Cantos vocales

Under normal extracellular and intracellular environment conditions in cell physiology, membrane transporters such as solute carriers (SLC) and ATP- binding cassette (ABC) transporters can transport and pass solutes, macromolecules, and ions into and out of cells. In some unusual environments, for example under chemotherapy treatment conditions, extracellular matrix microenvironment may be remodelled and some transporters can pump drugs outside the cell too (Gottesman and Pastan 1993, Scotto 2003). SLC is a group of membrane proteins which includes 43 families and 298 transporter genes that mediate the transporter and control uptake and efflux processes of crucial compounds such as inorganic ions, nucleotides and amino acids (Hediger, Romero et al. 2004). Further, ABC transporters which include 8 subfamilies and 49 ABC genes can mediate transport of different substances such as lipids, carbohydrates, ions and xenobiotics compounds (Hediger, Romero et al. 2004). To achieve this aim, membrane transporters use energy from the hydrolysis of ATP (Li and Shu 2014). This transport generates and maintains electrochemical ion gradients across membranes of the cell and it also can act by itself as a self- defence mechanism of the cell by actively transporting toxic substances. Drugs can be pumped out from the cell to the extracellular space by these transporter proteins (Hediger, Romero et al. 2004). So, the cytotoxic agent inside the cell tends to decrease due to the efflux by itself and the decrease in both the influx

15

process of the agent into the cell and the cell permeability (Gottesman and Pastan 1993).

Frequently, MDR is identified by a high increase in the expression of ABC transmembrane transporters like P-glycoprotein (Pgp) (MDR1) and the overexpression of the multidrug resistance-related protein (MRP1) (Gottesman and Pastan 1993, Scotto 2003, Schumacher, Nehmann et al. 2012). MDR1 is a 170 kDa transmembrane glycoprotein. It is one of the best characterized proteins involved in avoiding the cytotoxic effects of multiple drugs on tumour cells (Mechetner, Kyshtoobayeva et al. 1998, Perez-Herrero and Fernandez- Medarde 2015). It is overexpressed in malignant cells in order to remove chemotherapeutic drugs such as IRI from the cells by an active pathway (Xu and Villalona-Calero 2002). However, the specific role that transmembrane transporters play during the development of drug resistance mechanisms is still unknown. Focusing on MDR1, CRC cell lines with low levels of MDR1 protein were more sensitive to drugs and high levels of MDR1 was an important factor during establishment of resistant sublines (Kramer, Weber et al. 1993).

Other studies have shown that MDR1 positive HT 29 CRC cells proliferate much slower than MDR1 negative cells and metastasize less frequently in severe combined immunodeficiency (SCID) mice models (Schumacher, Nehmann et al. 2012). A deceleration of cell growth could be a mechanism of resistance against any drug whose target was the proliferation process. In addition, HT 29 CRC cell line with high expression of MDR1 would have more time to repair damages in DNA, because MDR1 cells proliferate slower than MDR1 negative cell lines.

Whilst the MDR1 transporter accepts amphipathic cationic compounds and neutral compounds as substrates (Chu, Suzuki et al. 1999), the MRP1 transporter family are organic anion transporters that transport anionic drugs and neutral drugs. Normally, MRP1 mediates glutathione S-conjugate export pump, playing its main role in the glutathione S-conjugates transporting and in the transport of glutathione disulfide to the extracellular space, taking a very important part in detoxification and defence against oxidative stress (Chu, Suzuki et al. 1999). Moreover, it has been shown that ABC proteins can play an

16

important role in camptothecins efflux. It has been demonstrated that both MDR1 and MRP1 overexpression are involved in the active efflux of IRI and its activated form SN-38 (Chu, Suzuki et al. 1999). Some tumour suppressor proteins such as p53 can repress the expression MDR1 and MRP1 proteins (Xu and Villalona-Calero 2002, Cai, Miao et al. 2013). Furthermore, decreasing p53 expression is correlated with MRP1 expression in CRC cancer (Scotto 2003) and some p53 mutations can activate MDR1 promotor (Scotto 2003). Another gene involved in drug transporter regulation is c-MYC. c-MYC is a gene that encodes a transcription factor involved in cell cycle progression, apoptosis and cellular transformation (Kugimiya, Nishimoto et al. 2015). Besides, p53 regulates ABCB5 (MRP5) expression, another transmembrane transport protein which is involved in development of resistance to 5-FU in CRC cell lines (Kugimiya, Nishimoto et al. 2015).

Finally, MDR1 and MRP1 are not the unique members of the ABC family that can act like transporters and mediate resistance to cytotoxic agents used in CRC therapy, like camptothecins and CPT-11 (IRI) (Chu, Suzuki et al. 1999). An additional identified ATP-binding cassette transporter is the breast cancer resistance protein (BCRP), that is overexpressed in colon, breast, gastric, and ovarian cancer cell lines (Maliepaard, van Gastelen et al. 2001). BCRP

recognizes and transports a large number of anticancer agents (IRI, SN-38, 9 amino camptothecin (9-AC) and topotecan) and increases MDR expression

that mediates efflux of toxic chemical compounds (Ismaili 2011). BCRP is involved in camptothecins resistance and its inhibition by GF120918 (a BCRP inhibitor) overcomes the resistant phenotype in human ovarian tumours (Maliepaard, van Gastelen et al. 2001). BCRP has been described also as a stem cell marker (Nakanishi and Ross 2012).

To sum up, overexpression of certain family ABC transporter proteins such as MRP1, MDR1, and BCRP play crucial roles in the active excretion and efflux of chemical agents involved in CRC therapy and consequently, they are potential targets for modulation of pharmaceutical therapy (Gottesman and Pastan 1993).

17