Mechanism of cell death in Burkitt lymphomas: apoptosis or mitotic catastrophe

Burkitt lymphoma is a fast-growing Non-Hodgkin's Lymphoma that originates from mature B cells. It is generally associated with chromosomal translocations or/and Epstein– Barr virus infection leading to deregulated activation of the c-myc oncogene. Combination chemotherapy with a vincristine-containing regimen is the treatment of choice in Burkitt lymphoma, however, only 50% of the patients respond to treatment due to the high genetic instability, selection of resistant tumor cell subclones and development of clinical resistance to therapy. Apoptosis resistance is considered as the major cause of resistance in Burkitt lymphomas. To elucidate molecular abnormalities that are responsible for resistance, 15 Burkitt lymphoma cell lines were investigated for apoptosis induction upon treatment with the microtubule inhibitors taxol (paclitaxel), nocodazole and vincristine. Interestingly, cell lines, which are highly resistant to apoptosis induction showed development of polyploidy (>4N cellular DNA content) and vice versa, displaying an inverse relationship between apoptosis and polyploidy induction upon treatment with taxol or nocodazole. The underlying mechanism was characterized in three resistant and four sensitive prototypic cell lines. In sensitive cell lines, taxoland nocodazole-induced apoptosis was accompanied by caspase activation, Bid cleavage and Mcl-1 downregulation. In contrast, most apoptotic resistant cell lines exhibited a loss of Bax/Bak and showed prolonged mitotic arrest with >4N DNA content upon treatment. Interestingly, inhibition of apoptosis in sensitive cells by caspase inhibition promoted polyploidy and subsequent death by a mitotic catastrophe confirming the inverse relationship between apoptosis and polyploidisation. To gain mechanistic insights into microtubule inhibitor-induced cell death, the role of the mitotic kinase PLK1 was addressed. PLK1 has multiple functions during the cell cycle and is known to be overexpressed in 80% of human tumors of diverse origins. Elevated levels of PLK1 correlate with poor prognosis for a wide range of human cancers, including Non-Hodgkin Lymphoma. Here, abrogation of PLK1 function in Burkitt lymphomas induces cell cycle checkpoint activation at G2 and M phase. Blocking the function of PLK1, however, did not interfere with cell death induced by the spindle toxins indicating that cell death induced by microtubule disrupting agents does not require coordinated transition through mitosis. Moreover, a dominant negative PLK1 mutant induced apoptosis. Additional treatment with microtubule inhibitors failed to show synergism in induction of apoptosis indicating that PLK1 inhibition and spindle toxins might