Induction of BCL2 family member MCL1 as an early response to DNA damage

When ML-1 human myeloid leukemia cells are exposed to DNA damaging agents, they exhibit dramatic changes in the expression of a variety of gene products. This includes an increase in p53 (wild-type), a decrease in BCL2, a p53-dependent increase in the BCL2 family member BAX, and increases in Growth Arrest and DNA Damage-inducible (GADD) genes such as GADD45; these changes occur as early events in a sequence that culminates in DNA damage-induced apoptosis. DNA damaging agents have now been tested for effects on expression of another BCL2 family member, MCL1, a gene expressed during ML-1 cell differentiation. Expression of MCL1 was found to increase upon exposure of ML-1 cells to various types of DNA damaging agents, including ionizing radiation, ultraviolet radiation, and alkylating drugs. The increase in MCL1 occurred rapidly and was transient, levels of the MCL1 mRNA being elevated within 4 h and having returned to near baseline within 24 h. An increase in the Mcl1 protein was also seen, with the maximal increase occurring at an intermediate dose of IR (5 Gray) and lesser increases occurring at either lower or higher doses. The increase in expression of MCL1 was further studied using a panel of human cell lines that includes cells containing or not containing alterations in p53 as well as cells sensitive or insensitive to the apoptosis-inducing effects of DNA damage. The DNA damage-induced increase in MCL1 mRNA did not depend upon p53 as it was seen in cells lacking functional p53. However, the increase did depend upon susceptibility to apoptosis as it was not seen in cells insensitive to apoptosis-induction by DNA damaging agents. These findings demonstrate that cytotoxic DNA damage causes an increase in the expression of MCL1 along with increases in GADD45 and BAX and a decrease in BCL2. Furthermore, while the increase in GADD45 is seen both in cells that undergo growth arrest and in cells that undergo apoptosis in response to DNA damage, alterations in the profile of expression of BCL2 family members occur exclusively in cells that undergo the apoptotic response, with some family members increasing through p53-dependent (BAX) and others through p53-independent (MCL1) pathways. Overall, expression MCL1 can increase during the induction of cell death as well as during the induction of differentiation.