Regulation of KLF4 by posttranslational modification circuitry in endocrine resistance

KLF4 plays an important role in orchestrating a variety of cellular events, including cell-fate decision, genome stability and apoptosis. Its deregulation is correlated with human diseases such as breast cancer and gastrointestinal cancer. Results from recent biochemical studies have revealed that KLF4 is tightly regulated by posttranslational modifications. Here we report a new finding that KLF4 orchestrates estrogen receptor signaling and facilitates endocrine resistance. We also uncovered the underlying mechanism that alteration of KLF4 by posttranslational modifications such as phosphorylation and ubiquitylation changes tumor cell response to endocrine therapy drugs. IHC analyses using based on human breast cancer specimens showed the accumulation of KLF4 protein in ER-positive breast cancer tissues. Elevated KLF4 expression significantly correlated with prognosis and endocrine resistance. Our drug screening for suppressing KLF4 protein expression led to identification of Src kinase to be a critical player in modulating KLF4-mediated tamoxifen resistance. Depletion of VHL (von Hippel-Lindau tumor suppressor), a ubiquitin E3 ligase for KLF4, reduces tumor cell sensitivity to tamoxifen. We demonstrated phosphorylation of VHL by Src enhances proteolysis of VHL that in turn leads to upregulation of KLF4 and increases endocrine resistance. Suppression of Src-VHL-KLF4 cascade by Src inhibitor or enhancement of VHL-KLF4 ubiquitination by TAT-KLF4 (371-420AAa) peptides re-sensitizes tamoxifen-resistant breast cancer cells to tamoxifen treatment. Taken together, our findings demonstrate a novel role for KLF4 in modulating endocrine resistance via the Src-VHL-KLF4 axis.

Abstract SY20-02: The SIRT3-MnSOD-Ac axis in carcinogenesis and tumor resistance

ABSTRACT The molecular mechanism by which acetylation regulates manganese superoxide dismutase (MnSOD) activity, and its oncogenic and/or physiological significance, is unclear. Here we show that expression of the MnSOD-K68 acetylation (Ac) mimic mutant (MnSODK68Q) transformed wild-type primary mouse embryonic fibroblasts (pMEFs) when co-infected with either c-Myc or Ras, as measured by in vitro transformation assays and xenograft formation. Immortalized MnSOD-/- mouse embryonic fibroblasts (MEFs) infected with lenti-MnSODK68Q also exhibited an in vitro transformation permissive phenotype. In addition, MCF7 and T47D cells expressing MnSODK68Q demonstrated increased in vitro and xenograft oncogenic properties. These findings collectively identify MnSODK68Q as a tumor promoter. MCF7, T47D, and MnSOD-/- cells expressing MnSODK68Q displayed a monomeric form of MnSOD, distinct from the well-established homotetramer, and MnSOD-K68-Ac functions as a peroxidase, determined using a bacterial system for site-specific incorporation of N-(ε)-acetyl-l-lysine. Finally, tamoxifen resistant MCF7 and T47D cells, and a subgroup of human breast malignancies, displayed a MnSOD-K68-Ac signature.

Citation Format: David R. Gius, Yueming Zhu. The SIRT3-MnSOD-Ac axis in carcinogenesis and tumor resistance [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr SY20-02.

Efferocytosis Fuels Requirements of Fatty Acid Oxidation and the Electron Transport Chain to Polarize Macrophages for Tissue Repair

During wound injury, efferocytosis fills the macrophage with a metabolite load nearly equal to the phagocyte itself. A timely question pertains to how metabolic phagocytic signaling regulates the signature anti-inflammatory macrophage response. Here we report the metabolome of activated macrophages during efferocytosis to reveal an interleukin-10 (IL-10) cytokine escalation that was independent of glycolysis yet bolstered by apoptotic cell fatty acids and mitochondrial β-oxidation, the electron transport chain, and heightened coenzyme NAD+. Loss of IL-10 due to mitochondrial complex III defects was remarkably rescued by adding NAD+ precursors. This activated a SIRTUIN1 signaling cascade, largely independent of ATP, that culminated in activation of IL-10 transcription factor PBX1. Il-10 activation by the respiratory chain was also important in vivo, as efferocyte mitochondrial dysfunction led to cardiac rupture after myocardial injury. These findings highlight a new paradigm whereby macrophages leverage efferocytic metabolites and electron transport for anti-inflammatory reprogramming that culminates in organ repair.

