Regulation of ST6GAL1 sialyltransferase expression in cancer cells

The ST6GAL1 sialyltransferase, which adds α2-6 linked sialic acids to N-glycosylated proteins, is overexpressed in a wide range of human malignancies. Recent studies have established the importance of ST6GAL1 in promoting tumor cell behaviors such as invasion, resistance to cell stress and chemoresistance. Furthermore, ST6GAL1 activity has been implicated in imparting cancer stem cell characteristics. However, despite the burgeoning interest in the role of ST6GAL1 in the phenotypic features of tumor cells, insufficient attention has been paid to the molecular mechanisms responsible for ST6GAL1 upregulation during neoplastic transformation. Evidence suggests that these mechanisms are multifactorial, encompassing genetic, epigenetic, transcriptional and posttranslational regulation. The purpose of this review is to summarize current knowledge regarding the molecular events that drive enriched ST6GAL1 expression in cancer cells.

Abstract TMIM-071: SOX2 DRIVES ST6GAL-I EXPRESSION AND ACTIVITY TO PROMOTE A CSC PHENOTYPE IN OVARIAN CANCER

This study elucidates ST6Gal-I, a sialyltransferase, as a functional driver of a cancer stem cell (CSC) phenotype regulated by the stem cell transcription factor Sox2. ST6Gal-I is upregulated in 98% of ovarian cancers (OC) and its high expression correlates with reduced overall and progression free survival in high-grade serous ovarian carcinoma. ST6Gal-I functions to add an α2-6 sialic acid, a large, negatively charged sugar, to N-glycosylated proteins bound for the cell surface. Normal differentiated epithelia have very low expression of ST6Gal-I, however, expression is turned on in stem cell compartments and transformed tissues. Furthermore, our work has shown that ST6Gal-I plays a causal role in conferring hallmark CSC properties including greater tumor-initiating capabilities, and resistance to tumor-associated stressors like chemotherapies, serum deprivation, and hypoxia. However, despite this stark upregulation and functional importance of ST6Gal-I in cancer, very little work has been done to identify the transcriptional drivers of ST6Gal-I expression. We identified that Sox2 and ST6Gal-I are both located on one of the most commonly enriched amplicons in human cancer, amplicon 3q26. We then examined the TCGA databases and found that these two genes are co-amplified in 48/73 cancer cohorts, including ovarian cancer. Furthermore, in those 48 cohorts with co-amplification, there is also co-occurrence per individual patient samples. In addition to being genetically co-amplified, we identified Sox2 response elements in the ST6Gal-I promoter and performed a chromatin immunoprecipitation assay (ChIP) to confirm Sox2 binding. These data implicated Sox2, a key stem-associated transcription factor in CSCs, as a transcriptional activator of ST6Gal-I expression. To confirm this hypothesis, Sox2 was overexpressed or knocked-down in ovarian cancer cells, and it was consistently found that high expression of Sox2 directly induces expression of ST6Gal-I mRNA and protein. Finally, given the relationship between Sox2 and ST6Gal-I in the promotion of a CSC phenotype, we profiled the reprogramming of the cell population into a more stem-like state by quantifying expression of stemness markers Oct4 and Nanog. Overexpression or knockdown of Sox2 resulted in up- or down- regulation of CSC markers Oct4 and Nanog, respectively. Importantly, forced ST6Gal-I knockdown in cells with high Sox2 inhibited the Sox2-induced ST6Gal-I upregulation and subsequently prevented the enhanced expression of Oct4 and Nanog. These data suggest that Sox2 requires ST6Gal-I expression to promote a CSC phenotype. Collectively, our results highlight a novel, glycosylation-dependent mechanism that drives a CSC phenotype.

Citation Format: Kaitlyn A. Dorsett and Susan L. Bellis. SOX2 DRIVES ST6GAL-I EXPRESSION AND ACTIVITY TO PROMOTE A CSC PHENOTYPE IN OVARIAN CANCER [abstract]. In: Proceedings of the 12th Biennial Ovarian Cancer Research Symposium; Sep 13-15, 2018; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2019;25(22 Suppl):Abstract nr TMIM-071.

