The goal of this study was to investigate never whether a similar xc?-mediated system was also used by pancreatic cancer cells for growth, survival and drug-related resistance. In the present study, the human pancreatic cancer cell lines MIA PaCa-2, PANC-1, and BxPC-3 were determined to be critically dependent on extracellular cystine for growth (Figure 1). This finding demonstrates that the biochemical cysteine synthesis pathway known as the transsulphuration pathway does not participate in alleviating cystine depletion in the cell lines of this study. Some cell types, such as neurons and astrocytes, do not rely on the xc? transporter for growth (Chung et al, 2005).

As such, these cells may (i) possess a functional transsulphuration pathway, (ii) express other transporters that mediate the transport of cystine or cysteine into the cell, for example, excitatory amino-acid transporters (McBean and Flynn, 2001), or (iii) not express the xc? transporter, but instead rely solely on uptake of extracellular cysteine secreted by certain somatic cells (Gout et al, 2001; Chung et al, 2005; Lo et al, 2008). The potential utility of screening tumour biopsies for the presence of enzymes in the transsulphuration pathway (i.e., for ��-cystathionase) to determine whether a patient may benefit from cystine starvation therapy remains a possibility. Because our results indicate that pancreatic cancer cell lines are dependent on extracellular cystine/cysteine for growth, these cell lines may exhibit sensitivity towards depletion of the amino acid in their environment.

Indeed, at least two of the three cell lines tested showed an inverse correlation between a low extracellular cystine environment (0.01mM) and expression of the xc? transporter, suggesting that these cells can modulate their expression of the xc? transporter to accommodate their growth needs. Consistent with published reports, cell proliferation is strongly associated with cystine/cysteine availability and intracellular GSH levels (Godwin et al, 1992; Noda et al, 2002). Besides manipulating cysteine/cystine levels in the extracellular microenvironment to effect a change in xc? transporter expression, the addition of the oxidative stressor DEM was also used.

In accordance with other studies (Bannai, 1984a; Kim et al, 2001; Hosoya et al, 2002), DEM treatment induced xCT mRNA expression with a corresponding increase in total GSH, indicating that increased intracellular GSH synthesis enables pancreatic cancer cells to survive in the presence of oxidative stress. In human embryonic kidney (HEK) cells, Batimastat xCT expression has been reported at the plasma membrane (Shih and Murphy, 2001). Of interest is the BxPC-3 cell line, which upon treatment with DEM, clearly exhibited localisation of the xCT subunit to the plasma membrane (Figure 3D).