Osteopontin-4 (OPN-4) Suppresses Tumor Progression Features Whilst Sensitizing c643 Anaplastic Thyroid Cells to Sorafenib

Background/Objectives: Anaplastic thyroid carcinoma (ATC) is one of the most aggressive and lethal forms of malignant neoplasm of the endocrine system, and osteopontin (OPN) has been shown to be aberrantly expressed in this tumor type. Among the five OPN splicing isoforms (OPN-SI), OPN-4 has been recently reported in several tumor types, including ATC, but its functional role(s) have not yet been elucidated. Methods: To characterize OPN-4 roles in ATC cells, OPN-4 was ectopically overexpressed in the c643 ATC cell line, generating the c643/OPN-4 cells. OPN-roles were evaluated by cell functional assays, including cell proliferation and viability, using Carboxyfluorescein Succinimidyl Ester (CFSE), crystal violet, and trypan blue assays. For migration, clonogenicity, cell cycle and apoptosis assays were used. For assessment, c643/OPN-4 cells were cultured in two-dimensional (2D) monolayers or three-dimensional (3D) spheroids with the latter being maintained in a bespoke microfluidic system. Results: OPN-4 overexpression led to a significant reduction in cell proliferation, viability, migration and clonogenicity. c643/OPN-4 cells displayed a significant accumulation in the G0/G1 phase and a decrease in the S phase of the cell cycle; however this did not affect cell death or the expression levels of other OPN-SI. In a spheroid model of c643/OPN-4 cells, no significant differences were found in spheroid size or viability when compared to those formed by control cells. Notably, OPN-4 overexpression enhanced the effects of sorafenib on cell viability under dynamic treatment conditions involving continuous perfusion. Conclusions: These early findings point to the fact that OPN-4 may reduce some aspects of tumor progression features in ATC cells and open new avenues for investigating OPN-4 as a biomarker of therapeutic response in personalized treatment strategies.