Abstract 5883: Role of major facilitative folate transporters SLC19A1 and SLC46A1 in cyclic dinucleotide transport and STING signaling
Authors: McQuithey Holly , Brzezinski Madelyn , O’Connor Carrie , Matherly Larry , Hou Zhanjun 
Published: March 2024

Abstract
Activation of stimulator of interferon genes (STING) activates the transcription factor pIRF3, promoting gene transcription encoding type I interferons including IFNβ that stimulate broader immune responses. Targeting the STING pathway for cancer is especially compelling given its importance to the malignant phenotype. Cyclic dinucleotides (CDNs) function as STING agonists. Inspired by preclinical studies, two phase I clinical trials with CDNs including ADU-S100 were initiated but concluded early due to lack of anti-tumor efficacy. The reduced folate carrier (RFC; SLC19A1) has been identified as one of the primary means to transport CDNs through the cell membrane, although a study suggested that CDN and folate binding to RFC was as least partly distinct. While depletion of the proton-coupled folate transporter (PCFT; SLC46A1) in THP-1 cells had nominal effects on CDN stimulation, overexpressing PCFT increased STING signaling. Limited clinical responses may reflect the impact of physiologic variables governing RFC and PCFT not previously considered. Understanding the mechanisms of CDN uptake by RFC and PCFT under physiologic conditions may elucidate the role of CDNs on STING signaling. To study the impact of RFC levels on CDN transport and STING signaling, we used an engineered R1-11/Tet-on-RFC HeLa cell model induced with doxycycline. In R1-11/Tet-on-RFC cells, ADU-S100 showed a low affinity for binding to RFC, as measured by direct competition for transport with [3H]methotrexate. RFC dose-dependence of ADU-S100 uptake and STING activation was confirmed by monitoring IFN-β (qPCR) and pIRF3 (Westerns). Maximum induction of pIRF3/IFNβ was seen in THP-1 and R1-11/Tet-on-RFC cells by ADU-S100 treatment at pH 7.2 (RFC pH optimum) in the presence of 25 nM leucovorin. Induction of pIRF3/IFNβ by ADU-S100 in THP-1 and R1-11/Tet-on-RFC cells were partially abolished by treatment with 200 μM of leucovorin or 10 μM of PT523 (RFC-specific inhibitor), suggesting the transport of folates and CDNs may share common albeit distinct mechanisms. ADU-S100 treatment showed no pIRF3 or IFN-β production in an engineered R1-11/Tet-on-PCFT HeLa model expressing high levels of PCFT, suggesting no direct transport by PCFT, even under optimal pH conditions. Interestingly, pIRF3/IFN-β induction was enhanced in R1-11/Tet-on-RFC/PCFT cells expressing constitutive PCFT in addition to RFC, compared to R1-11/Tet-on-RFC cells that express comparable RFC without PCFT. This may reflect previously unrecognized functional interactions between RFC and PCFT. In conclusion, RFC mediates ADU-S100 uptake and STING activation under physiologic conditions and this process is enhanced by concomitant expression of PCFT. Additional characterization of key molecular and biochemical determinants of CDN-based cancer therapeutics may lead to improved approaches for CDN therapy based on enhanced cellular uptake.
Citation Format: Holly McQuithey, Madelyn Brzezinski, Carrie O’Connor, Larry Matherly, Zhanjun Hou. Role of major facilitative folate transporters SLC19A1 and SLC46A1 in cyclic dinucleotide transport and STING signaling [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 5883.