Inhibition of HDAC6 Protein Enhances Bortezomib-induced Apoptosis in Head and Neck Squamous Cell Carcinoma (HNSCC) by Reducing Autophagy

Inhibition of HDAC6 elicits anticancer effects on head and neck cancer cells through Sp1/SOD3/MKP1 signaling axis to downregulate ERK phosphorylation

Oxidative stress caused by reactive oxygen species (ROS) and superoxides is linked to various cancer-related biological events. Extracellular superoxide dismutase (SOD3), an antioxidant enzyme that removes superoxides, contributes to redox homeostasis and has the potential to regulate tumorigenesis. Histone deacetylase 6 (HDAC6), a major HDAC isoform responsible for mediating the deacetylation of non-histone protein substrates, also plays a role in cancer progression. In this study, we examined the potential effects of HDAC6 inhibition on SOD3 expression in head and neck cancer (HNC) cells and its impact on cell proliferation, which remains unaddressed. We found that functional inactivation of HDAC6, through the use of chemical inhibitors such as tubastatin A (TubA), gene knockdown, or overexpression of an inactive mutant, strongly upregulated protein and mRNA levels of SOD3 in HNC cell lines FaDu and Detroit562. Mechanistically, TubA induced acetylation of the transcription factor Sp1 at Lys703, which consequently enhanced its binding to the SOD3 proximal promoter region and increased SOD3 expression. An acetylation-defective Sp1 mutant (K703R) was much less effective in inducing SOD3 expression compared to wild-type Sp1. TubA reduced intracellular ROS and superoxide levels, and this antioxidative effect was attenuated in SOD3 knockdown cells. Similar to the changes in ROS levels, HDAC6 inhibition as well as SOD3 overexpression suppressed cell proliferation and the stimulatory phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2), whereas SOD3 knockdown produced opposite effects in both resting and TubA-treated conditions. In addition, SOD3 overexpression prevented ROS-induced ERK1/2 phosphorylation and enhanced the protein stability of mitogen-activated protein kinase phosphatase 1 (MKP1), thereby counteracting ERK1/2 phosphorylation. We further showed that SOD3-mediated ERK1/2 dephosphorylation was moderated in MKP1 knockdown cells. Collectively, these results suggest that HDAC6 inhibition elicits anticancer effects on HNC cells by promoting Sp1 acetylation-dependent SOD3 upregulation, leading to MKP1 stabilization and subsequent ERK1/2 inactivation.

Combinatorial functionomics identifies HDAC6-dependent molecular vulnerability of radioresistant head and neck cancer

BACKGROUND

Radiotherapy is the primary treatment modality for most head and neck cancers (HNCs). Despite the addition of chemotherapy to radiotherapy to enhance its tumoricidal effects, almost a third of HNC patients suffer from locoregional relapses. Salvage therapy options for such recurrences are limited and often suboptimal, partly owing to divergent tumor and microenvironmental factors underpinning radioresistance. In this study, we utilized a combinatorial functionomics approach, the Quadratic Phenotypic Optimization Platform (QPOP), to rationally design drug pairings that exploit the molecular fingerprint and vulnerability of established in vitro isogenic radioresistant (RR)-HNC models.

METHODS

A QPOP-specific protocol was applied to RR-HNC models to rank and compare all possible drug combinations from a 12-drug set comprising standard chemotherapy, small molecule inhibitors and targeted therapies specific to HNC. Drug combination efficacy was evaluated by computing combination index scores, and by measuring apoptotic response. Drug targeting was validated by western blot analyses, and the Comet assay was used to quantify DNA damage. Enhanced histone deacetylase inhibitor (HDACi) efficacy in RR models was further examined by in vivo studies, and genetic and chemical inhibition of major Class I/II HDACs. Regulatory roles of HDAC6/SP1 axis were investigated using immunoprecipitation, gel shift and ChIP-qPCR assays. Comparative transcriptomic analyses were employed to determine the prognostic significance of targeting HDAC6.

RESULTS

We report the therapeutic potential of combining panobinostat (pan-HDAC inhibitor) with AZD7762 (CHK1/2 inhibitor; AstraZeneca) or ionizing radiation (IR) to re-sensitize RR-HNC cells and showed increased DNA damage underlying enhanced synergy. We further refined this RR-specific drug combination and prioritized HDAC6 as a targetable dependency in reversing radioresistance. We provide mechanistic insights into HDAC6-mediated regulation via a crosstalk involving SP1 and oncogenic and repair genes. From two independent patient cohorts, we identified a four-gene signature that may have discriminative ability to predict for radioresistance and amenable to HDAC6 inhibition.

CONCLUSION

We have uncovered HDAC6 as a promising molecular vulnerability that should be explored to treat RR-HNC.

