Abstract

Background

Argininosuccinate synthase 1 (ASS1) is the rate-limiting enzyme for arginine biosynthesis and is lost in ~50% of all malignant pleural mesotheliomas (MPM). Arginine deprivation can be used as a strategy to selectively kill mesothelioma tumours that lack ASS1. The clinical use of this arginine deprivation therapy (ADI-PEG20) has encountered a significant problem as with time, tumour cells are able to re-express ASS1 and therefore become resistant to the drug. Recent studies from our lab have shown that this resistance leads to alterations in the levels of polyamines biosynthesis enzymes (eg. ODC1, SMOX). A combination treatment with ADI-PEG20 and polyamine enzyme inhibitors might emerge as a novel therapeutic strategy to overcome resistance for the treatment of MPM.

Method

To uncover the molecular biology driving this resistance, we analysed expression of ASS1, ODC1 and SMOX upon treatment of ASS1-negative cells with ADI-PEG20, over time.  Next, we silenced a range of polyamine enzymes individually or in combination in MPM cells and analysed ASS1 expression. We also determined the effect of ADI-PEG20 upon knockdown of the polyamine enzymes in both sensitive and resistant cell lines.

Results

We have shown for the first time that, when upon ADI-PEG20 treatment, ASS1-deficient cells initially induce expression of polyamine enzymes. This is then followed by ASS1 re-expression and an increase of cell proliferation. Furthermore, we show that upon silencing of ODC1 or SMOX the levels of ASS1 expression is increased, in ASS1-deficient cells. Our results suggest that inhibition of polyamine enzymes upon ADI-PEG20 treatment induced ASS1 re-expression in ADI-PEG20-resistant cells. Our results also show that the addition of the polyamine metabolites, Spermidine or Spermine to the cells, after ODC1/SMOX silencing, reverses ASS1 re-expression.

Conclusion

Taken together, these results give insight into how resistance to arginine deprivation occurs in ASS1-deficient cells and how a therapeutic combination might overcome this resistance.