Abstract 4462: Immunoregulatory effects of NNMT-expressing cancer-associated fibroblasts

Nicotinamide N-methyltransferase (NNMT) is highly expressed in the stroma of several malignancies and has been shown to drive the transformation of resting fibroblasts into cancer-associated fibroblasts (CAFs) through epigenetic changes. Given that CAFs can promote immunosuppression in cancer, the aim of our study was to investigate the effect of NNMT-expressing CAFs on the tumor-immune environment.

Using a co-culture system of normal fibroblasts and activated PBMCs, we found that IFN-γ produced by PBMCs upregulates NNMT expression in fibroblasts. We confirmed this mechanism in vivo in IFN-γ knockout mice and identified T cells as the main source of IFN-γ in the tumor. Gene expression and protein analysis showed that upregulation of NNMT in CAFs induces the secretion of the chemokine CXCL1, a critical chemoattractant for immunosuppressive myeloid-derived suppressor cells (MDSCs). Upregulation of NNMT induces CXCL1 secretion directly through promoter hypomethylation, and in vitro migration assays show that NNMT-expressing CAFs recruit high numbers of MDSCs. Using spectral flow cytometry, we analyzed the tumor-immune cell infiltration in whole-body NNMT-knockout mice and discovered that NNMT knockout of the stroma significantly reduced the abundance of MDSCs and increased the number of functional CD8+ T cells. Increased T cell cytotoxicity was associated with a significant reduction in tumor burden in the syngeneic ID8 ovarian and MC38 colon cancer models. These findings were confirmed after adoptive transfer of NNMT-wildtype immune cells into irradiated whole-body NNMT-knockout mice. Moreover, in scRNA-seq data of various human malignancies, we found a strong correlation between NNMT and CXCL1 expression in CAFs.

In summary, our results support a model in which cancer cells induce IFN-γ secretion by T-cells, thereby upregulating NNMT expression in fibroblasts, leading to CXCL1 secretion. CXCL1 secreted by CAFs recruits MDSCs to the tumor, which in turn suppress the anti-tumor T cell response.

Citation Format: Janna Heide, Andras Piffko, Agnes J. Bilecz, Ethan A. Teich, Lisa Schweizer, Sayed R. Alhunayan, Kaiting Yang, Ernst Lengyel. Immunoregulatory effects of NNMT-expressing cancer-associated fibroblasts. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4462.

The adipocyte microenvironment and cancer

Many epithelial tumors grow in the vicinity of or metastasize to adipose tissue. As tumors develop, crosstalk between adipose tissue and cancer cells leads to changes in adipocyte function and paracrine signaling, promoting a microenvironment that supports tumor growth. Over the last decade, it became clear that tumor cells co-opt adipocytes in the tumor microenvironment, converting them into cancer-associated adipocytes (CAA). As adipocytes and cancer cells engage, a metabolic symbiosis ensues that is driven by bi-directional signaling. Many cancers (colon, breast, prostate, lung, ovarian cancer, and hematologic malignancies) stimulate lipolysis in adipocytes, followed by the uptake of fatty acids (FA) from the surrounding adipose tissue. The FA enters the cancer cell through specific fatty acid receptors and binding proteins (e.g., CD36, FATP1) and are used for membrane synthesis, energy metabolism (β-oxidation), or lipid-derived cell signaling molecules (derivatives of arachidonic and linolenic acid). Therefore, blocking adipocyte-derived lipid uptake or lipid-associated metabolic pathways in cancer cells, either with a single agent or in combination with standard of care chemotherapy, might prove to be an effective strategy against cancers that grow in lipid-rich tumor microenvironments.