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ABSTRACT
Arsenic can have different roles in human pathology: i) Inorganic arsenic is a known and widespread environmental contaminant responsible for human diseases such as tumors, heart disease and diabetes; ii) in the form of arsenic trioxide (ATO), arsenic is used in cancer therapy, especially for the treatment of acute promyelocytic leukemia and solid tumors such as gliomas.
Human glutathione S-transferase1 (GSTO1) might modulate arsenic citotoxicity by means of two opposite mechanisms. It has been demonstrated that GSTO1 shows monomethylarsonic acid reductase activity, thus producing the most toxic metabolite of arsenic, methylarsonous acid. In this context, some authors have speculated that GSTO1 overexpression may increase the toxicity of ATO, but so far clear evidence has not been provided. On the other hand, it has been shown in the laboratory where I carried out my thesis that GSTO1 overexpression efficiently prevents cisplatin induced apoptosis. Thus the antiapoptotic mechanisms highlighted with cisplatin could also apply to ATO and GSTO1 overexpression could, conversely, protect rather than sensitizeagainst ATO cytotoxicity. In a first set of experiments we have shown that GSTO1 transfection in HeLa cells confers a marked protection against ATO cytotoxicity, suggesting that antiapoptotic effects of GSTO1 may be effective. An opposite result was however obtained with endogenous GSTO1. When the activity spontaneously increased in high-density control HeLa cells, no protection against ATO was observed. Clear protection was observed instead when spontaneously expressed GSTO1 was silenced in these cells with siRNA. It seems that in the case of spontaneous expression, the role of GSTO1 in activating ATO cytotoxicity prevails on its antiapoptotic action. It therefore appears that the two circumstances – transfection vs. spontaneous expression – lead to opposite roles of the enzyme. It is possible that the discrepancy may depend on HeLa cells harbouring one of the already known polymorphisms of GSTO1 or some other mutation, favouring the metabolic properties of GSTO1 over its antiapoptotic effects. Experiments with other cell lines (A2780, U937 and HL60) seem to confirm this interpretation: at high density these cell lines also overexpress GSTO1 and are indeed more resistant to ATO toxicity as compared to low density cells. In addition, GSTO1 silencing in these cells removes ATO resistance, again suggesting that
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the anti-apoptotic role of GSTO1 can prevail when the enzyme is spontaneously overexpressed.
All these data strongly suggest the presence of a mutated form of GSTO1 in HeLa cells. This hypothesis is presently under investigation.
Preliminary experiments have been performed to verify the activation of the main signaling pathways involved in the apoptotic process (JNK, ERK, p38, AKT). A slight increase in ERK, p38 and AKT activation has been observed at 7 hours after treatment in transfected cells, but not in control cells. On the contrary in transfected cells the strong activation of JNK1 induced by ATO at later times (15hrs) was completely prevented.
