Methionine restriction-induced sulfur deficiency impairs antitumour immunity partially through gut microbiota

Restriction of methionine (MR), a sulfur-containing essential amino acid, has been reported to repress cancer growth and improve therapeutic responses in several preclinical settings. However, how MR impacts cancer progression in the context of the intact immune system is unknown. Here we report that while inhibiting cancer growth in immunocompromised mice, MR reduces T cell abundance, exacerbates tumour growth and impairs tumour response to immunotherapy in immunocompetent male and female mice. Mechanistically, MR reduces microbial production of hydrogen sulfide, which is critical for immune cell survival/activation. Dietary supplementation of a hydrogen sulfide donor or a precursor, or methionine, stimulates antitumour immunity and suppresses tumour progression. Our findings reveal an unexpected negative interaction between MR, sulfur deficiency and antitumour immunity and further uncover a vital role of gut microbiota in mediating this interaction. Our study suggests that any possible anticancer benefits of MR require careful consideration of both the microbiota and the immune system.

Trending topics of SIRT1 in tumorigenicity

BACKGROUND

Carcinogenesis is governed by a series of genetic alterations and epigenetic changes that lead to aberrant patterns in neoplastic cells. Sirtuin-1(SIRT1), an NAD⁺-dependent protein deacetylase, is capable of deacetylating histones and non-histone substrates that regulate various physiological activities during tumorigenesis. Recent studies have identified the role of SIRT1 in different stages of cancer, including genome instability, tumor initiation, proliferation, metabolism, and therapeutic response. However, the action of SIRT1 has been reported to be both oncogenic and tumor suppressive during carcinogenesis. Consequently, the biological functions of SIRT1 in cancer remain controversial.

SCOPE OF REVIEW

We highlight the most recent findings on SIRT1 in different stages of tumorigenesis, and update the current status of SIRT1 small molecule modulators in clinical application of cancer treatment.

MAJOR CONCLUSION

By targeting both tumor suppressors and oncogenic proteins, SIRT1 has a bifunctional role at different stages of tumorigenesis. The impact of SIRT1 on tumorigenesis is also distinct at different stages and is dependent on its dosages. SIRT1 suppresses tumor initiation through its functions in promoting DNA repair, increasing genome stability, and inhibiting inflammation at the pre-cancer stage. However, SIRT1 enhances tumor proliferation, survival, and drug resistance through its roles in anti-apoptosis, pro-tumor metabolism, and anti-inflammation (inhibition of anti-tumor immunity) at the stages of tumor progression, metastasis, and relapse. Consequently, both SIRT1 inhibitors and activators have been explored for cancer treatment.

GENERAL SIGNIFICANCE

Better understanding the dose- and stage-dependent roles of SIRT1 in each cancer type can provide new avenues of exploration for therapy development.

Abstract 4052: Effects of CRISPR/Cas9-mediated knockout of SIRT6 in human melanoma cells, in vitro and in vivo

Melanoma is one of the most aggressive and fatal forms of skin cancer, with dismal 5-year survival if not diagnosed early. Although recent advances have led to better targeted- and immuno- therapeutic approaches, many patients develop resistance and recurrence. Therefore, it is crucial to develop newer druggable molecular targets that could be exploited towards the management of this deleterious neoplasm. Earlier studies from our laboratory have shown that SIRT6, a member of the sirtuin family of class III histone deacetylases, possesses a potential pro-proliferative role in melanoma. Extending on these novel findings, the objective of this study was to determine the effects of CRISPR/Cas9-mediated knockout (KO) of SIRT6 in melanoma cells, both in vitro and in vivo. First, we evaluated the effects of SIRT6 KO on growth, viability, and clonogenic survival in the A375 human melanoma cell line. We found that KO of SIRT6 resulted in a significant anti-proliferative response in melanoma cells, as measured by trypan blue and RealTime-Glo assays. Further, SIRT6 KO also resulted in a significant decrease in the long-term clonogenic survival of melanoma cells, as measured by a colony formation assay. Additionally, we used DNA cell cycle analysis by flow cytometry to assess the effect of SIRT6 KO on cell cycle perturbation in melanoma cells. Our data demonstrated that SIRT6 KO induced a G1-phase arrest in the A375 melanoma cells. Furthermore, we employed a PCR array (RT2 Profiler Cancer Pathway Array; with 84 genes) to understand the molecular mechanisms associated with the observed anti-proliferative response of SIRT6 KO. We found that SIRT6 KO caused statistically significant alteration in genes involved in important cellular pathways, including angiogenesis (CCL2, KDR, SERPINF1, and FLT1), hypoxia signaling (ADM and CA9), cellular senescence (IGFBP5 and TBX2), apoptosis (CFLAR), epithelial-to-mesenchymal transition (EMT) (SNAI2), and telomere maintenance (TEP1). Finally, to validate the in vitro findings to in vivo situation, we determined the growth of SIRT6 KO A375 human melanoma cells-implanted tumors in immunocompromised mice. We observed a significant slow growth of SIRT6 KO tumors compared to wild-type A375 tumors, suggesting that SIRT6 reduction decreases cell survival in melanoma both in vitro and in vivo. Overall, our results re-emphasize the tumor promoter role of SIRT6 in melanoma. Further detailed studies are required to determine the potential of SIRT6 inhibition as a novel treatment regimen against melanoma.

Citation Format: Liz M. Garcia-Peterson, Mary A. Ndiaye, Glorimar Guzman-Perez, Chandra K. Singh, Gagan Chhabra, Nihal Ahmad. Effects of CRISPR/Cas9-mediated knockout of SIRT6 in human melanoma cells, in vitro and in vivo [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4052.