Sox2 promotes expression of the ST6Gal-I glycosyltransferase in ovarian cancer cells

Background

The ST6Gal-I glycosyltransferase, which adds α2–6-linked sialic acids to N-glycosylated proteins is upregulated in a wide range of malignancies including ovarian cancer. Prior studies have shown that ST6Gal-I-mediated sialylation of select surface receptors remodels intracellular signaling to impart cancer stem cell (CSC) characteristics. However, the mechanisms that contribute to ST6Gal-I expression in stem-like cancer cells are poorly understood.

Results

Herein, we identify the master stem cell transcription factor, Sox2, as a novel regulator of ST6Gal-I expression. Interestingly, SOX2 and ST6GAL1 are located within the same tumor-associated amplicon, 3q26, and these two genes exhibit coordinate gains in copy number across multiple cancers including ~ 25% of ovarian serious adenocarcinomas. In conjunction with genetic co-amplification, our studies suggest that Sox2 directly binds the ST6GAL1 promoter to drive transcription. ST6Gal-I expression is directed by at least four distinct promoters, and we identified the P3 promoter as the predominant promoter utilized by ovarian cancer cells. Chromatin Immunoprecipitation (ChIP) assays revealed that Sox2 binds regions proximal to the P3 promoter. To confirm that Sox2 regulates ST6Gal-I expression, Sox2 was either overexpressed or knocked-down in various ovarian cancer cell lines. Sox2 overexpression induced an increase in ST6Gal-I mRNA and protein, as well as surface α2–6 sialylation, whereas Sox2 knock-down suppressed levels of ST6Gal-I mRNA, protein and surface α2–6 sialylation.

Conclusions

These data suggest a process whereby SOX2 and ST6GAL1 are coordinately amplified in cancer cells, with the Sox2 protein then binding the ST6GAL1 promoter to further augment ST6Gal-I expression. Our collective results provide new insight into mechanisms that upregulate ST6Gal-I expression in ovarian cancer cells, and also point to the possibility that some of the CSC characteristics commonly attributed to Sox2 may, in part, be mediated through the sialyltransferase activity of ST6Gal-I.

Transfer of Functional Cargo in Exomeres

Exomeres are a recently discovered type of extracellular nanoparticle with no known biological function. Herein, we describe a simple ultracentrifugation-based method for separation of exomeres from exosomes. Exomeres are enriched in Argonaute 1-3 and amyloid precursor protein. We identify distinct functions of exomeres mediated by two of their cargo, the β-galactoside α2,6-sialyltransferase 1 (ST6Gal-I) that α2,6- sialylates N-glycans, and the EGFR ligand, amphiregulin (AREG). Functional ST6Gal-I in exomeres can be transferred to cells, resulting in hypersialylation of recipient cell-surface proteins including β1-integrin. AREG-containing exomeres elicit prolonged EGFR and downstream signaling in recipient cells, modulate EGFR trafficking in normal intestinal organoids, and dramatically enhance the growth of colonic tumor organoids. This study provides a simplified method of exomere isolation and demonstrates that exomeres contain and can transfer functional cargo. These findings underscore the heterogeneity of nanoparticles and should accelerate advances in determining the composition and biological functions of exomeres.

Abstract 1129: Sox2 drives ST6Gal-I expression and activity to promote a CSC phenotype in ovarian cancer

This study elucidates ST6Gal-I, a sialyltransferase, as a functional driver of a cancer stem cell (CSC) phenotype regulated by the stem cell transcription factor Sox2. ST6Gal-I is upregulated in 98% of ovarian cancers (OC); it functions to add an α2-6 sialic acid, a large, negatively charged sugar, to N-glycosylated proteins bound for the cell surface. Normal differentiated epithelia have very low expression of ST6Gal-I, however, expression is turned on in stem cell compartments and transformed tissues. Furthermore, our work has shown that ST6Gal-I plays a causal role in conferring hallmark CSC properties including greater tumor-initiating capabilities, promotion of recurrence, and resistance to tumor-associated stressors like chemotherapies, serum deprivation, and hypoxia. Even given this stark upregulation and apparent functional import of ST6Gal-I in cancers, very little work has been done to identify the transcriptional driver of ST6Gal-I expression. We identified that Sox2 and ST6Gal-I are both part of one of the most commonly enriched amplicons in human cancer, amplicon 3q26. We then examined the TCGA database and found that these two genes are co-amplified in 48/73 cancer cohorts, including ovarian cancer. In addition to being genetically co-amplified, Sox2 has been shown to bind to the promoter of ST6Gal-I, though no further analyses have been performed to assess a functional link. Herein we address a novel, glycosylation-dependent mechanism that drives a CSC phenotype. We hypothesize that Sox2, a key stem-associated transcription factor in CSCs, directly induces expression of ST6Gal-I. In our ovarian cancer cell model systems, we forced overexpression or knockdown of Sox2 and found that Sox2 expression directly correlated with ST6Gal-I expression. Overexpression or knockdown of Sox2 resulted in up- or down- regulation of CSC markers Oct4 and Nanog respectively. We then forced ST6Gal-I knockdown to inhibit the Sox2-induced ST6Gal-I upregulation and found that this prevented the enhanced expression of Oct4 and Nanog. Here we show that Sox2 requires ST6Gal-I expression to promote a CSC phenotype.

