Heterogeneity of MGMT expression in corticotroph adenomas and mechanism of acquired temozolomide resistance.

CONTEXT: Temozolomide (TMZ) can be an effective medical treatment for aggressive pituitary tumors. In case of disease recurrence following TMZ treatment, however, treatment with the drug generally does not control tumor regrowth.

OBJECTIVE

This work aimed to better understand the mechanisms of resistance of corticotroph tumors to TMZ in the context of heterogeneity of methylguanine-DNA methyltransferase (MGMT) expression.

METHODS

We performed immunohistochemical analysis of the MGMT expression pattern in 25 corticotroph tumors to evaluate intratumoral heterogeneity. In addition, we created in vitro models of AtT20 corticotroph tumor cells with acquired TMZ-resistance after exposure to high- and low-dose TMZ, the latter representing a clinically achievable TMZ level.

RESULTS

MGMT immunostaining in corticotroph tumors showed a considerable heterogeneous intertumoral and intratumoral distribution pattern in 80% of tumors. In the in vitro model, high- and low-dose TMZ challenges induced a 6.3- and 3.4-fold decreased sensitivity to the growth inhibitory effect of TMZ. TMZ-induced changes in cell cycle phases were lower in TMZ-resistant cells than in vehicle-challenged cells. TMZ-resistant cells had higher Mgmt messenger RNA and protein expression and 1.8-fold higher number of Mgmt-positive cells. No difference was observed in the level of Mgmt promoter methylation.

CONCLUSION

Corticotroph pituitary tumors demonstrate a high intertumoral and intratumoral heterogeneity in MGMT expression. In an acquired TMZ-resistant corticotroph pituitary tumor cell model, TMZ resistance was associated with strong increase in MGMT expression and percentage of MGMT-positive cells. We hypothesize that clonal selection of high MGMT-expressing cells is involved in this acquired TMZ resistance.