Spermidine/Spermine N1-Acetyltransferase 2 (SSAT2) functions as a coactivator for NF-kappaB and cooperates with CBP and P/CAF to enhance NF-kappaB-dependent transcription

HDAC10 inhibition represses melanoma cell growth and BRAF inhibitor resistance via upregulating SPARC expression

Secreted protein acidic and rich in cysteine (SPARC), a conserved secreted glycoprotein, plays crucial roles in regulating various biological processes. SPARC is highly expressed and has profound implications in several cancer types, including melanoma. Understanding the mechanisms that govern SPARC expression in cancers has the potential to lead to improved cancer diagnosis, prognosis, treatment strategies, and patient outcomes. Here, we demonstrate that histone deacetylase 10 (HDAC10) is a key regulator of SPARC expression in melanoma cells. Depletion or inhibition of HDAC10 upregulates SPARC expression, whereas overexpression of HDAC10 downregulates it. Mechanistically, HDAC10 coordinates with histone acetyltransferase p300 to modulate the state of acetylation of histone H3 at lysine 27 (H3K27ac) at SPARC regulatory elements and the recruitment of bromodomain-containing protein 4 (BRD4) to these regions, thereby fine-tuning SPARC transcription. HDAC10 depletion and resultant SPARC upregulation repress melanoma cell growth primarily by activating AMPK signaling and inducing autophagy. Moreover, SPARC upregulation due to HDAC10 depletion partly accounts for the resensitization of resistant cells to a BRAF inhibitor. Our work reveals the role of HDAC10 in gene regulation through indirect histone modification and suggests a potential therapeutic strategy for melanoma or other cancers by targeting HDAC10 and SPARC.

HDAC10 blockade upregulates SPARC expression thereby repressing melanoma cell growth and BRAF inhibitor resistance

Secreted Protein Acidic and Rich in Cysteine (SPARC), a highly conserved secreted glycoprotein, is crucial for various bioprocesses. Here we demonstrate that histone deacetylase 10 (HDAC10) is a key regulator of SPARC expression. HDAC10 depletion or inhibition upregulates, while overexpression of HDAC10 downregulates, SPARC expression. Mechanistically, HDAC10 coordinates with histone acetyltransferase p300 to modulate the acetylation state of histone H3 lysine 27 (H3K27ac) at SPARC regulatory elements and the recruitment of bromodomain-containing protein 4 (BRD4) to these regions, thereby tuning SPARC transcription. HDAC10 depletion and resultant SPARC upregulation repress melanoma cell growth, primarily by induction of autophagy via activation of AMPK signaling. Moreover, SPARC upregulation due to HDAC10 depletion partly accounts for the resensitivity of resistant cells to a BRAF inhibitor. Our work reveals the role of HDAC10 in gene regulation through epigenetic modification and suggests a potential therapeutic strategy for melanoma or other cancers by targeting HDAC10 and SPARC.

Highlights

HDAC10 is the primary HDAC member that tightly controls SPARC expression. HDAC10 coordinates with p300 in modulating the H3K27ac state at SPARC regulatory elements and the recruitment of BRD4 to these regions. HDAC10 depletion and resultant SPARC upregulation inhibit melanoma cell growth by inducing autophagy via activation of AMPK signaling.SPARC upregulation as a result of HDAC10 depletion resensitizes resistant cells to BRAF inhibitors.

Identification of a ferroptosis-related gene signature (FRGS) for predicting clinical outcome in lung adenocarcinoma

Background

Lung adenocarcinoma (LUAD) is the most common pathological subtype of lung cancer. Ferroptosis, an oxidative, iron-dependent form of necrotic cell death, is highly associated with tumorigenesis and cancer progression. However, the prognostic value of ferroptosis progress in LUAD was still rarely be investigated.

Methods

Herein, we collected three mRNA expression profiles and 85 ferroptosis-related genes from public databases. The “limma” package was used to identify ferroptosis-related differentially expressed genes (DEGs). Univariate Cox regression analysis and LASSO regression analysis were applied to screen and develop a ferroptosis-related gene signature (FRGS) and a formula to calculate the risk score. Multivariate Cox regression analysis was implemented to determine independent prognostic predictors of overall survival (OS). The area under the receiver operating characteristic curve (AUC) and calibration plot were used to evaluate the predictive accuracy of the FRGS and nomogram.

Results

We developed a FRGS with five genes (CYBB, CISD1, FADD, SAT2, VDAC2). The AUC of the FRGS in TCGA cohort was 0.777 at 1-year, 0.721 at 3-year and 0.725 at 5-year, significantly superior to the AUC of TNM stage (1-year: 0.701, 3-year: 0.691, 5-year: 0.686). A similar phenomenon was observed in GEO cohort 1 and 2. Multivariate Cox regression analysis indicted TNM stage and risk score were independent prognostic predictors. Finally, we built a nomogram with TNM stage and FRGS, the AUCs of which markedly higher than that of FRGS or TNM stage alone.