NF-кB promotes aggresome formation via upregulating HDAC6 and in turn maintaining Vimentin cage

Proteasome inhibitors have been applied to anticancer therapy by accumulating toxic misfolded proteins. However, chemical inactivation of proteasome generates aggresome, a Vimentin cage-enclosed subcellular structure quarantining HDAC6-Dynein-transported misfolded proteins before the protein toxicants are degraded by autophagy. Hence, aggresome may attenuate proteasome inhibitor drug-induced cytotoxicity. To solve the problem, it is imperative to characterize how cells assemble aggresome. By examining aggresomes in six cell lines, A549 cells were selectively studied for their bigger cell size and moderate aggresome-forming activity. Aggresome grew in size upon continuous exposure of A549 cells to proteasome inhibitor MG132 and reached a mature size around the 16th to 24th hour of treatment. Mechanistic studies revealed that NF-кB translocated to the nucleus in MG132-treated cells, and chemical activation or knockdown of NF-кB enhanced or prohibited aggresome assembly. Further analyses showed that NF-кB upregulated HDAC6, and HDAC6 maintained the Vimentin cage by interacting with Vimentin p72, a key modification of the intermediate filament contributing to aggresome formation. Remarkably, chemical inactivation of NF-кB synergized MG132-induced cell mortality. All the findings suggest that NF-кB dictates aggresome assembly via upregulating HDAC6, and NF-кB inhibitor may serve as a potential drug potentiating proteasome inhibitor medicine-induced cytotoxicity during the treatment of cancer cells.NEW & NOTEWORTHY The study reveals a new mechanism guiding MG132-triggered aggresome formation. NF-кB is quickly activated upon exposure to MG132, and NF-кB upregulates the misfolded protein recognizing factor HDCA6. In addition to collecting misfolded proteins, HDAC6 also binds Vimentin and maintains the Vimentin cage, which quarantines toxic misfolded proteins and protects cells from being toxified by those protein toxicants. Therapeutically, chemical inactivation of NF-кB synergizes MG132-induced cytotoxicity, providing a new strategy to defeat cancers.

Targeting histone deacetylases in head and neck squamous cell carcinoma: molecular mechanisms and therapeutic targets

The onerous health and economic burden associated with head and neck squamous cell carcinoma (HNSCC) is a global predicament. Despite the advent of novel surgical techniques and therapeutic protocols, there is an incessant need for efficacious diagnostic and therapeutic targets to monitor the invasion, metastasis and recurrence of HNSCC due to its substantial morbidity and mortality. The differential expression patterns of histone deacetylases (HDACs), a group of enzymes responsible for modifying histones and regulating gene expression, have been demonstrated in neoplastic tissues. However, there is limited knowledge regarding the role of HDACs in HNSCC. Consequently, this review aims to summarize the existing research findings and explore the potential association between HDACs and HNSCC, offering fresh perspectives on therapeutic approaches targeting HDACs that could potentially enhance the efficacy of HNSCC treatment. Additionally, the Cancer Genome Atlas (TCGA) dataset, CPTAC, HPA, OmicShare, GeneMANIA and STRING databases are utilized to provide supplementary evidence on the differential expression of HDACs, their prognostic significance and predicting functions in HNSCC patients.

Inhibition of SQSTM1 S403 phosphorylation facilitates the aggresome formation of ubiquitinated proteins during proteasome dysfunction

BACKGROUND

Ubiquitin-proteasome-system-mediated clearance of misfolded proteins is essential for cells to maintain proteostasis and reduce the proteotoxicity caused by these aberrant proteins. When proteasome activity is inadequate, ubiquitinated proteins are sorted into perinuclear aggresomes, which is a significant defense mechanism employed by cells to combat insufficient proteasome activity, hence mitigating the proteotoxic crisis. It has been demonstrated that phosphorylation of SQSTM1 is crucial in regulating misfolded protein aggregation and autophagic degradation. Although SQSTM1 S403 phosphorylation is essential for the autophagic degradation of ubiquitinated proteins, its significance in proteasome inhibition-induced aggresome formation is yet unknown. Herein, we investigated the influence of SQSTM1 S403 phosphorylation on the aggresome production of ubiquitinated proteins during proteasome suppression.

METHODS

We examined the phosphorylation levels of SQSTM1 S403 or T269/S272 in cells after treated with proteasome inhibitors or/and autophagy inhibitors, by western blot and immunofluorescence. We detected the accumulation and aggresome formation of ubiquitinated misfolded proteins in cells treated with proteasome inhibition by western blot and immunofluorescence. Furthermore, we used SQSTM1 phosphorylation-associated kinase inhibitors and mutant constructs to confirm the regulation of different SQSTM1 phosphorylation in aggresome formation. We examined the cell viability using CCK-8 assay.

RESULTS

Herein, we ascertained that phosphorylation of SQSTM1 S403 did not enhance the autophagic degradation of ubiquitinated proteins during proteasome inhibition. Proteasome inhibition suppresses the phosphorylation of SQSTM1 S403, which facilitated the aggresome production of polyubiquitinated proteins. Interestingly, we found proteasome inhibition-induced SQSTM1 T269/S272 phosphorylation inhibits the S403 phosphorylation. Suppressing S403 phosphorylation rescues the defective aggresome formation and protects cells from cell death caused by unphosphorylated SQSTM1 (T269/S272).

CONCLUSIONS

This study shows that inhibition of SQSTM1 S403 phosphorylation facilitates the aggresome formation of ubiquitinated proteins during proteasome dysfunction. SQSTM1 T269/S272 phosphorylation inhibits the S403 phosphorylation, boosting the aggresome formation of ubiquitinated protein and shielding cells from proteotoxic crisis.