Exosomes as a Drug Delivery System in Cancer Therapy: Potential and Challenges

Exosomes play a pivotal role in mediating intercellular communications and package delivery. They have recently been discovered to serve as diagnostic biomarkers as well as a possible drug delivery vehicle based on their nanometer size range and capability to transfer biological materials to recipient cells. Their unique biocompatibility, high stability, preferred tumor homing, and adjustable targeting efficiency can make exosomes an attractive and potentially effective tool of drug delivery in cancer therapy. While exosomes possess properties that make them uniquely suitable for delivery of bioactive molecules, there remains a to-be-filled gap between the current understanding about exosome biology and the ideal application scenarios. In this review, we summarize the characteristics enabling the potential of exosomes for drug delivery as well as the outstanding questions related to exosome composition and function, production and purification, bioengineering and targeting, uptake and biodistribution, efficacy and immune regulation, etc. Advanced technologies are demanded to visualize, characterize, and sort heterogeneous exosome populations. We are positive that the deeper and more comprehensive understanding of exosome biology as well as advanced nanotechnology will certainly accelerate its therapeutic applications.

Manganese superoxide dismutase (SOD2): is there a center in the universe of mitochondrial redox signaling?

It is becoming increasingly clear that mitochondria drive cellular functions and in vivo phenotypes by directing the production rate and abundance of metabolites that are proposed to function as signaling molecules (Chandel 2015; Selak et al. 2005; Etchegaray and Mostoslavsky 2016). Many of these metabolites are intermediates that make up cellular metabolism, part of which occur in mitochondria (i.e. the TCA and urea cycles), while others are produced "on demand" mainly in response to alterations in the microenvironment in order to participate in the activation of acute adaptive responses (Mills et al. 2016; Go et al. 2010). Reactive oxygen species (ROS) are well suited for the purpose of executing rapid and transient signaling due to their short lived nature (Bae et al. 2011). Hydrogen peroxide (H₂O₂), in particular, possesses important characteristics including diffusibility and faster reactivity with specific residues such as methionine, cysteine and selenocysteine (Bonini et al. 2014). Therefore, it is reasonable to propose that H₂O₂ functions as a relatively specific redox signaling molecule. Even though it is now established that mtH₂O₂ is indispensable, at least for hypoxic adaptation and energetic and/or metabolic homeostasis (Hamanaka et al. 2016; Guzy et al. 2005), the question of how H₂O₂ is produced and regulated in the mitochondria is only partially answered. In this review, some roles of this indispensable signaling molecule in driving cellular metabolism will be discussed. In addition, we will discuss how H₂O₂ formation in mitochondria depends on and is controlled by MnSOD. Finally, we will conclude this manuscript by highlighting why a better understanding of redox hubs in the mitochondria will likely lead to new and improved therapeutics of a number of diseases, including cancer.

G Protein-Coupled Bile Acid Receptor TGR5 Activation Inhibits Kidney Disease in Obesity and Diabetes

