Abstract 996: Fibronectin exposes a targetable DHPS/SLC3A2 vulnerability that can be leveraged to decrease cytoplasmic levels of eIF5A1/2 and proliferation/survival in TNBC
Authors: Geller Cameron , Maddela Joanna , Tuplano Ranel , Cantor Joseph , Kelber Jonathan A. 
Published: June 2022

Abstract
Breast cancer ranks first among female cancer-related deaths in 2021, and fewer than 20% of patients survive five years after metastasis. Triple negative breast cancer (TNBC or basal) is highly aggressive and particularly difficult to treat due to the lack therapeutic target receptors. Notably, elevated fibronectin expression within the microenvironment of TNBC is an indicator of advanced disease stage. In this regard, we previously reported that the two-step, hypusination reaction that activates eukaryotic initiation factor 5A 1/2 (eIF5A1/2) is necessary for PEAK1 pseudokinase translation and fibronectin/TGFβ-mediated TNBC metastasis. In our current study, we observed intratumoral heterogeneity of DOHH, DHPS, eIF5A and eIF5A2 expression in TNBC patient tumor samples, suggesting a role for this pathway in disease progression and therapy resistance. Using bioinformatics tools, we identified a correlation between elevated solute carrier 3A2 (SLC3A2 or CD98) and decreased survival in breast cancer patients with amplified SMAD3 loci. Given that SMAD3-dependent transcription is one mechanism by which canonical TGFβ signaling elicits tumor suppressive outcomes, we asked whether SLC3A2 may function to prime TNBC cells for tumor promoting outcomes in response to TGFβ. In this regard, inhibition of SLC3A2 or DHPS decreased TNBC spheroid growth. Notably, spiking fibronectin into 3D culture media partially rescued spheroid growth from SLC3A2 inhibition - an effect blocked by dual DHPS/SLC3A2 inhibition. In agreement with these data, dual DHPS/SLC3A2 inhibition in TNBC cells cultured with exogenous fibronectin significantly reduced the percentage of cells in S phase and subsequently increased the percentage of cells in either G0/G1 or Sub-G0/G1 phases, suggesting that DHPS and SLC3A2 cooperatively promote TNBC cell proliferation/survival in fibronectin-rich contexts. eIF5A1/2 carries out hypusination-dependent functions in both the nucleus (i.e., mRNA export) and cytoplasm (i.e., translation). Interestingly, it undergoes nucleocytoplasmic transport via exportin four (XPO4) - the same nuclear pore complex protein that is used for nuclear export of SMAD3. Using structured illumination microscopy, we discovered that fibronectin promotes an increase in cytoplasmic eIF5A1/2 levels - an effect blocked by dual DHPS/SLC3A2 inhibition. In contrast, DHPS inhibition alone increased nuclear SMAD3 levels independent of extracellular matrix proteins. Taken together, these data provide a possible mechanism for the antiproliferative/cytotoxic effects of dual DHPS/SLC3A2 inhibition and suggest that this treatment strategy may be efficacious in TNBC patients with advanced stage disease.
Citation Format: Cameron Geller, Joanna Maddela, Ranel Tuplano, Joseph Cantor, Jonathan A. Kelber. Fibronectin exposes a targetable DHPS/SLC3A2 vulnerability that can be leveraged to decrease cytoplasmic levels of eIF5A1/2 and proliferation/survival in TNBC [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 996.