21(-/-) isogenic HCT116 colon carcinoma cells. In HCT116 (p21(+/+)) cells, SC-560 dose-dependently induced growth inhibition and cell cycle arrest at the G1 phase without significant apoptosis induction. SC-560-induced cell cycle arrest was accompanied by upregulation of p21CIP1. However, the extent of SC-560-induced accumulation at the G1 phase was approximately equal in the p21(+/+) and the p21(-/-) cells. Nonetheless, the growth inhibition by SC-560 was increased in p21(-/-) cells than p21(+/+)cells. SC-560-induced reactive oxygen species (ROS) generation did not differ between p21(+/+) and p21(-/-) cells but the subsequent activation of apoptotic caspase cascade was more pronounced in p21(-/-) cells compared with p21(+/+) cells. These results suggest that p21CIP1 blocks the SC-560-induced apoptotic response of HCT116 cells. SC-560 combined with other therapy that can block p21 CIP1 expression or function may contribute to the effective treatment of colon cancer.

Alpha-tocopheryl succinate, in contrast to alpha-tocopherol and alpha-tocopheryl acetate, inhibits prostaglandin E2 production in human lung epithelial cells.

The production of prostaglandin E2 (PGE2), a key proinflammatory mediator, is regulated by the availability of its substrate, arachidonic acid (AA), and the activity of the enzyme cyclooxygenase (COX). Increased PGE2 production and COX-2 expression have been observed frequently in specimens from lung cancer patients. Agents that decrease PGE2 production may prevent the initiation and progression of lung cancer. We, therefore, tested the effects of alpha-tocopherol (alphaTOL) analogs on PGE2 production in human lung epithelial cells. Alpha-tocopheryl succinate (alphaTOS), but not alphaTOL or alpha-tocopheryl acetate (alphaTOA), inhibited the phorbol 12-myristate 13-acetate (PMA)-stimulated PGE2 production in three human lung epithelial cell lines (BEAS-2B, H460 and A549 cells). The effect of these compounds on PGE2 production was not correlated with their antioxidant activities, since alphaTOS alone did not inhibit PMA-induced generation of reactive oxygen species. alphaTOS had no effect on PMA-induced AA release or COX-2 expression, although post-incubation with alphaTOS inhibited COX activity and prostaglandin (PGE2 and PGF(2alpha)) production in PMA-stimulated cells. alphaTOS also blocked the COX activity in A549 cells with endogenous high levels of COX enzymes in the absence of PMA stimulation. In addition, the ability of alphaTOS to inhibit COX was affected by AA concentration, suggesting that alphaTOS may compete with AA for interaction with COX proteins. These results suggest that alphaTOS inhibits COX activity, thereby inhibiting PGE2 production in human lung epithelial cells, despite the lack of antioxidant activity. Administration of alphaTOS may block inflammatory responses mediated by PGE2, thereby inhibiting the initiation and progression of lung cancer.

5-(4-chlorophenyl)-1-(4-methoxyphenyl)-3-trifluoromethylpyrazole acts in a reactive oxygen species-dependent manner to suppress human lung cancer growth.

PURPOSE: 5-(4-chlorophenyl)-1-(4-methoxyphenyl)-3-trifluoromethylpyrazole (SC-560) is a structural analog of celecoxib. Recent studies suggested that SC-560 inhibits the in vivo proliferation of colon and breast cancer cells to an extent similar to that observed in celecoxib, and that SC-560 exerts their growth inhibitory effects in a cyclooxygenase-independent manner. METHODS: In the current study, we sought to investigate the mechanism by which SC-560 inhibits the growth of human lung cancer cells. RESULTS: SC-560 more potently inhibited the growth of human A549, H460, and H358 lung cancer cell lines compared with that of human BEAS-2B normal bronchial epithelial cells. SC-560-induced growth inhibition was mainly due to the induction of cell-cycle arrest at the G1 phase without apoptosis induction. SC-560 rapidly and dose-dependently induced the generation of reactive oxygen species (ROS), followed by accumulation of cells at the G1 phase. Antioxidant pretreatment blocked the cell-cycle arrest and growth inhibition induced by SC-560. Combination treatment with other ROS-inducing agents such as alpha-tocopheryl succinate (TOS) augmented cellular response against SC-560, leading to synergistic apoptosis induction and growth suppression. Our data also showed that the apoptosis induced by combination treatment with SC-560 and TOS was mediated through ROS-dependent caspase activation. CONCLUSION: Collectively, our results demonstrate that SC-560 acts in a ROS-dependent manner to induce growth suppression in human lung cancer cells.

Cyclooxygenase-independent down-regulation of multidrug resistance-associated protein-1 expression by celecoxib in human lung cancer cells.