Obesity and diabetes mellitus are the leading causes of renal disease. In this study, we determined the regulation and role of the G protein-coupled bile acid receptor TGR5, previously shown to be regulated by high glucose and/or fatty acids, in obesity-related glomerulopathy (ORG) and diabetic nephropathy (DN). Treatment of diabetic db/db mice with the selective TGR5 agonist INT-777 decreased proteinuria, podocyte injury, mesangial expansion, fibrosis, and CD68 macrophage infiltration in the kidney. INT-777 also induced renal expression of master regulators of mitochondrial biogenesis, inhibitors of oxidative stress, and inducers of fatty acid β-oxidation, including sirtuin 1 (SIRT1), sirtuin 3 (SIRT3), and Nrf-1. Increased activity of SIRT3 was evidenced by normalization of the increased acetylation of mitochondrial superoxide dismutase 2 (SOD2) and isocitrate dehydrogenase 2 (IDH2) observed in untreated db/db mice. Accordingly, INT-777 decreased mitochondrial H2O2 generation and increased the activity of SOD2, which associated with decreased urinary levels of H2O2 and thiobarbituric acid reactive substances. Furthermore, INT-777 decreased renal lipid accumulation. INT-777 also prevented kidney disease in mice with diet-induced obesity. In human podocytes cultured with high glucose, INT-777 induced mitochondrial biogenesis, decreased oxidative stress, and increased fatty acid β-oxidation. Compared with normal kidney biopsy specimens, kidney specimens from patients with established ORG or DN expressed significantly less TGR5 mRNA, and levels inversely correlated with disease progression. Our results indicate that TGR5 activation induces mitochondrial biogenesis and prevents renal oxidative stress and lipid accumulation, establishing a role for TGR5 in inhibiting kidney disease in obesity and diabetes.

CDK1-Mediated SIRT3 Activation Enhances Mitochondrial Function and Tumor Radioresistance

Tumor adaptive resistance to therapeutic radiation remains a barrier for further improvement of local cancer control. SIRT3, a member of the sirtuin family of NAD(+)-dependent protein deacetylases in mitochondria, promotes metabolic homeostasis through regulation of mitochondrial protein deacetylation and plays a key role in prevention of cell aging. Here, we demonstrate that SIRT3 expression is induced in an array of radiation-treated human tumor cells and their corresponding xenograft tumors, including colon cancer HCT-116, glioblastoma U87, and breast cancer MDA-MB231 cells. SIRT3 transcriptional activation is due to SIRT3 promoter activation controlled by the stress transcription factor NF-κB. Posttranscriptionally, SIRT3 enzymatic activity is further enhanced via Thr150/Ser159 phosphorylation by cyclin B1-CDK1, which is also induced by radiation and relocated to mitochondria together with SIRT3. Cells expressing Thr150Ala/Ser159Ala-mutant SIRT3 show a reduction in mitochondrial protein lysine deacetylation, Δψm, MnSOD activity, and mitochondrial ATP generation. The clonogenicity of Thr150Ala/Ser159Ala-mutant transfectants is lower and significantly decreased under radiation. Tumors harboring Thr150Ala/Ser159Ala-mutant SIRT3 show inhibited growth and increased sensitivity to in vivo local irradiation. These results demonstrate that enhanced SIRT3 transcription and posttranslational modifications in mitochondria contribute to adaptive radioresistance in tumor cells. CDK1-mediated SIRT3 phosphorylation is a potential effective target to sensitize tumor cells to radiotherapy.

Antioxidant inhibition of steady-state reactive oxygen species and cell growth in neuroblastoma

BACKGROUND

Reactive oxygen species (ROS) contribute to adult tumorigenesis; however, their roles in pediatric solid tumors are unknown. Here, we sought to define the steady-state ROS levels in neuroblastoma and to examine whether aggressive cellular behavior, which may predict treatment failure, is regulated by ROS.

METHODS

Neuroblastoma sections were assessed for 4-hydroxynonenal (4-HNE), a marker of intracellular lipid peroxidation and a byproduct of increased levels of ROS. Human neuroblastoma cell lines, MYCN-amplified BE(2)-C and MYCN-nonamplified SK-N-SH, were examined in our study. Superoxide and hydroperoxide oxidation products were detected by staining for dihydroethidium (DHE) and 5, 6-carboxy-2', 7'-dichlorodihydrofluorescein diacetate (CDCFH2), using the oxidation-insensitive analog CDCF as a negative control. Cells were treated with N-acetylcysteine (NAC; 10 mmol/L) daily for 5 days and analyzed.

