Effect of tumor heterogeneity in antitumor treatments based on NIS protein expression

The use of iodine radioisotopes in nuclear medicine is a very effective tool for the diagnosis and treatment of thyroid pathologies, such as cancer, based on the activity of the sodium/iodine co-carrier (NIS), a glycoprotein located in the basement membrane of the thyroid cells, which catalyzes active iodine transport. The capture of radioactive iodine (I131, I125) and other radioisotopes such as technetium (Tc99m) in tumor cells is achieved by the same mechanism, allowing the visualization of tumors by the emission of gamma particles from I125 or Tc99m and the radiotherapeutic destruction of them by the emission of particles β from I131.

Recent developments in gene therapy have allowed the transfer of NIS to other cell types by means of viral vectors, with the promising prospect of using these treatments in tumours of extra-throid origin. Such experimental approaches have shown additional advantages over conventional treatments, such as reducing extra-tumour toxicity.

In the TIRO-MATOs Unit (Faculty of Medicine, University of Nice, France), a line of colon carcinoma cells (HT29) that express the nis gene (HT29-NIS) in a stable way was developed. These cells, introduced into immuno-deficient mice, form tumors that can be studied by single photon emission computed tomography (SPECT) imaging techniques. Among the most important findings, an unexpected heterogeneous distribution of iodine incorporation and NIS expression in the cells that form the tumor was evident, with greater uptake in the external part of the tumor (oxygenated zones with the presence of proliferating cells), while this uptake and NIS expression decreased markedly inside the tumor (hypoxic zones that could contain mainly quiescent cells). These quiescent cells are attributed to radioresistance to treatments and relapses and metastases suffered by non-responders.

These results suggest that the decrease in iodine uptake and possibly NIS expression in the in vivo model may be related to heterogeneous tumor characteristics, particularly the presence of different cell types (proliferating and quiescent cells), the non-uniform distribution of oxygen (presence of hypoxic zones), and changes in nutrient availability that alter the metabolic pathways used by tumor cells, which affect the

The results obtained by the French group make clear the importance of understanding how the microenvironmental factors of the tumour affect the metabolism of cancer cells, particularly associated with the cellular response to stress, and how these metabolic changes induce resistance even to novel treatments such as metabolic radiotherapy based on gene therapy, and to conventional radiotherapy from external sources. In order to answer these questions, a cooperation alliance in applied basic research was established between the TIRO-MATOs Unit, the Cancer Control Center Ltda., and the Biochemistry and Biotechnology Research Group (Bio-Bio) of the University of Rosario. Based on the results obtained in the TIRO-MATOs Unit, an in vitro study will be carried out to imitate in culture the microenvironmental conditions found in in vivo tumours that determine the heterogeneity of the tumours, especially the variation in oxygen concentration and changes in cell proliferation. The effect of these microenvironmental conditions on cellular metabolic behaviour (glycolytic pathways and cellular stress response pathways) and the expression of NIS protein will be analysed. Likewise, it will be studied how heterologous expression of NIS protein and tumor heterogeneity affect the sensitivity of cells to ionizing radiation. In this way, this work is expected to provide knowledge on some tumor metabolic pathways that may be affecting the cellular response to stressors used in cancer treatments, with a view to identifying possible therapeutic targets to increase the sensitivity of tumors to such treatments.