Advances in the Mechanistic Study of the Control of Oxidative Stress Injury by Modulating HDAC6 Activity

Oxidative stress is defined as an injury resulting from a disturbance in the dynamic equilibrium of the redox environment due to the overproduction of active/radical oxygen exceeding the antioxidative ability of the body. This is a key step in the development of various diseases. Oxidative stress is modulated by different factors and events, including the modification of histones, which are the cores of nucleosomes. Histone modification includes acetylation and deacetylation of certain amino acid residues; this process is catalyzed by different enzymes. Histone deacetylase 6 (HDAC6) is a unique deacetylating protease that also catalyzes the deacetylation of different nonhistone substrates to regulate various physiologic processes. The intimate relationship between HDAC6 and oxidative stress has been demonstrated by different studies. The present paper aims to summarize the data obtained from a mechanistic study of HDAC6 and oxidative stress to guide further investigations on mechanistic characterization and drug development.

Effect of regulatory cell death on the occurrence and development of head and neck squamous cell carcinoma

Head and neck cancer is a malignant tumour with a high mortality rate characterized by late diagnosis, high recurrence and metastasis rates, and poor prognosis. Head and neck squamous cell carcinoma (HNSCC) is the most common type of head and neck cancer. Various factors are involved in the occurrence and development of HNSCC, including external inflammatory stimuli and oncogenic viral infections. In recent years, studies on the regulation of cell death have provided new insights into the biology and therapeutic response of HNSCC, such as apoptosis, necroptosis, pyroptosis, autophagy, ferroptosis, and recently the newly discovered cuproptosis. We explored how various cell deaths act as a unique defence mechanism against cancer emergence and how they can be exploited to inhibit tumorigenesis and progression, thus introducing regulatory cell death (RCD) as a novel strategy for tumour therapy. In contrast to accidental cell death, RCD is controlled by specific signal transduction pathways, including TP53 signalling, KRAS signalling, NOTCH signalling, hypoxia signalling, and metabolic reprogramming. In this review, we describe the molecular mechanisms of nonapoptotic RCD and its relationship to HNSCC and discuss the crosstalk between relevant signalling pathways in HNSCC cells. We also highlight novel approaches to tumour elimination through RCD.

Pharmacological inhibition of HDAC6 overcomes cisplatin chemoresistance by targeting cancer stem cells in oral squamous cell carcinoma

BACKGROUND

Chemoresistance is associated with recurrence and metastasis in oral squamous cell carcinoma (OSCC). The cancer stem cell (CSC) subpopulation is highly resistant to therapy, and they are regulated by epigenetic mechanisms. HDACs are histone deacetylase enzymes that epigenetically regulate gene expression. HDAC6 acts on several physiological processes, including oxidative stress, autophagy and DNA damage response, and its accumulation is associated with cancer. Here, we investigate the role of HDAC6 in CSC-mediated chemoresistance in oral carcinoma in addition to its application as a therapeutic target to reverse chemoresistance.

METHODS

Wild-type oral carcinoma cell lines (CAL27 WT and SCC9 WT), cisplatin-resistant (CAL27 CisR and SCC9 CisR), and the subpopulations of cancer stem cells (CSC+) and non-stem (CSC-) derived from CisR cells were investigated. HDAC6 accumulation was analyzed by Western blot and immunofluorescence; DNA damage was evaluated by immunofluorescence of phospho-H2A.X; the qPCR for PRDX2, PRDX6, SOD2, and TXN and ROS assay assessed oxidative stress. Apoptosis and CSC accumulation were investigated by flow cytometry.

RESULTS

We identified the accumulation of HDAC6 in CisR cell lines and CSC. Cisplatin-resistant cell lines and CSC demonstrated a reduction in DNA damage and ROS and elevated expression of PRDX2. The administration of tubastatin A (a specific HDAC6 inhibitor) increased oxidative stress and DNA damage and decreased PRDX2. Tubastatin A as a monotherapy induced apoptosis in CisR and CSC and reduced the stemness phenotype.

CONCLUSION

High levels of HDAC6 sustain CSC subpopulation and chemoresistance in OSCC, suggesting HDAC6 as a pharmacological target to overcome resistance and perhaps prevent recurrence in OSCC.

Blocking autophagy overcomes resistance to dual histone deacetylase and proteasome inhibition in gynecologic cancer

Histone deacetylase (HDAC) inhibitors and proteasome inhibitors have been approved by the FDA for the treatment of multiple myeloma and lymphoma, respectively, but have not achieved similar activity as single agents in solid tumors. Preclinical studies have demonstrated the activity of the combination of an HDAC inhibitor and a proteasome inhibitor in a variety of tumor models. However, the mechanisms underlying sensitivity and resistance to this combination are not well-understood. This study explores the role of autophagy in adaptive resistance to dual HDAC and proteasome inhibition. Studies focus on ovarian and endometrial gynecologic cancers, two diseases with high mortality and a need for novel treatment approaches. We found that nanomolar concentrations of the proteasome inhibitor ixazomib and HDAC inhibitor romidepsin synergistically induce cell death in the majority of gynecologic cancer cells and patient-derived organoid (PDO) models created using endometrial and ovarian patient tumor tissue. However, some models were not sensitive to this combination, and mechanistic studies implicated autophagy as the main mediator of cell survival in the context of dual HDAC and proteasome inhibition. Whereas the combination of ixazomib and romidepsin reduces autophagy in sensitive gynecologic cancer models, autophagy is induced following drug treatment of resistant cells. Pharmacologic or genetic inhibition of autophagy in resistant cells reverses drug resistance as evidenced by an enhanced anti-tumor response both in vitro and in vivo. Taken together, our findings demonstrate a role for autophagic-mediated cell survival in proteasome inhibitor and HDAC inhibitor-resistant gynecologic cancer cells. These data reveal a new approach to overcome drug resistance by inhibiting the autophagy pathway.

