Resistance to lithium-induced apoptosis in a lithium tolerant clone of HL-60 promyelocytes

It is well documented that lithium, at concentrations of 10 mM and above, can cause apoptosis in HL-60 promyelocytic leukemia cells in vitro. However, the cellular mechanism underlying this action has not been determined. To assist in the elucidation of the biochemical modes of action of lithium, a lithium resistant sub-clone of HL-60 (HL-60 LiR) was developed. HL-60 cells were gradually exposed to increasing concentrations of lithium until they proliferated normally and remained more than 95% viable in media supplemented with 12.5 mM lithium (LiR). These LiR cells showed a marked increase in their ability to survive at 20 mM lithium when compared to «normal» HL-60 cells. Preliminary characterisation of apoptosis related gene expression by immunocytochemistry indicated that HL-60 LiR cells expressed increased levels of the anti-apoptotic gene product, Bcl-2, than normally grown HL-60's. After treatment with 20 mM lithium for 2 days, this expression was reduced and the levels of the pro-apoptotic gene product, Bax, were enhanced in both subclones. The observed changes in expression of Bcl-2 and Bax were much more pronounced in normal HL-60's compared to HL-60 LiR's. Expression of Retinoblastoma and p53 proteins was also enhanced after 20 mM lithium treatment in both HL-60 and HL-60 LiR, but the expression was less evident in the lithium resistant cells. It would appear as if lithium induces apoptosis in HL-60 cells through typical apoptotic mechanisms which are characterised by increased levels of Bax and decreased levels of Bcl-2 and that the observed lithium tolerance of the HL-60 LiR cells is at least partially due to increased expression of bcl-2.