Citation Format: Kaitlyn A. Dorsett, Susan L. Bellis. Sox2 drives ST6Gal-I expression and activity to promote a CSC phenotype in ovarian cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1129.

Abstract 2435: ST6Gal-I sialylation potentiates hypoxia adaptation by enhancing HIF-1a signaling

Aberrant surface glycosylation is a well-known hallmark of a tumor cell, however the functional role of glycans in cancer cell biology remains unclear. Tumor cells frequently display an increase in surface α2-6 sialylation, a modification added to N-glycosylated proteins by the ST6Gal-I sialyltransferase. ST6Gal-I sialylates a select cohort of surface receptors, which correspondingly modulates receptor-induced intracellular signaling cascades. Emerging evidence suggests that ST6Gal-I-mediated sialylation promotes the survival of tumor cells exposed to a variety of cell stressors. In the current study we identify a new function for ST6Gal-I in protecting against hypoxic stress. In response to low oxygen tension, the hypoxia inducible factor, HIF-1α, becomes stabilized in tumor cells. In turn, HIF-1α stimulates the transcription of genes important for cell survival. To interrogate a role for ST6Gal-I in hypoxic response, we evaluated HIF-1α accumulation in ovarian and pancreatic cancer cells with ST6Gal-I overexpression or knockdown. We find that ST6Gal-I activity augments HIF-1α accumulation in cells treated with chemical hypoxia mimetics (DFO and DMOG), or alternatively grown in a hypoxic environment. Furthermore, hypoxic cells with high ST6Gal-I expression have increased mRNA levels of HIF-1α transcriptional targets including the glucose transporters, GLUT1 GLUT3, and the glycolytic enzyme, PDHK1. Interestingly, cells with high ST6Gal-I expression also have an increased pool of HIF-1α mRNA, suggesting that ST6Gal-I may influence the biosynthesis of HIF-1α. Finally, cells grown in hypoxia for several weeks display an enrichment in ST6Gal-I expression, consistent with the concept that ST6Gal-I acts as a pro-survival factor. Taken together, these findings highlight a novel, glycosylation-dependent mechanism that facilitates tumor cell adaptation to a hypoxic milieu.

Citation Format: Robert B. Jones, Kaitlyn A. Dorsett, Anita B. Hjelmeland, Susan L. Bellis. ST6Gal-I sialylation potentiates hypoxia adaptation by enhancing HIF-1a signaling [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2435.

The ST6Gal-I sialyltransferase protects tumor cells against hypoxia by enhancing HIF-1α signaling

Aberrant cell surface glycosylation is prevalent in tumor cells, and there is ample evidence that glycans have functional roles in carcinogenesis. Nonetheless, many molecular details remain unclear. Tumor cells frequently exhibit increased α2-6 sialylation on N-glycans, a modification that is added by the ST6Gal-I sialyltransferase, and emerging evidence suggests that ST6Gal-I-mediated sialylation promotes the survival of tumor cells exposed to various cell stressors. Here we report that ST6Gal-I protects cancer cells from hypoxic stress. It is well known that hypoxia-inducible factor 1α (HIF-1α) is stabilized in hypoxic cells, and, in turn, HIF-1α directs the transcription of genes important for cell survival. To investigate a putative role for ST6Gal-I in the hypoxic response, we examined HIF-1α accumulation in ovarian and pancreatic cancer cells in ST6Gal-I overexpression or knockdown experiments. We found that ST6Gal-I activity augmented HIF-1α accumulation in cells grown in a hypoxic environment or treated with two chemical hypoxia mimetics, deferoxamine and dimethyloxalylglycine. Correspondingly, hypoxic cells with high ST6Gal-I expression had increased mRNA levels of HIF-1α transcriptional targets, including the glucose transporter genes GLUT1 and GLUT3 and the glycolytic enzyme gene PDHK1 Interestingly, high ST6Gal-I-expressing cells also had an increased pool of HIF-1α mRNA, suggesting that ST6Gal-I may influence HIF-1α expression. Finally, cells grown in hypoxia for several weeks displayed enriched ST6Gal-I expression, consistent with a pro-survival function. Taken together, these findings unravel a glycosylation-dependent mechanism that facilitates tumor cell adaptation to a hypoxic milieu.