The recent finding of a link between cyclooxygenase-2 (COX-2) and p-glycoprotein expression suggests that COX-2 is involved in the development of the multidrug resistance (MDR) phenotype. MDR-associated protein 1 (MRP1) is another major MDR-related protein that is frequently overexpressed in cancer patients, including those with lung cancer. Based on our observation that among four human epithelial lung cell lines both MRP1 and COX-2 protein were highly expressed only in A549 cells, we have investigated whether COX-2 regulates the expression of MRP1. The COX-2 inhibitor celecoxib down-regulated the expression of MRP1 protein in A549 cells, which was accompanied by increased accumulation and enhanced cytotoxicity of doxorubicin, an MRP1 substrate. However, enforced expression of COX-2 in human H460 lung carcinoma cell lines, which express minimal level of COX-2, did not cause enhancement in MRP1 expression. Celecoxib down-regulation of MRP1 was observed independent of COX-2 expression. Moreover, in COX-2-overexpressing cell lines, celecoxib down-regulation of MRP1 was observed only at a concentration far exceeding that required for inhibiting COX activity, and exogenous addition of prostaglandin E(2) did not restore MRP1 expression. These results suggest that celecoxib down-regulates MRP1 expression in human lung cancer cells in a COX-independent manner. The use of celecoxib for adjuvant therapy in lung cancer patients may contribute to their decreased resistance to chemotherapeutic drugs transported by MRP1.

Enhanced anticancer efficacy of alpha-tocopheryl succinate by conjugation with polyethylene glycol.

Alpha-tocopheryl polyethylene glycol succinate (TPGS) has been used to enhance the bioavailability of poorly absorbed drugs and as a vehicle for drug delivery systems. In response to recent reports that alpha-tocopheryl succinate (TOS) acts as an anticancer agent, we investigated whether its polyethylene glycol (PEG) conjugate, TPGS, also possesses anticancer activity. TPGS inhibited the growth of human lung carcinoma cells implanted in nude mice, and in an in vitro cell culture, even more potently than TOS. The time-dependent uptake of TPGS into cells did not differ from that of TOS, indicating that the enhanced antitumor efficacy of TPGS was not due to its increased uptake into cells. Compared with TOS, TPGS was more effective at inducing apoptosis and the generation of reactive oxygen species, suggesting that the superior anticancer efficacy of TPGS is associated with its increased ability to induce apoptosis. Our data suggest that further studies assessing the potential usefulness of TPGS in cancer therapeutics are warranted, since its use as a vehicle in the formulation of anticancer drugs may provide an effective way to improve their therapeutic efficacy.

Potentiation by alpha-tocopheryl succinate of the etoposide response in multidrug resistance protein 1-expressing glioblastoma cells.

Multidrug resistance protein 1 (MRP1) is one of the representative members of the ATP-binding cassette superfamily of transporters that is involved in resistance to chemotherapeutic agents in cancer patients. MRP1 functions as an efflux pump of drugs, primarily those conjugated to glutathione (GSH). Decreases in the intracellular concentration of GSH have been shown to enhance the response of MRP1-overexpressing cells to MRP1-substrate drugs by limiting the available drug-GSH conjugates. We report here that alpha-tocopheryl succinate (TOS), a vitamin E analogue, decreased intracellular GSH concentration and blocked MRP1 function in glioblastoma cells. Functional blockade by TOS of MRP1 was confirmed by the enhanced accumulation of etoposide (VP-16), an MRP1-substrate drug. As a result, co-treatment of TOS with VP-16 or treatment with liposomes containing both TOS and VP-16 greatly enhanced the response of MRP1-expressing glioblastoma cells to VP-16. TOS may be a promising adjuvant for enhancing the therapeutic efficacy of VP-16 in patients with MRP1-expressing glioblastomas.

Role of reactive oxygen species in the induction of apoptosis by alpha-tocopheryl succinate.

Alpha-tocopheryl succinate (TOS), a vitamin E analog, is a promising anticancer agent due to its abilities to inhibit proliferation and to induce apoptosis in a variety of human malignant cell lines, while being relatively less active toward normal cells. However, the molecular mechanisms underlying the apoptotic effects of TOS are not precisely understood. Reports that TOS can generate reactive oxygen species (ROS) prompted us to investigate the role of ROS in TOS-induced apoptosis in cancer cells. We found that the human lung cancer A549 and H460 cell lines were much more sensitive to TOS-induced apoptosis than the human glioblastoma T98G and U87MG cell lines. Our data suggested that the differential TOS sensitivity was not caused by differences in the uptake and retention of TOS between TOS-sensitive and -resistant cancer cells. The differential ability of cancer cells to generate ROS in response to TOS appears to be an important factor in determining the susceptibility of cells to TOS-induced apoptosis. Our results further suggest that TOS-induced generation of ROS is involved in caspase-independent apoptosis. Taken together, our findings suggest an important role of ROS generation in TOS-induced, caspase-independent apoptosis of cancer cells.