Abstract A17: Non-RGD-based strategies to target the thyroid hormone receptor-integrin αvβ3: Lessons from myeloma cells.

Abstract

Abstract Introduction. Integrin αvβ3 is a plasma membrane structural protein expressed/activated in tumor cells and dividing blood vessel cells. The extracellular domain of αvβ3 has an Arg-Gly-Asp (RGD) recognition site that enables the integrin to bind specific extracellular matrix (ECM) proteins that contain the RGD sequence and to transduce the binding into discrete intracellular signals. Such ECM proteins have pivotal roles in solid tumors and the RGD site has been tested as a chemotherapeutic target. Recently, a receptor for thyroid hormone (L-thyroxine, T4; 3, 5, 3'-triiodo-L-thyronine, T3) has been described on αvβ3 that is distinct from the RGD recognition site and has been shown to mediate proliferative effects of T4 and T3 on solid tumor cells via the MAPK pathway. We have recently shown that similar T4- and T3-induced proliferation occurs in multiple myeloma (MM) cells. Actions of T4 and T3 at the non-RGD site can be blocked by tetraiodothyroacetic acid (tetrac), a naturally-occurring deaminated T4 analog. In addition, tetrac has anti-tumor effects that are independent of blockade of actions of T4 and T3. In this report, we define mechanisms of tetrac action in MM cells. Methods. MM cell lines (CAG, RPMI 8226, ARK, ARP-1, U266) and primary cultures of marrow cells from MM patients were grown with T4/T3 and with/without tetrac (100 nM-50 μM). Harvested cells were subjected to the following analyses: cell counts (FACS, CyQuant), viability (WST-1), cell cycle (FACS, PI), cell death (Annexin-PI ± pan-caspase inhibitor, ZVAD, FACS; cleaved caspase-3, cleaved PARP, western blots), DNA damage response (pATM and PARP, western blots) and transcription of apoptosis-relevant genes (RT-PCR). Results. Early (4-8 h) and late (24 h) effects of tetrac were observed. In the early phase, tetrac reduced cell proliferation (p <<0.05) and induced a DNA damage response (ATM and PARP) and apoptosis (cleaved caspase-3 and cleaved PARP). After 24 h, tetrac significantly (p < 0.05) reduced cell number and viability and induced apoptotic cell death, with a parallel increase in pro-apoptotic gene expression. Conclusion. The thyroid hormone-binding (non-RGD) site on integrin αvβ3 in myeloma cells appears to be a novel target for chemotherapy with tetrac. Citation Format: Keren Cohen, Martin Ellis, Paul J. Davis, Aleck Hercbergs, Osnat Ashur-Fabian. Non-RGD-based strategies to target the thyroid hormone receptor-integrin αvβ3: Lessons from myeloma cells. [abstract]. In: Proceedings of the AACR Special Conference on Hematologic Malignancies: Translating Discoveries to Novel Therapies; Sep 20-23, 2014; Philadelphia, PA. Philadelphia (PA): AACR; Clin Cancer Res 2015;21(17 Suppl):Abstract nr A17.