Salmonella Impacts Tumor-Induced Macrophage Polarization, and Inhibits SNAI1-Mediated Metastasis in Melanoma.

Christian R Pangilinan,Che-Hsin Lee,Li-Hsien Wu

doi:10.3390/cancers13122894

Christian R Pangilinan, Che-Hsin Lee + Show 1 more

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https://doi.org/10.3390/cancers13122894

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Abstract

Simple SummaryCancer treatment using Salmonella is deemed a viable option since Salmonella impedes expression of proteins relevant to tumor survival and progression. In essence, the present study investigated the effect of Salmonella on melanoma metastasis. We found that Salmonella reduces Akt/mTOR activity, resulting in downregulation of SNAI1—an EMT inducer vital for cell migration. Furthermore, Salmonella increases HMGB1 secretion in tumors, thereby influencing macrophage reprogramming toward an M1-like phenotype. We propose that these two processes, coaxed by Salmonella, work concurrently to prevent melanoma metastasis.Targeting metastasis is a vital strategy to improve the clinical outcome of cancer patients, specifically in cases with high-grade malignancies. Here, we employed a Salmonella-based treatment to address metastasis. The potential of Salmonella as an anticancer agent has been extensively studied; however, the mechanism through which it affects metastasis remains unclear. This study found that the epithelial-to-mesenchymal transition (EMT) inducer SNAI1 was markedly reduced in Salmonella-treated melanoma cells, as revealed by immunoblotting. Furthermore, wound healing and transwell assays showed a reduced in vitro cell migration following Salmonella treatment. Transfection experiments confirmed that Salmonella acted against metastasis by suppressing protein kinase B (Akt)/mammalian target of rapamycin (mTOR) signaling, which in turn inhibited SNAI1 expression. Since it is known that metastasis is also influenced by inflammation, we partly characterized the immune infiltrates in melanoma as affected by Salmonella treatment. We found through tumor-macrophage co-culture that Salmonella treatment increased high mobility group box 1 (HMGB1) secretion in tumors to coax the polarization of macrophages in favor of an M1-like phenotype, as shown by increased inducible nitric oxide synthase (iNOS) expression and Interleukin 1 Beta (IL-1β) secretion. Data from our animal study corroborated the in vitro findings, wherein the Salmonella-treated group obtained the lowest lung metastases, longer survival, and increased iNOS-expressing immune infiltrates.

Highlights

Melanoma is a malignant tumor regarded as the most fatal amongst the known forms of skin cancer [1]
Given that Salmonella influences gene expression in tumors by modulating Akt/mammalian target of rapamycin (mTOR) and MAPK signaling [19], we examined the effect of Salmonella treatment on the expression of SNAI1 in a panel of melanoma cell lines, as SNAI1 was known to be regulated, in part, by Akt signaling [9,10]
The same reduction pattern of SNAI1 brought about by Salmonella treatment was observed in the levels of Akt and mTOR phosphorylation, with an apparent drop at 100 M.O.I. in all cells tested (Figures 1B,C and S1A,B); these results indicate the probable action of Salmonella on SNAI1 by disrupting intracellular Akt/mTOR signaling

Summary

Introduction

Melanoma is a malignant tumor regarded as the most fatal amongst the known forms of skin cancer [1]. Cancer metastasis occurs when cancer cells gain the potential to migrate through the circulation, extravasation, and eventually invasion of distant tissues [2,3]. These phenomena are directed by the expression of various transcription factors, including members of the Snail family, SNAI1, a known regulator of the epithelial-to-mesenchymal transition during early development [4]. SNAI1 expression has been implicated in high-grade malignancies and correlated with poor prognosis, as it is one of the drivers of cancer progression to metastasis [5,6,7]. SNAI1 expression was reported to be mediated by several signaling mechanisms, including phosphoinositide 3-kinase (PI3K)/protein kinase B (Akt) signaling, which is known to modulate behavioral phenotypes in tumors such as anti-apoptosis, drug resistance, and metastasis [9,10]

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