RESULTS

Greater expression of 4-HNE was observed in undifferentiated tumor sections as compared with the more differentiated tumors. Interestingly, increased levels of ROS were detected in MYCN-amplified BE(2)-C cells. Moreover, gastrin-releasing peptide receptor-induced ROS production stimulated upregulation of the hypoxia inducible factor (HIF)-1α/vascular endothelial growth factor (VEGF) pathway and an increase in cell growth. Antioxidant NAC decreased HIF-1α/VEGF expression and inhibited BE(2)-C cell growth.

CONCLUSION

We report a novel observation that shifting the redox balance toward greater ROS levels results in a more aggressive neuroblastoma phenotype. Our data suggest that ROS play a critical role in refractory neuroblastoma.

Site-specific processing of Ras and Rap1 Switch I by a MARTX toxin effector domain

Ras (Rat sarcoma) protein is a central regulator of cell growth and proliferation. Mutations in the RAS gene are known to occur in human cancers and have been shown to contribute to carcinogenesis. In this study, we show that the multifunctional-autoprocessing repeats-in-toxin (MARTX) toxin-effector domain DUF5_Vv from Vibrio vulnificus to be a site-specific endopeptidase that cleaves within the Switch 1 region of Ras and Rap1. DUF5_Vv processing of Ras, which occurs both biochemically and in mammalian cell culture, inactivates ERK1/2, thereby inhibiting cell proliferation. The ability to cleave Ras and Rap1 is shared by DUF5_Vv homologues found in other bacteria. In addition, DUF5_Vv can cleave all Ras isoforms and KRas with mutations commonly implicated in malignancies. Therefore, we speculate that this new family of Ras/Rap1-specific endopeptidases (RRSPs) has potential to inactivate both wild-type and mutant Ras proteins expressed in malignancies.

V. vulnificus, a bacteria that cause life-threatening septicaemia following wound infections or tainted food consumption, utilizes MARTX toxins for toxic effector delivery. Here the authors show that the MARTX virulence factor DUF5 targets the cellular MAP kinase pathway as a Ras and Rap1 site-specific protease.

Transduced p16INK4a peptides inhibit hypophosphorylation of the retinoblastoma protein and cell cycle progression prior to activation of Cdk2 complexes in late G1

Progression of cells through the G1 phase of the cell cycle requires cyclin D:Cdk4/6 and cyclin E:Cdk2 complexes; however, the duration and ordering of these complexes remain unclear. To address this, we synthesized a peptidyl mimetic of the Cdk4/6 inhibitor, p16INK4a that contained an NH2-terminal TAT protein transduction domain. Transduction of TAT-p16 wild-type peptides into cells resulted in the loss of active, hypophosphorylated pRb and elicited an early G1 cell cycle arrest, provided cyclin E:Cdk2 complexes were inactive. We conclude that cyclin D:Cdk4/6 activity is required for early G1 phase cell cycle progression up to, but not beyond, activation of cyclin E:Cdk2 complexes at the restriction point and is thus nonredundant with cyclin E:Cdk2 in late G1.

Hypo-phosphorylation of the retinoblastoma protein (pRb) by cyclin D:Cdk4/6 complexes results in active pRb

In cycling cells, the retinoblastoma protein (pRb) is un- and/or hypo-phosphorylated in early G1 and becomes hyper-phosphorylated in late G1. The role of hypo-phosphorylation and identity of the relevant kinase(s) remains unknown. We show here that hypo-phosphorylated pRb associates with E2F in vivo and is therefore active. Increasing the intracellular concentration of the Cdk4/6 specific inhibitor p15(INK4b) by transforming growth factor beta treatment of keratinocytes results in G1 arrest and loss of hypo-phosphorylated pRb with an increase in unphosphorylated pRb. Conversely, p15(INK4b)-independent transforming growth factor beta-mediated G1 arrest of hepatocellular carcinoma cells results in loss of Cdk2 kinase activity with continued Cdk6 kinase activity and pRb remains only hypo-phosphorylated. Introduction of the Cdk4/6 inhibitor p16(INK4a) protein into cells by fusion to a protein transduction domain also prevents pRb hypo-phosphorylation with an increase in unphosphorylated pRb. We conclude that cyclin D:Cdk4/6 complexes hypo-phosphorylate pRb in early G1 allowing continued E2F binding.