ST6Gal-I sialyltransferase promotes chemoresistance in pancreatic ductal adenocarcinoma by abrogating gemcitabine-mediated DNA damage

Pancreatic ductal adenocarcinoma (PDAC) is an aggressive malignancy with a poor prognosis. Gemcitabine, as a single agent or in combination therapy, remains the frontline chemotherapy despite its limited efficacy due to de novo or acquired chemoresistance. There is an acute need to decipher mechanisms underlying chemoresistance and identify new targets to improve patient outcomes. Here, we report a novel role for the ST6Gal-I sialyltransferase in gemcitabine resistance. Utilizing MiaPaCa-2 and BxPC-3 PDAC cells, we found that knockdown (KD) of ST6Gal-I expression, as well as removal of surface α2-6 sialic acids by neuraminidase, enhances gemcitabine-mediated cell death assessed via clonogenic assays and cleaved caspase 3 expression. Additionally, KD of ST6Gal-I potentiates gemcitabine-induced DNA damage as measured by comet assays and quantification of γH2AX foci. ST6Gal-I KD also alters mRNA expression of key gemcitabine metabolic genes, RRM1, RRM2, hENT1, and DCK, leading to an increased gemcitabine sensitivity ratio, an indicator of gemcitabine toxicity. Gemcitabine-resistant MiaPaCa-2 cells display higher ST6Gal-I levels than treatment-naïve cells along with a reduced gemcitabine sensitivity ratio, suggesting that chronic chemotherapy selects for clonal variants with more abundant ST6Gal-I. Finally, we examined Suit2 PDAC cells and Suit2 derivatives with enhanced metastatic potential. Intriguingly, three metastatic and chemoresistant subclones, S2-CP9, S2-LM7AA, and S2-013, exhibit up-regulated ST6Gal-I relative to parental Suit2 cells. ST6Gal-I KD in S2-013 cells increases gemcitabine-mediated DNA damage, indicating that suppressing ST6Gal-I activity sensitizes inherently resistant cells to gemcitabine. Together, these findings place ST6Gal-I as a critical player in imparting gemcitabine resistance and as a potential target to restore PDAC chemoresponse.

Ypt4 and lvs1 regulate vacuolar size and function in Schizosaccharomyces pombe

The yeast vacuole plays key roles in cellular stress responses. Here, we show that deletion of lvs1, the fission yeast homolog of the Chediak-Higashi Syndrome CHS1/LYST gene, increases vacuolar size, similar to deletion of the Rab4 homolog ypt4. Overexpression of lvs1-YFP rescued vacuolar size in ypt4Δ cells, but ypt4-YFP did not rescue lvs1Δ, suggesting that lvs1 may act downstream of ypt4. Vacuoles were capable of hypotonic shock-induced fusion and recovery in both ypt4Δ and lvs1Δ cells, although recovery may be slightly delayed in ypt4Δ. Endocytic and secretory trafficking were not affected, but ypt4Δ and lvs1Δ strains were sensitive to neutral pH and CaCl₂, consistent with vacuolar dysfunction. In addition to changes in vacuolar size, deletion of ypt4 also dramatically increased cell size, similar to tor1 mutants. These results implicate ypt4 and lvs1 in maintenance of vacuolar size and suggest that ypt4 may link vacuolar homeostasis to cell cycle progression.