Conclusion

We constructed a prognostic FRGS with five ferroptosis-related genes and a nomogram for predicting the 1-, 3- and 5-year survival rate of LUAD patients, which may provide a new understanding of the prognostic value of ferroptosis progress in LUAD and will benefit prognosis assessment of LUAD patients.

The minimal gap-junction network among melanophores and xanthophores required for stripe-pattern formation in zebrafish

Connexin39.4 (Cx39.4) and Connexin41.8 (Cx41.8), two gap-junction proteins expressed in both melanophores and xanthophores, are critical for the intercellular communication among pigment cells that is necessary for generating the stripe pigment pattern of zebrafish. We previously characterized the gap-junction properties of Cx39.4 and Cx41.8, but how these proteins contribute to stripe formation remains unclear; this is because distinct types of connexins potentially form heteromeric gap junctions, which precludes accurate elucidation of individual connexin functions in vivo. Here, by arranging Cx39.4 and Cx41.8 expression in pigment cells, we identified the simplest gap-junction network required for stripe generation: Cx39.4 expression in melanophores is required but expression in xanthophores is not necessary for stripe patterning, whereas Cx41.8 expression in xanthophores is sufficient for the patterning, and Cx41.8 expression in melanophores might stabilize the stripes. Moreover, patch-clamp recordings revealed that Cx39.4 gap junctions exhibit spermidine-dependent rectification property. Our results suggest that Cx39.4 facilitates the critical cell-cell interactions between melanophores and xanthophores that mediate a unidirectional activation-signal transfer from xanthophores to melanophores, which is essential for melanophore survival.

Genetic Drivers of Pancreatic Islet Function

The majority of gene loci that have been associated with type 2 diabetes play a role in pancreatic islet function. To evaluate the role of islet gene expression in the etiology of diabetes, we sensitized a genetically diverse mouse population with a Western diet high in fat (45% kcal) and sucrose (34%) and carried out genome-wide association mapping of diabetes-related phenotypes. We quantified mRNA abundance in the islets and identified 18,820 expression QTL. We applied mediation analysis to identify candidate causal driver genes at loci that affect the abundance of numerous transcripts. These include two genes previously associated with monogenic diabetes (PDX1 and HNF4A), as well as three genes with nominal association with diabetes-related traits in humans (FAM83E, IL6ST, and SAT2). We grouped transcripts into gene modules and mapped regulatory loci for modules enriched with transcripts specific for α-cells, and another specific for δ-cells. However, no single module enriched for β-cell-specific transcripts, suggesting heterogeneity of gene expression patterns within the β-cell population. A module enriched in transcripts associated with branched-chain amino acid metabolism was the most strongly correlated with physiological traits that reflect insulin resistance. Although the mice in this study were not overtly diabetic, the analysis of pancreatic islet gene expression under dietary-induced stress enabled us to identify correlated variation in groups of genes that are functionally linked to diabetes-associated physiological traits. Our analysis suggests an expected degree of concordance between diabetes-associated loci in the mouse and those found in human populations, and demonstrates how the mouse can provide evidence to support nominal associations found in human genome-wide association mapping.

Complex N-acetylation of triethylenetetramine

Triethylenetetramine (TETA) is an efficient copper chelator that has versatile clinical potential. We have recently shown that spermidine/spermine-N(1)-acetyltransferase (SSAT1), the key polyamine catabolic enzyme, acetylates TETA in vitro. Here, we studied the metabolism of TETA in three different mouse lines: syngenic, SSAT1-overexpressing, and SSAT1-deficient (SSAT1-KO) mice. The mice were sacrificed at 1, 2, or 4 h after TETA injection (300 mg/kg i.p.). We found only N(1)-acetyltriethylenetetramine (N(1)AcTETA) and/or TETA in the liver, kidney, and plasma samples. As expected, SSAT1-overexpressing mice acetylated TETA at an accelerated rate compared with syngenic and SSAT1-KO mice. It is noteworthy that SSAT1-KO mice metabolized TETA as syngenic mice did, probably by thialysine acetyltransferase, which had a K(m) value of 2.5 ± 0.3 mM and a k(cat) value of 1.3 s(-1) for TETA when tested in vitro with the human recombinant enzyme. Thus, the present results suggest that there are at least two N-acetylases potentially metabolizing TETA. However, their physiological significance for TETA acetylation requires further studies. Furthermore, we detected chemical intramolecular N-acetyl migration from the N(1) to N(3) position of N(1)AcTETA and N(1),N(8)-diacetyltriethylenetetramine in an acidified high-performance liquid chromatography sample matrix. The complex metabolism of TETA together with the intramolecular N-acetyl migration may explain the huge individual variations in the acetylation rate of TETA reported earlier.