Proteasome inhibition by bortezomib parallels a reduction in head and neck cancer cells growth, and an increase in tumor-infiltrating immune cells

Head and neck cancer (HNC) has frequently an aggressive course for the development of resistance to standard chemotherapy. Thus, the use of innovative therapeutic drugs is being assessed. Bortezomib is a proteasome inhibitor with anticancer effects. In vitro antitumoral activity of Bortezomib was investigated employing human tongue (SCC-15, CAL-27), pharynx (FaDu), salivary gland (A-253) cancer cell lines and a murine cell line (SALTO-5) originated from a salivary gland adenocarcinoma arising in BALB-neuT male mice transgenic for the oncogene neu. Bortezomib inhibited cell proliferation, triggered apoptosis, modulated the expression and activation of pro-survival signaling transduction pathways proteins activated by ErbB receptors and inhibited proteasome activity in vitro. Intraperitoneal administration of Bortezomib delayed tumor growth of SALTO-5 cells transplanted in BALB-neuT mice, protracted mice survival and adjusted tumor microenvironment by increasing tumor-infiltrating immune cells (CD4⁺ and CD8⁺ T cells, B lymphocytes, macrophages, and Natural Killer cells) and by decreasing vessels density. In addition, Bortezomib modified the expression of proteasome structural subunits in transplanted SALTO-5 cells. Our findings further support the use of Bortezomib for the treatment of HNC and reveal its ineffectiveness in counteracting the activation of deregulated specific signaling pathways in HNC cell lines when resistance to proteasome inhibition is developed.

A therapeutic approach with combination of interferon-gamma and autophagy inhibitor for oral squamous cell carcinoma

Former clinical trials and experimental research have indicated that Interferon-gamma therapy does not achieve an ideal effect in solid tumors. Autophagy has been associated with tumor chemoresistance. The aim of this study was to explore the efficacy of Interferon-gamma and autophagy inhibitor in the combination treatment of oral squamous cell carcinoma. Interferon-gamma-induced apoptosis was evaluated by the expression of relative proteins (cleaved-PARP and caspase-3) and flow cytometry. Interferon-gamma induced autophagy was assessed by the expression of Beclin1, LC3B, and P62. The synergistic effect of interferon-gamma and autophagy inhibitor (chloroquine) was evaluated in vitro and in vivo. Interferon-gamma induced anti-proliferation, apoptosis, and autophagy in oral squamous cell carcinoma cells. Autophagy-related protein 5 was a key feature in Interferon-gamma-induced autophagy flux. Interferon-gamma and chloroquine had obvious synergistic effects on cellular growth inhibition and apoptosis promotion in oral squamous cell carcinoma cells and xenograft models. Our findings suggest that Interferon-gamma-induced autophagy plays a cellular protective role, and blocking autophagy flux can promote Interferon-gamma mediated oral squamous cell carcinoma cell apoptosis. The combination of Interferon-gamma and autophagy inhibitors represents a novel strategy for oral squamous cell carcinoma therapy.

Inhibition of HDAC6 by Tubastatin A reduces chondrocyte oxidative stress in chondrocytes and ameliorates mouse osteoarthritis by activating autophagy

The aim of this study was to determine the effect of HDAC6 inhibition using the selective inhibitor Tubastatin A (TubA) on the regulation of tert-butyl hydroperoxide (TBHP)-treated chondrocytes and a mouse OA model. Using conventional molecular biology methods, our results showed that the level of HDAC6 increases both in the cartilage of osteoarthritis (OA) mice and TBHP-treated chondrocytes in vitro. TubA treatment effectively inhibits the expression of HDAC6, attenuates oxidative stress, reduces the level of apoptotic proteins to maintain chondrocyte survival, and suppresses the extracellular matrix (ECM) degradation. In addition, our results also revealed that HDAC6 inhibition by TubA activates autophagy in chondrocytes, whereas the protective effects of TubA were abolished by autophagy inhibitor intervention. Subsequently, the positive effects of HDAC6 inhibition by TubA were also found in a mouse OA model. Therefore, our study provide evidence that HDAC6 inhibition prevents OA development, and HDAC6 could be applied as a potential therapeutic target for OA management.

Resistance of osteosarcoma cells to the proapoptotic effects of carfilzomib involves activation of mitogen activated protein kinase pathways

NEW FINDINGS

What is the central question of this study? Carfilzomib, a second-generation proteasome inhibitor approved for the treatment of multiple myeloma, shows efficacy against osteosarcoma. However, drug resistance remains a major challenge. What is the role of carfilzomib-induced changes in mitogen-activated protein kinase (MAPK) pathways in the sensitivity of osteosarcoma cells to the proapoptotic effects of the drug? What is the main finding and its importance? The dose-dependent antiapoptotic effects in osteosarcoma are associated with activation of MAPK signalling. Combinational targeting of MAPK signalling pathways can synergistically enhance carfilzomib-induced cell apoptosis, suggesting that MAPK inhibitors in combination with proteasome inhibitors can serve as a novel therapeutic tool for osteosarcoma.

