Inducer-and cell type-specific regulation of antiapoptotic MCL1 in myeloid leukemia and multiple myeloma cells exposed to differentiation-inducing or microtubule-disrupting agents.

The antiapoptotic BCL2 family member MCL1 is rapidly upregulated upon exposure of ML-1 myeloid leukemia cells to either differentiation-inducing phorbol 12'-myristate 13'-acetate (PMA) or chemotherapeutic microtubule disrupting agents (MTDAs). This report examined how signaling for MCL1 upregulation is coupled to these two different phenotypic changes, and tested for upregulation in other hematopoietic cancers. With PMA, ERK stimulated MCL1 mRNA expression and ML-1 cell differentiation, and ERK additionally stabilized expression of the MCL1 protein. However, with MTDAs, transient ERK and ensuing JNK activation contributed to initial MCL1 upregulation and viability-retention, but sustained JNK activation eventually resulted in cell death. MCL1 was upregulated by PMA in THP-1 and U937 myeloid leukemia cells, but by MTDAs only in THP-1 cells. MCL1 expression was constitutively elevated in multiple myeloma cell lines, and was not affected by PMA/ERK or MTDAs. Thus, MCL1 expression level and sensitivity to regulation are important considerations in selecting approaches for targeting this antiapoptotic gene product to kill cancer cells.

An MCL1-overexpressing Burkitt lymphoma subline exhibits enhanced survival on exposure to serum deprivation, topoisomerase inhibitors, or staurosporine but remains sensitive to 1-beta-D-arabinofuranosylcytosine.

Members of the BCL2 gene family influence cell viability and can, therefore, affect the susceptibility of cancer cells to multiple chemotherapeutic agents. Thus, it is a challenge to devise approaches for inducing the death of tumor cells in which the expression of prosurvival family members is elevated or deregulated. BL41-3, a spontaneously derived subline of BL41 Burkitt lymphoma cells, was found to have amplified the prosurvival MCL1 gene (3-fold) and overexpressed the MCL1 protein. The level of MCL1 protein was 5-fold elevated compared with ML-1 cells expressing maximal MCL1 on exposure to phorbol-12-myristate-13- acetate. To assess whether this increase in MCL1 expression was associated with enhanced protection from cell death, cells were exposed to conditions of growth factor deprivation or to various cytotoxic agents. Whereas BL41-3 and BL41 cells exhibited similar growth rates in logarithmic phase, BL41-3 cells remained largely viable on reaching saturation phase in contrast to BL41 cells, which began to die. Similarly, the BL41-3 subline remained viable for an extended period under conditions of reduced serum. BL41-3 cells were also more resistant to the apoptosis-inducing effects of etoposide, camptothecin, and staurosporine (>3-fold more than BL41 cells). Unexpectedly, these cells exhibited enhanced sensitivity to 1-beta-D-arabinofuranosylcytosine, but only on exposure for an extended period (>10-fold more sensitive than BL41 cells with a 24-h but not a 6-h exposure). Thus, whereas cells expressing prosurvival BCL2 family members are frequently resistant to a variety of chemotherapeutic agents, the findings presented here, using a cell line exhibiting amplification and overexpression of MCL1, indicate that such cells may exhibit increased sensitivity to certain chemotherapeutic regimens.