Antitumor agent PX-12 inhibits HIF-1α protein levels through an Nrf2/PMF-1-mediated increase in spermidine/spermine acetyl transferase

PURPOSE

Thioredoxin-1 (Trx-1) redox signaling regulates multiple aspects of cell growth and survival, and elevated tumor levels of Trx-1 have been associated with decreased patient survival. PX-12, an inhibitor of Trx-1 currently in clinical development, has been found to decrease tumor levels of the HIF-1α transcription factor. SSAT1 has been reported to bind to HIF-1α and RACK1, resulting in oxygen-independent HIF-1 ubiquitination and degradation. SSAT2, a related protein, stabilizes the interaction of the VHL protein and elongin C with HIF-1 leading to oxygen-dependent HIF-1α ubiquitination and degradation. We investigated the effects of PX-12 and Trx-1 on SSAT1, SSAT2, and inhibition of HIF-1α.

METHODS

A panel of cell lines was treated with PX-12 to investigate its effects on SSAT1 and SSAT2 expression, and on HIF-1α protein levels. We also evaluated the regulation of SSAT1 through the Nrf2 and PMF-1, two trans-acting transcription factors.

RESULTS

We found that PX-12 increased nuclear Nrf2 activity and antioxidant response element binding. PX-12 also increased the expression of SSAT1 but not SSAT2 in a PMF-1-dependent manner that was independent of Trx-1. Inhibition of Nrf2 or PMF-1 prevented the increase in SSAT1 caused by PX-12.

CONCLUSIONS

The results show that PX-12, acting independently of Trx-1, increases nuclear Nrf2, which interacts with PMF-1 to increase the expression of SSAT1. The degradation of HIF-1α that results from binding with SSAT1 may explain the decrease in HIF-1α caused by PX-12 and could contribute to the antitumor activity of PX-12.

Spermidine/spermine-N(1)-acetyltransferase: a key metabolic regulator

Spermidine/spermine-N(1)-acetyltransferase (SSAT) regulates cellular polyamine content. Its acetylated products are either excreted from the cell or oxidized by acetylpolyamine oxidase. Since polyamines play critical roles in normal and neoplastic growth and in ion channel regulation, SSAT is a key enzyme in these processes. SSAT is very highly regulated. Its content is adjusted in response to alterations in polyamine content to maintain polyamine homeostasis. Certain polyamine analogs can mimic the induction of SSAT and cause a loss of normal polyamines. This may have utility in cancer chemotherapy. SSAT activity is also induced via a variety of other stimuli, including toxins, hormones, cytokines, nonsteroidal anti-inflammatory agents, natural products, and stress pathways, and by ischemia-reperfusion injury. These increases are initiated by alterations in Sat1 gene transcription reinforced by alterations at the other regulatory steps, including protein turnover, mRNA processing, and translation. Transgenic manipulation of SSAT activity has revealed that SSAT activity links polyamine metabolism to lipid and carbohydrate metabolism by means of alterations in the content of acetyl-CoA and ATP. A high level of SSAT stimulates flux through the polyamine biosynthetic pathway, since biosynthetic enzymes are induced in response to the fall in polyamines. This sets up a futile cycle in which ATP is used to generate S-adenosylmethionine for polyamine biosynthesis and acetyl-CoA is consumed in the acetylation reaction. A variety of other effects of increased SSAT activity include death of pancreatic cells, blockage of regenerative tissue growth, behavioral changes, keratosis follicularis spinulosa decalvans, and hair loss. These are very likely due to changes in polyamine and putrescine levels, although increased oxidative stress via the oxidation of acetylated polyamines may also contribute. Recently, it was found that the SSAT protein and/or a related protein, thialysine acetyltransferase, interacts with a number of other important proteins, including the hypoxia-inducible factor-1 alpha-subunit, the p65 subunit of NF-kappaB, and alpha9beta1-integrin, altering the function of these proteins. It is not yet clear whether this functional alteration involves protein acetylation, local polyamine concentration changes, or other effects. It has been suggested that SSAT may also be a useful target in diseases other than cancer, but the wide-ranging physiological and pathophysiological effects of altered SSAT expression will require very careful limitation of such strategies to the relevant cells to avoid toxic effects.

Spermidine/Spermine-N1-Acetyltransferase 2 Is an Essential Component of the Ubiquitin Ligase Complex That Regulates Hypoxia-inducible Factor 1α

Hypoxia-inducible factor 1 (HIF-1) is a heterodimeric transcription factor that functions as a master regulator of oxygen homeostasis. The HIF-1alpha subunit is subjected to O(2)-dependent prolyl hydroxylation leading to ubiquitination by the von Hippel-Lindau protein (VHL)-Elongin C ubiquitin-ligase complex and degradation by the 26 S proteasome. In this study, we demonstrate that spermidine/spermine-N(1)-acetyltransferase (SSAT) 2 plays an essential role in this process. SSAT2 binds to HIF-1alpha, VHL, and Elongin C and promotes ubiquitination of hydroxylated HIF-1alpha by stabilizing the interaction of VHL and Elongin C. Multivalent interactions by SSAT2 provide a mechanism to ensure efficient complex formation, which is necessary for the extremely rapid ubiquitination and degradation of HIF-1alpha that is observed in oxygenated cells.