*Review* **Structural and Functional Characterization of the ABCC6 Transporter in Hepatic Cells: Role on PXE, Cancer Therapy and Drug Resistance**

**Faustino Bisaccia, Prashant Koshal, Vittorio Abruzzese, Maria Antonietta Castiglione Morelli and Angela Ostuni \***

> Department of Sciences, University of Basilicata, 85100 Potenza, Italy; faustino.bisaccia@unibas.it (F.B.); prashantkoshal240@gmail.com (P.K.); v.abruzz@hotmail.it (V.A.); maria.castiglione@unibas.it (M.A.C.M.) **\*** Correspondence: angela.ostuni@unibas.it

**Abstract:** Pseudoxanthoma elasticum (PXE) is a complex autosomal recessive disease caused by mutations of ABCC6 transporter and characterized by ectopic mineralization of soft connective tissues. Compared to the other ABC transporters, very few studies are available to explain the structural components and working of a full ABCC6 transporter, which may provide some idea about its physiological role in humans. Some studies suggest that mutations of *ABCC6* in the liver lead to a decrease in some circulating factor and indicate that PXE is a metabolic disease. It has been reported that ABCC6 mediates the efflux of ATP, which is hydrolyzed in PPi and AMP; in the extracellular milieu, PPi gives potent anti-mineralization effect, whereas AMP is hydrolyzed to Pi and adenosine which affects some cellular properties by modulating the purinergic pathway. Structural and functional studies have demonstrated that silencing or inhibition of ABCC6 with probenecid changed the expression of several genes and proteins such as *NT5E* and TNAP, as well as Lamin, and CDK1, which are involved in cell motility and cell cycle. Furthermore, a change in cytoskeleton rearrangement and decreased motility of HepG2 cells makes ABCC6 a potential target for anti-cancer therapy. Collectively, these findings suggested that ABCC6 transporter performs functions that modify both the external and internal compartments of the cells.

**Keywords:** ABCC6; TNAP; *NT5E*; Pseudoxanthoma elasticum (PXE); cancer