CRISPR/Cas9-mediated Knockout of SIRT6 Imparts Remarkable Antiproliferative Response in Human Melanoma Cells in vitro and in vivo

Melanoma is one of the most aggressive, potentially fatal forms of skin cancer and has been shown to be associated with solar ultraviolet radiation-dependent initiation and progression. Despite remarkable recent advances with targeted and immune therapeutics, lasting and recurrence-free survival remain significant concerns. Therefore, additional novel mechanism-based approaches are needed for effective melanoma management. The sirtuin SIRT6 appears to have a pro-proliferative function in melanocytic cells. In this study, we determined the effects of genetic manipulation of SIRT6 in human melanoma cells, in vitro and in vivo. Our data demonstrated that CRISPR/Cas9-mediated knockout (KO) of SIRT6 in A375 melanoma cells resulted in a significant (1) decrease in growth, viability and clonogenic survival and (2) induction of G1-phase cell cycle arrest. Further, employing a RT² Profiler PCR array containing 84 key transformation and tumorigenesis genes, we found that SIRT6 KO resulted in modulation of genes involved in angiogenesis, apoptosis, cellular senescence, epithelial-to-mesenchymal transition, hypoxia signaling and telomere maintenance. Finally, we found significantly decreased tumorigenicity of SIRT6 KO A375 cells in athymic nude mice. Our data provide strong evidence that SIRT6 promotes melanoma cell survival, both in vitro and in vivo, and could be exploited as a target for melanoma management.

The sirtuin 6: An overture in skin cancer

In the recent past, the sirtuins have been under intense investigation for their roles in biology and disease, including cancer. The sirtuin SIRT6 is comparatively a lesser studied member of this family of seven proteins. Like certain other sirtuins, SIRT6 is emerging to have an oncogenic function as well as tumor suppressor roles in cancer. Limited studies have been conducted assessing the role and functional significance of SIRT6 in melanoma and non-melanoma skin cancers. In this review, we have attempted to critically dissect the potential role and significance of SIRT6 in skin carcinogenesis. With limited available information to date, SIRT6 appears to have a pro-proliferative function in non-melanoma skin cancers (NMSCs), including squamous cell carcinoma (SCC) and basal cell carcinoma (BCC). In addition, SIRT6 is also emerging to have an oncogenic function in melanoma. Moreover, we have provided information regarding the available SIRT6 inhibitors. Conclusively, it appears that additional comprehensive studies are needed to establish the role of SIRT6 in skin biology and skin diseases, including cancer. Further, concerted efforts are needed to characterize the stage-specific role of SIRT6 in skin cancers.

Abstract 548: The effects of Sirtuin 6 inhibition on autophagy-related pathways in melanoma

Melanoma is an aggressive skin cancer that can rapidly metastasize to become fatal, if not diagnosed early. Despite therapeutic advances, clinical management of melanoma remains challenging. The existing chemotherapeutic drugs either fail to achieve greater than 25% response or ultimately develop resistance to therapy. Therefore, novel molecular targets and treatments are required for an effective management of melanoma. In order to design novel and more effective approaches for lasting cure of melanoma, a deeper understanding of the mechanisms involved in melanomagenesis are needed. In our laboratory, we are assessing the role and functional and therapeutic significance of sirtuin proteins in melanoma. We have previously demonstrated that Sirtuin 6 (SIRT6) is significantly overexpressed in human melanoma cells and clinical tissues, and its genetic knockdown resulted in a marked anti-proliferative response in human melanoma cells (Genes & Cancer; In Press). A limited number of studies have implicated SIRT6 in autophagy regulation. Autophagy forms part of a basic cellular process that copes with cellular stress and is considered to be an important metabolic mechanistic essential for degradation and recycling of unnecessary intracellular components. In melanoma autophagy is believed to be a tumor suppressing process in early stages of cancer, but tumor-promoting in established tumors. This study was designed to determine the connection between SIRT6 and autophagy in melanoma. Employing lentiviral short hairpin RNA-mediated knockdown of SIRT6 in A375 melanoma cells, we determined the modulation in autophagy related pathways. We employed a PCR array containing 84 genes that are involved in the regulation of autophagy. Our data demonstrated that 17 of the 84 genes were significantly modulated (two-fold or more) upon SIRT6 knockdown. Of these 17 genes, 2 were upregulated (ATG10 and GAA) and 15 were downregulated (AKT1, ATG12, ATG3, ATG7, BAK1, BCL2L1, CLN3, CTSB, CTSS, DRAM2, HSP90AA1, IRGM, NPC1, SQSTM1, and TNF). Modulated genes were analyzed using Ingenuity Pathway Analysis (IPA) for predicted gene-gene interactions and functional networks. Many of the network genes were found to have definitive links to cancer. These genes were found to be associated with cell transformation and tumor invasion. Further validation at mRNA and protein levels, confirmed the significant modulations in autophagy markers BECN1, SQSTM1, ATG3, ATG7, ATG10 and GAA, upon SIRT6 knockdown. Moreover, SIRT6 knockdown was found to cause a marked decrease in the LC3 II protein level, which is a central protein in the autophagy pathway. Taken together, our data suggest that SIRT6 knockdown significantly alters pathways related to autophagy in melanoma cells. Future studies are required to carefully study the role of SIRT6 and autophagy in early versus late melanomas.

Citation Format: Liz M. Garcia-Peterson, Mary A. Ndiaye, Chandra K. Singh, Gagan Chhabra, Nihal Ahmad. The effects of Sirtuin 6 inhibition on autophagy-related pathways in melanoma [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 548.