ABSTRACT

Osteosarcoma is the most common primary bone malignancy. Despite efforts to improve outcomes, the overall survival rates for osteosarcoma have remained unchanged over the past three decades. In this study, we assessed the proapoptotic effects of the second-generation proteasome inhibitor carfilzomib on osteosarcoma and investigated the potential mechanisms underlying the synergistic proapoptotic action when combined with mitogen-activated protein kinase (MAPK) inhibitors. We found that carfilzomib alone significantly inhibited cell proliferation and induced apoptosis in a dose-dependent manner, characterized by the induction of cleaved caspase 3 and poly (ADP-ribose) polymerase. More importantly, focusing on the changes of antiapoptotic B-cell lymphoma 2 (Bcl-2) family members and signalling pathways, we found a striking induction of myeloid cell leukaemia 1 (Mcl-1) and the activation of MAPK pathways. Furthermore, we observed that combinational targeting of the MAPK pathways using the specific inhibitors U0126, SP600125 or SB203580 synergistically enhanced carfilzomib-induced cell apoptosis. Notably, we found that the combinational inhibition of extracellular signal-regulated kinase or c-Jun N-terminal kinase MAPK pathways significantly decreased the expression of the three antiapoptotic Bcl-2 family proteins, and in particular this reversed induction of Mcl-1 by carfilzomib. Collectively, our findings show that activation of the MAPK pathways contributes to the mechanisms of drug resistance to carfilzomib. In addition, the synergistic proapoptotic action of MAPK and proteasome inhibitors in osteosarcoma cells suggests that combinational therapy with both drug types may serve as a novel strategy for the clinical management of osteosarcoma.

Combination of ACY-241 and JQ1 Synergistically Suppresses Metastasis of HNSCC via Regulation of MMP-2 and MMP-9

Overexpression of histone deacetylase 6 (HDAC6) and bromodomain-containing protein 4 (BRD4) is related to aggressiveness of head and neck squamous carcinoma (HNSCC). Based on studies that HDAC6 and BRD4 are potential therapeutic targets of HNSCC, we hypothesized that the combination treatment of BET inhibitor JQ1 and HDAC6-selective inhibitor ACY-241 could exhibit synergistic anticancer effects in human papillomavirus (HPV)-positive and HPV-negative HNSCC cells. In this study, HNSCC cell growth and viability were measured by CCK-8 assay, apoptosis was analyzed by flow cytometry, and metastasis was studied by wound healing and transwell assays. Furthermore, immunoblotting is conducted to investigate proteins that modulate apoptosis or metastasis. Here, we report that the combination of ACY-241 and JQ1 shows synergistic cell growth inhibition, viability reduction, and apoptosis induction in HNSCC cells through inactivation of AKT and NF-κB signaling. Importantly, we demonstrate that combined treatment of ACY-241 and JQ1 synergistically suppresses TNF-α-induced migration and invasion via dysregulating matrix metalloproteinase (MMP)-2, MMP-9, and MT1-MMP. Overall, the combination of ACY-241 and JQ1 significantly suppresses proliferation and metastasis in HPV-positive and HPV-negative HNSCC. Collectively, these findings suggest that the co-inhibition of BET and HDAC6 can be a new therapeutic strategy in HNSCC.

Dual Inhibition of Autophagy and PI3K/AKT/MTOR Pathway as a Therapeutic Strategy in Head and Neck Squamous Cell Carcinoma

Genomic analyses of head and neck squamous cell carcinoma (HNSCC) have highlighted alterations in the phosphatidylinositol 3-kinase (PI3K) signaling pathway, presenting a therapeutic target for multiple ongoing clinical trials with PI3K or PI3K/MTOR inhibitors. However, these inhibitors can potentially increase autophagy in HNSCC and indirectly support cancer cell survival. Here, we sought to understand the relationship between the PI3K signaling pathway and autophagy during their dual inhibition in a panel of HNSCC cell lines. We used acridine orange staining, immunoblotting, and tandem sensor Red Fluorescent Protein- Green Fluorescent Protein-, microtubule-associated protein 1 light chain 3 beta (RFP-GFP-LC3B) expression analysis to show that PI3K inhibitors increase autophagosomes in HNSCC cells, but that chloroquine treatment effectively inhibits the autophagy that is induced by PI3K inhibitors. Using the Bliss independence model, we determined that the combination of chloroquine with PI3K inhibitors works in synergy to decrease cancer cell proliferation, independent of the PIK3CA status of the cell line. Our results indicate that a strategy focusing on autophagy inhibition enhances the efficacy of therapeutics already in clinical trials. Our results suggest a broader application for this combination therapy that can be promptly translated to in vivo studies.

Oxidative Inactivation of the Proteasome Augments Alveolar Macrophage Secretion of Vesicular SOCS3

Extracellular vesicles (EVs) contain a diverse array of molecular cargoes that alter cellular phenotype and function following internalization by recipient cells. In the lung, alveolar macrophages (AMs) secrete EVs containing suppressor of cytokine signaling 3 (SOCS3), a cytosolic protein that promotes homeostasis via vesicular transfer to neighboring alveolar epithelial cells. Although changes in the secretion of EV molecules-including but not limited to SOCS3-have been described in response to microenvironmental stimuli, the cellular and molecular machinery that control alterations in vesicular cargo packaging remain poorly understood. Furthermore, the use of quantitative methods to assess the sorting of cytosolic cargo molecules into EVs is lacking. Here, we utilized cigarette smoke extract (CSE) exposure of AMs as an in vitro model of oxidative stress to address these gaps in knowledge. We demonstrate that the accumulation of reactive oxygen species (ROS) in AMs was sufficient to augment vesicular SOCS3 release in this model. Using nanoparticle tracking analysis (NTA) in tandem with a new carboxyfluorescein succinimidyl ester (CFSE)-based intracellular protein packaging assay, we show that the stimulatory effects of CSE were at least in part attributable to elevated amounts of SOCS3 packaged per EV secreted by AMs. Furthermore, the use of a 20S proteasome activity assay alongside treatment of AMs with conventional proteasome inhibitors strongly suggest that ROS stimulated SOCS3 release via inactivation of the proteasome. These data demonstrate that tuning of AM proteasome function by microenvironmental oxidants is a critical determinant of the packaging and secretion of cytosolic SOCS3 protein within EVs.

