Endocrine Abstracts

Introduction: Temozolomide (TMZ) treatment has been used for aggressive pituitary tumours with positive results, but has proven ineffective in controlling tumour regrowth in cases of disease recurrence. TMZ resistance has been linked to the expression of O6-methyl-guanine-DNA-methyltransferase (MGMT) protein and mismatch repair components (MMR). In the present study, we describe the development of an in vitro model of acquired resistance to TMZ and the mechanisms involved in such resistance.

Methods: Mouse AtT20 corticotroph pituitary adenoma cells were used. The TMZ resistance protocol consisted of three challenges of TMZ using a previously determined EC₅₀ (50% of the maximal effect) dose. AtT20 cells treated with challenges of the vehicle dimethyl sulfoxide were used as control.

Results: The first TMZ challenge already induced a significant increase in EC₅₀ (decreased sensitivity) in TMZ-challenged cells (17 μM vs control 3 μM, P<0.001). After 9 weeks following the third TMZ challenge, TMZ-challenged cells remained resistant (18 μM vs control 2 μM, P<0.001). In control cells, TMZ treatment caused an accumulation of cells in G2/M phase (P<0.001), while in TMZ-challenged cells no accumulation was observed (P=0.91). In control cells, 50 μM of TMZ induced maximum 11-fold stimulation of apoptosis (1132±241% vs 100±5%, P=0.0008), whereas in TMZ-challenged cells a significantly lower (2.4-fold) increase in apoptosis was observed (244±55% vs 100±26%, P=0.003). The mRNA expression of MGMT was higher and of MMR components (MSH6, MSH2 and PMS2) was lower in TMZ-challenged cells compared to control cells (P<0.001).

Conclusion: The present study describes an acquired temozolomide resistant cell corticotroph pituitary adenoma model. The TMZ resistance is demonstrated by a sustained increase in TMZ EC₅₀, a lack of cell cycle changes and lower TMZ-induced increase in apoptosis. Acquired TMZ resistance is associated with strongly increased MGMT expression and lowered expression of components of the MMR system.