The Glycosyltransferase ST6Gal-I Protects Tumor Cells against Serum Growth Factor Withdrawal by Enhancing Survival Signaling and Proliferative Potential

A hallmark of cancer cells is the ability to survive and proliferate when challenged with stressors such as growth factor insufficiency. In this study, we report a novel glycosylation-dependent mechanism that protects tumor cells from serum growth factor withdrawal. Our results suggest that the β-galactoside α-2,6-sialyltransferase 1 (ST6Gal-I) sialyltransferase, which is up-regulated in numerous cancers, promotes the survival of serum-starved cells. Using ovarian and pancreatic cancer cell models with ST6Gal-I overexpression or knockdown, we find that serum-starved cells with high ST6Gal-I levels exhibit increased activation of prosurvival signaling molecules, including pAkt, p-p70S6K, and pNFκB. Correspondingly, ST6Gal-I activity augments the expression of tumor-promoting pNFκB transcriptional targets such as IL-6, IL-8, and the apoptosis inhibitor cIAP2. ST6Gal-I also potentiates expression of the cell cycle regulator cyclin D2, leading to increased phosphorylation and inactivation of the cell cycle inhibitor pRb. Consistent with these results, serum-starved cells with high ST6Gal-I expression maintain a greater number of S phase cells compared with low ST6Gal-I expressors, reflecting enhanced proliferation. Finally, selective enrichment in clonal variants with high ST6Gal-I expression is observed upon prolonged serum deprivation, supporting the concept that ST6Gal-I confers a survival advantage. Collectively, these results implicate a functional role for ST6Gal-I in fostering tumor cell survival within the serum-depleted tumor microenvironment.

Abstract 3336: ST6Gal-I glycosyltransferase promotes an undifferentiated cell phenotype and enhances c-kit signaling

Acute myelogenous leukemia (AML) is characterized by an increased number of myeloid cells in the bone marrow. These cells are arrested in their maturation process, frequently resulting in hematopoietic insufficiency. Recent advancements in molecular genetics have identified new markers that may be useful in diagnosing AML, including mutant c-kit (CD117). c-kit is a receptor tyrosine kinase and activating mutations in this receptor have been identified as a functional driver of AML and other myeloproliferative diseases. c-kit also plays a major role in maintaining normal stem cell pools. Prior studies suggest that ST6Gal-I, a unique glycosyltransferase upregulated in many cancers, promotes a stem-like cell phenotype in epithelial tumor cells. ST6Gal-I is a sialyltransferase that adds a negatively charged sialic acid in an α2-6 linkage to membrane glycoproteins. In this study, we show c-kit to be a substrate of ST6Gal-I, with increased sialylation correlating to increased c-kit activation. In order to study the effects of sialylation on signaling, we forced constitutive expression of ST6Gal-I in the U937 monocytic leukemia cell line. c-kit signaling was then induced by treating U937 cells with stem cell factor (SCF), the c-kit ligand. Here we show enhanced SCF-mediated c-kit activation, indicated by increased levels of phospho-c-kit in ST6Gal-I overexpressing cells. This, in turn, is associated with increased activation of downstream effecters of c-kit signaling, phospho-Erk and phospho-Akt. We further hypothesize that ST6Gal-I might play a role in the cell differentiation process. To test the effects of sialylation on differentiation, we used a well-established model to induce monocyte cell differentiation, treatment with the phorbol ester, PMA. PMA is known to induce macrophage differentiation in U937 cells, evidenced by exit from the cell cycle and upregulation of differentiation markers, including CD11b. Macrophage differentiation induces downregulation of endogenous ST6Gal-I, thereby reducing surface sialylation of specific receptors. However, forced overexpression of ST6Gal-I inhibits PMA-induced exit from the cell cycle, suggesting that ST6Gal-I acts to maintain cells in an undifferentiated state. To further understand the effects of receptor sialylation on PMA-induced differentiation, we used flow cytometric analysis to evaluate surface markers on U937 cells with or without ST6Gal-I overexpression. We show that forced expression of ST6Gal-I inhibits PMA-induced upregulation of CD11b, whereas c-kit expression is elevated in ST6Gal-I overexpressing cells relative to parental cells. These results suggest a novel mechanism by which ST6Gal-I contributes to an undifferentiated state in part through the regulation of the c-kit receptor.

Citation Format: Kaitlyn A. Dorsett, Susan L. Bellis. ST6Gal-I glycosyltransferase promotes an undifferentiated cell phenotype and enhances c-kit signaling. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3336.