BET Inhibitors Synergize with Carfilzomib to Induce Cell Death in Cancer Cells via Impairing Nrf1 Transcriptional Activity and Exacerbating the Unfolded Protein Response

Currently, proteasome inhibitors bortezomib, carfilzomib, and ixazomib are successfully used in clinics to treat multiple myeloma. However, these agents show limited efficacy against solid tumors. Identification of drugs that can potentiate the action of proteasome inhibitors could help expand the use of this therapeutic modality to solid tumors. Here, we found that bromodomain extra-terminal (BET) family protein inhibitors such as JQ1, I-BET762, and I-BET151 synergize with carfilzomib in multiple solid tumor cell lines. Mechanistically, BET inhibitors attenuated the ability of the transcription factor Nrf1 to induce proteasome genes in response to proteasome inhibition, thus, impeding the bounce-back response of proteasome activity, a critical pathway by which cells cope with proteotoxic stress. Moreover, we found that treatment with BET inhibitors or depletion of Nrf1 exacerbated the unfolded protein response (UPR), signaling that was initiated by proteasome inhibition. Taken together, our work provides a mechanistic explanation behind the synergy between proteasome and BET inhibitors in cancer cell lines and could prompt future preclinical and clinical studies aimed at further investigating this combination.

Targeted inhibition of ULK1 enhances daunorubicin sensitivity in acute myeloid leukemia

PURPOSE

In acute myeloid leukemia (AML), complete remission can be achieved in parts of patients using cytarabine/anthracycline combination-based chemotherapy, however, drug resistance-related recurrence is still a common cause of treatment failure, leading to high mortality among patients. In our research, we revealed the molecular mechanisms that were sufficient to improve sensitivity of AML cells to the anthracycline daunorubicin (DNR).

METHODS

We evaluated the effects of autophagy and apoptosis induced by DNR using two AML cell lines HL60 and U937.Western blot was preformed to analyze the apoptotic pathway protein expression and flow cytometric analysis was used to detect the level of apoptosis in AML cells. The levels of autophagy-related proteins were detected by western blotting and autophagic vesicles were observed by electron microscopy.

RESULTS

DNR effectively induced autophagy in two AML cell lines HL60 and U937 confirming by upregulation of LC3-II lipidation, formation of autophagosomes. Inhibition of autophagy by pharmacologic inhibitor HCQ promoted apoptosis induced by DNR, suggesting that autophagy played a vital role in pro-survival in AML. Furthermore, ULK1 inhibition by a highly selective kinase inhibitor SBI-0206965 and shRNA enhanced cytotoxicity of DNR against AML cells. Independent of mTOR -ULK1 signaling pathway, activation of autophagy of DNR was proved to be mediated by AMPK (pThr172)/ULK1 pathway.

CONCLUSIONS

These results revealed that pro-survival autophagy induced by ULK1 activation was one of the potential mechanisms of AML resistance to DNR. Targeting ULK1 selectively could be a promising therapeutic strategy to enhance sensitivity of DNR for AML therapy.

A Novel Therapeutic Induces DEPTOR Degradation in Multiple Myeloma Cells with Resulting Tumor Cytotoxicity

Prior work indicates DEPTOR expression in multiple myeloma cells could be a therapeutic target. DEPTOR binds to mTOR via its PDZ domain and inhibits mTOR kinase activity. We previously identified a drug, which prevented mTOR-DEPTOR binding (NSC126405) and induced multiple myeloma cytotoxicity. We now report on a related therapeutic, drug 3g, which induces proteasomal degradation of DEPTOR. DEPTOR degradation followed drug 3g binding to its PDZ domain and was not due to caspase activation or enhanced mTOR phosphorylation of DEPTOR. Drug 3g enhanced mTOR activity, and engaged the IRS-1/PI3K/AKT feedback loop with reduced phosphorylation of AKT on T308. Activation of TORC1, in part, mediated multiple myeloma cytotoxicity. Drug 3g was more effective than NSC126405 in preventing binding of recombinant DEPTOR to mTOR, preventing binding of DEPTOR to mTOR inside multiple myeloma cells, in activating mTOR and inducing apoptosis in multiple myeloma cells. In vivo, drug 3g injected daily abrogated DEPTOR expression in xenograft tumors and induced an antitumor effect although modest weight loss was seen. Every-other-day treatment, however, was equally effective without weight loss. Drug 3g also reduced DEPTOR expression in normal tissues. Although no potential toxicity was identified in hematopoietic or hepatic function, moderate cardiac enlargement and glomerular mesangial hypertrophy was seen. DEPTOR protected multiple myeloma cells against bortezomib suggesting anti-DEPTOR drugs could synergize with proteasome inhibitors (PI). Indeed, combinations of drug NSC126405 + bortezomib were synergistic. In contrast, drug 3g was not and was even antagonistic. This antagonism was probably due to prevention of proteasomal DEPTOR degradation.

