The Role of ABCC6 Transporter in the Purinergic System: from Oncology to Regenerative Medicine

Abstract

ABCC6 is a member of ATP-binding cassette family of transporters, a class of membrane transporters sharing a common structural organization. Mutations in ABCC6 are associated with Pseudoxantoma elasticum (PXE) and with a minority of cases of Generalized Arterial Calcification of Infancy (GACI), two metabolic diseases characterized by an abnormal ectopic calcification of soft tissues. It is now recognized that ABCC6-mediated efflux of ATP from the liver is the main source of inorganic pyrophosphate, which is a strong anti- mineralization factor and is lacking in serum of patients with PXE or GACI. Extracellular ATP is not only important in controlling mineralization processes, as it is also a signaling molecule and is the main source of extracellular adenosine. Both ATP and adenosine can act as autocrine/paracrine signals, participating to the regulation of a variety of pathophysiological processes, through specific receptors. Purinergic signaling has a profound impact on tumor progression, affecting cancer cells proliferation, apoptosis, invasiveness and interaction with host immune response. Most of studies on ABCC6 focused on the identification of substrates in relation to its role in mineralization processes, but little information is available in regard of the consequences of transport activity in cells expressing ABCC6 protein. In this thesis we studied the effects of ABCC6 gene silencing and pharmacological inhibition on migration and cytoskeleton rearrangement of cancer cells, focusing at first on liver cancer cell line HepG2. Encouraged by results obtained in these cells, we tried to extend the relevance of our findings to extra hepatic tumors as well. We investigated the effects of pharmacological inhibition on two colorectal cancer cell lines, Caco2 and HT29, which significantly differ in the levels of expression of ABCC6. Finally, we evaluated the feasibility of using hepatocytes- like cells (HLCs) derived from amniotic epithelial stem cells as a substitute of primary hepatocytes to assess the potential toxic effects arising from ABCC6 inhibition in normal cells. Results reported here clearly show that in tumors where ABCC6 is highly expressed, it could have an important role in controlling cancer cells migration, through the modulation of extracellular purinergic signaling, thus representing a potential target for anti-cancer treatments aimed to reduce tumor invasiveness. HLCs derived from pluripotent stem cells represent a promising tool not only for regenerative medicine but also for disease modeling and toxicological studies. However, improvements in current protocols of in vitro differentiation are required to broaden the scope of this technology to routine toxicological studies. [edited by Author]