Abstract 5269: Evaluation of mitotic spindle inhibitors as cell autonomous inflammatory stimulants in MYC overexpressing breast cancers
Authors: Jacobo Mauricio , Goga Andrei 
Published: March 2024

Abstract
The oncogenic transcription factor MYC is overexpressed in most cancer types, including difficult to treat human malignancies such as receptor triple-negative breast cancers (TNBC). MYC overexpression induces aneuploidy and suppresses anticancer immunity. Elevated levels of MYC can induce mitotic spindle assembly defects and chromosomal instability (CIN) events, exemplified by an increase in cells with micronuclei. These defects are partially mitigated by concomitant upregulation of various mitotic spindle genes. Micronuclei result in cytosolic DNA which can activate cGAS-STING signaling, activating an interferon regulatory transcriptional program responsible for mounting an inflammatory immune response that potentially enhances immune cell recognition. We hypothesized that pharmacological inhibition of specific MYC deregulated mitotic spindle genes will further exacerbate mitotic defects in MYC high cells resulting in synthetic-lethal cell death and additional CIN to activate robust cGAS-STING activation. Interferon activation can increase surface expression of antigen presentation machinery, such as MHC-I, restoring the immunogenicity of MYC high cells. Here, we used mouse breast tumor cells in which MYC expression is conditionally expressed as well as human breast cancer cell lines with different levels of MYC expression. We assessed the anti-proliferative and cytotoxic effects of 9 mitotic small molecular inhibitors that affect spindle function. cGAS-STING activation in inhibitor treated and untreated samples was measured by immunoblotting for pathway proteins phosphorylated TBK1, total TBK1, and IFIT1. Differences in MHC-I cell surface expression was quantified using flow cytometry. Our findings revealed that inhibition of kinesin-like protein KIFC1 and transforming acidic coiled-coil-containing protein 3 (TACC3) induced greater levels of apoptosis in MYC high models. Moreover, inhibition of KIFC1 robustly triggered cGAS-STING activity in MYC high cells as evidenced by increased protein abundance of phospho-TBK1 and IFIT1 in a dose-dependent manner. In addition, sublethal KIFC1 inhibition resulted in increased levels of MHC-I surface expression in MYC high models. Our findings suggest that inhibition of MYC regulated mitotic spindle genes, specifically KIFC1, can serve to preferentially eliminate MYC overexpressing aggressive cancers. Furthermore, sublethal concentrations of a KIFC1 inhibitor can stimulate a tumor cell-intrinsic inflammatory-pathway. Future studies will determine if targeting KIFC1 can enhance the response of MYC high cancers to immunotherapy.
Citation Format: Mauricio Jacobo, Andrei Goga. Evaluation of mitotic spindle inhibitors as cell autonomous inflammatory stimulants in MYC overexpressing breast cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 5269.