Blocking autophagy flux promotes interferon-alpha-mediated apoptosis in head and neck squamous cell carcinoma

Despite multiple antitumor activities, interferon-alpha (IFNα) therapy alone is less effective in solid tumors. Autophagy has been reported to play a key role in tumor chemoresistance. Therefore, it is meaningful to explore whether autophagy can be activated by IFNα in head and neck squamous cell carcinoma (HNSCC) and serve as a potential target to improve efficacy of IFNα therapy. In this study, we report that IFNα not only exhibits anti-proliferation activity and induces apoptosis, but also activates autophagy in HNSCC cells. Moreover, silencing autophagy-related protein 5 (ATG5) and signal transducer and activator of transcription 1 (STAT1) suppresses autophagy flux. Furthermore, IFNα and autophagy inhibitors (hydroxychloroquine and wortmannin) show clear synergistic effects on inhibiting growth and promoting apoptosis in HNSCC cells and xenograft models. Our findings indicate that IFNα-induced autophagy plays a cytoprotective role and blocking autophagy flux promotes IFNα-mediated apoptosis in HNSCC. These results suggest that the combination of IFNα and autophagy inhibitors represents a novel strategy for HNSCC treatment.

Niclosamide Induces Cell Cycle Arrest in G1 Phase in Head and Neck Squamous Cell Carcinoma Through Let-7d/CDC34 Axis

Niclosamide is a traditional anti-tapeworm drug that exhibits potent anti-cancer activity. Our previous study showed that niclosamide induces cell cycle arrest in G1 phase. Nevertheless, the underlying mechanism remains unknown. The following study investigated the molecular mechanism through which niclosamide induced G1 arrest in head and neck squamous cell carcinoma (HNSCC) cell lines. The effect of niclosamide on human HNSCC cell line WSU-HN6 and CNE-2Z were analyzed using IncuCyte ZOOM^TM assay, flow cytometry (FCM), real-time PCR and western blot. Luciferase assay was conducted to demonstrate the interaction between let-7d (a let-7 family member which functions as a tumor suppressor by regulating cell cycle) and 3′UTR of CDC34 mRNA. Xenografts tumor model was established to evaluate the niclosamide treatment efficacy in vivo. Briefly, an exposure to niclosamide treatment led to an increased let-7d expression and a decreased expression of cell cycle regulator CDC34, finally leading to G1 phase arrest. Moreover, an overexpression of let-7d induced G1 phase arrest and downregulated CDC34, while the knockdown of let-7d partially rescued the niclosamide-induced G1 phase arrest. Luciferase assay confirmed the direct inhibition of CDC34 through the targeting of let-7d. Furthermore, niclosamide markedly inhibited the xenografts growth through up-regulation of let-7d and down-regulation of CDC34. To sum up, our findings suggest that niclosamide induces cell cycle arrest in G1 phase in HNSCC through let-7d/CDC34 axis, which enriches the anti-cancer mechanism of niclosamide.

In HPV-Positive HNSCC Cells, Functional Restoration of the p53/p21 Pathway by Proteasome Inhibitor Bortezomib Does Not Affect Radio- or Chemosensitivity

Human papillomavirus (HPV) associated squamous cell carcinomas of the head and neck region (HPV+ HNSCCs) harbor diverging biological features as compared to classical noxa-induced (HPV−) HNSCC. One striking difference between subtypes is that the tumor suppressor gene TP53 is usually not mutated in HPV+ HNSCCs. However, p53 is inhibited by viral oncoprotein E6, leading to premature proteasomal degradation. We asked whether bortezomib (BZM), a clinically approved inhibitor of the proteasome, can functionally restore p53 and investigated in how far this will result in an enhanced radio- or chemosensitivity of HPV+ HNSCC cell lines. For all four HPV+ cell lines tested, BZM led to functional restoration of p53 and transactivation of downstream protein p21. In HPV+ cells, BZM also restored the radiation-induced p53/p21 transactivation. Consistently, in HPV+ cells, a restored G1 arrest as well as enhanced apoptosis were seen when BZM was given prior to irradiation (IR) or cisplatin (CDDP). BZM alone reduced the clonogenic survival of both HPV− and HPV+ cells. However, if BZM was combined with IR or CDDP, BZM did not significantly enhance radio- or chemosensitivity of HPV+ or HPV− HNSCC cell lines.

Transactive response DNA binding protein of 43/histone deacetylase 6 axis alleviates H 2 O 2 -induced retinal ganglion cells injury through inhibiting apoptosis and autophagy

