Effect of Hypoxia on the Expression of BRIT1 in K562 Cell Line: Implication for Resistance to Imatinib.

BRIT1 (Microcephalin, MCPH1) is a chromatin binding protein that forms ionizing irradiation-induced nuclear foci (IRIF); it is a crucial DNA damage regulator in the ATM/ATR pathways. Here we have analyzed BRIT1 expression on K562 cell line after treatment with imatinib 0,1-1 μM, UV irradiation (12 μJ/cm²) and after exposure to an hypoxic condition (3% O²). K562 in standard culture conditions (21% O²) were used to establish BRIT1 basal expression. Total RNA from K562 cultures was prepared at 24 and 48 hs. 1 μg total RNA was reverse transcribed from each sample. TaqMan Gene expression assay (Applied Biosystems) in Real-Time PCR was employed to examine the expression of BRIT1. Cellular count was assayed by trypan blue. Treatment with IM (0,1 and 1 μM) and UV induced a 2–3 fold increased expression of BRIT1 at 24 h and a minor increase at 48 h. Cell mortality at 24 h was 30% for IM and 20% for UV while at 48 h cells started to grow again, possibly as a consequence of the DNA damage repair by BRIT1. Exposure of K562 cells to hypoxia increased BRIT1 expression about 5- and 6-folds at 24 and 48 h respectively. Cell mortality at 24 h was 30% while after 48 h it was 20%. We therefore concluded that BRIT1 expression increases in hypoxia more than after IM and UV exposure. Since we have found that K562 in hypoxic conditions are relatively resistant to imatinib, (K562 cell mortality after 24 h exposure to IM 1μM was 30% ± 0,3 in normoxia vs 9% ± 0,8 in hypoxia, P<0,005) we have hypothesized that BRIT1 could be involved in resistance to IM. Therefore, we exposed K562 to UV and, after 48 h we incubated the cells with imatinib 1 μM. Pre-exposure of K562 cells to UV reduced imatinib-induced mortality (9,7% ± 1,6 vs 21,3% ± 1,5 of control, p< 0,005) thus indicating that upregulation of BRIT1 (as we have observed especially in hypoxia) could contribute to resistance to imatinib in K562 cell line. Understanding of BRIT1 function may well contribute to novel therapeutic approaches for cancer.