Oxidative damage is believed to contribute to the pathogenesis of diabetic retinopathy (DR). The current study aimed to detect the effects of transactive response DNA binding protein of 43 (TDP-43) on cell damage induced by hydrogen peroxide (H₂ O₂ ) in retinal ganglion cells (RGCs) and to investigate the molecular mechanisms involved in this process. We observed that TDP-43 was highly expressed in RGC-5 cells induced by H₂ O₂ , and that repression of TDP-43 obviously ameliorated H₂ O₂ -induced RGC-5 cell injury. In addition, loss of TDP-43 profoundly mitigated H₂ O₂ -triggered oxidative stress by decreasing the production of intracellular reactive oxygen species and the activity of oxidative stress indicator malondialdehyde, as well as enhancing the content of antioxidant enzymes superoxide dismutase, glutathione peroxidase and catalase to restore the antioxidant defense system. Moreover, suppression of TDP-43 obviously obstructed H₂ O₂ -induced apoptosis. Meanwhile, knockdown of TDP-43 attenuated the expression of the proapoptotic proteins Bax and Cytochrome c, elevated the anti-apoptotic protein Bcl-2, and suppressed the activation of caspase 3 in H₂ O₂ -induced RGC-5 cells. Moreover, elimination of TDP-43 inhibited H₂ O₂ -triggered autophagy, which appeared as decreased expression of LC3II/I and Beclin-1, along with p62 degradation. Importantly, silencing of TDP-43 diminished the expression of histone deacetylase 6 (HDAC6), and HDAC6 also abolished the inhibitory effect of TDP-43 inhibition on H₂ O₂ -induced apoptosis and autophagy. Collectively, our findings demonstrated that depletion of TDP-43 may protect RGC-5 cells against oxidative stress-mediated apoptosis and autophagy by suppressing its target HDAC6. Thus, the TDP-43/HDAC6 axis might be a promising strategy for the treatment of DR.

Positioning of proteasome inhibitors in therapy of solid malignancies

Targeting of the protein degradation pathway, in particular, the ubiquitin-proteasome system, has emerged as an attractive novel cancer chemotherapeutic modality. Although proteasome inhibitors have been most successfully applied in the treatment of hematological malignancies, they also received continuing interest for the treatment of solid tumors. In this review, we summarize the current positioning of proteasome inhibitors in the treatment of common solid malignancies (e.g., lung, colon, pancreas, breast, and head and neck cancer), addressing topics of their mechanism(s) of action, predictive factors and molecular mechanisms of resistance.

Anti-cancer effects of naturally derived compounds targeting histone deacetylase 6-related pathways

Alterations of the epigenetic machinery, affecting multiple biological functions, represent a major hallmark enabling the development of tumors. Among epigenetic regulatory proteins, histone deacetylase (HDAC)6 has emerged as an interesting potential therapeutic target towards a variety of diseases including cancer. Accordingly, this isoenzyme regulates many vital cellular regulatory processes and pathways essential to physiological homeostasis, as well as tumor multistep transformation involving initiation, promotion, progression and metastasis. In this review, we will consequently discuss the critical implications of HDAC6 in distinct mechanisms relevant to physiological and cancerous conditions, as well as the anticancer properties of synthetic, natural and natural-derived compounds through the modulation of HDAC6-related pathways.

ClC5 Decreases the Sensitivity of Multiple Myeloma Cells to Bortezomib via Promoting Prosurvival Autophagy

Resistance to bortezomib (BZ) is the major problem that largely limits its clinical application in multiple myeloma treatment. In the current study, we investigated whether ClC5, a member of the chloride channel family, is involved in this process. The MTT assay showed that BZ treatment decreased cell viability in three multiple myeloma cell lines (ARH77, U266, and SKO-007), with IC₅₀ values of 2.83, 4.37, and 1.91 nM, respectively. Moreover, BZ increased the conversion of LC3B-I to LC3B-II and expressions of beclin-1 and ATG5, concomitantly with a decreased p62 expression. Pharmacological inhibition of autophagy with 3-MA facilitated cell death in response to BZ treatment. Additionally, BZ increased ClC5 protein expression in ARH77, U266, and SKO-007 cells. Knockdown of ClC5 with small interfering RNA sensitized cells to BZ treatment, and upregulation of ClC5 induced chemoresistance to BZ. Furthermore, ClC5 downregulation promoted BZ-induced LC3B-I to LC3B-II conversion and beclin-1 expression, whereas overexpression of ClC5 showed the opposite results in ARH77 cells. Finally, BZ induced dephosphorylation of AKT and mTOR, which was significantly attenuated by ClC5 inhibition. However, ClC5 upregulation further enhanced AKT and mTOR dephosphorylation induced by BZ. Our study demonstrates that ClC5 induces chemoresistance of multiple myeloma cells to BZ via increasing prosurvival autophagy by inhibiting the AKT–mTOR pathway. These data suggest that ClC5 may play a critical role in future multiple myeloma treatment strategies.

Epigenetic Modifications and Head and Neck Cancer: Implications for Tumor Progression and Resistance to Therapy

Head and neck squamous carcinoma (HNSCC) is the sixth most prevalent cancer and one of the most aggressive malignancies worldwide. Despite continuous efforts to identify molecular markers for early detection, and to develop efficient treatments, the overall survival and prognosis of HNSCC patients remain poor. Accumulated scientific evidences suggest that epigenetic alterations, including DNA methylation, histone covalent modifications, chromatin remodeling and non-coding RNAs, are frequently involved in oral carcinogenesis, tumor progression, and resistance to therapy. Epigenetic alterations occur in an unsystematic manner or as part of the aberrant transcriptional machinery, which promotes selective advantage to the tumor cells. Epigenetic modifications also contribute to cellular plasticity during tumor progression and to the formation of cancer stem cells (CSCs), a small subset of tumor cells with self-renewal ability. CSCs are involved in the development of intrinsic or acquired therapy resistance, and tumor recurrences or relapse. Therefore, the understanding and characterization of epigenetic modifications associated with head and neck carcinogenesis, and the prospective identification of epigenetic markers associated with CSCs, hold the promise for novel therapeutic strategies to fight tumors. In this review, we focus on the current knowledge on epigenetic modifications observed in HNSCC and emerging Epi-drugs capable of sensitizing HNSCC to therapy.