Apoptosis induced by DNA damage: Regulation of the response to specific alkylated bases.

다국어 초록 (Multilingual Abstract)

The alkylating agents are a group of DNA damaging agents that are important to human health because they can create cytotoxic, mutagenic and carcinogenic DNA damage. Moreover, they are found ubiquitously in the environment and within cells, and are used extensively in cancer chemotherapy. Among the various DNA lesions formed by these agents, <italic>O</italic><super>6</super>-alkylguanine (<italic>O</italic><super>6</super>AlkG) lesions are highly cytotoxic and mutagenic, and they turn out to be potent activators of apoptosis. Treating cells with certain alkylating agents triggers apoptosis in part through the formation of <italic>O</italic><super>6</super>AlkG lesions because expression of the <italic>O</italic><super>6</super>-methylguanine (<italic>O</italic><super> 6</super>MeG) DNA methyltransferase (MGMT) protein, which specifically repairs these lesions, can suppress the apoptosic response.
Here, the regulation of the apoptotic response to two such lesions, <italic> O</italic><super>6</super>MeG and <italic>O</italic><super>6</super>-chloroethylguanine (<italic>O</italic><super>6</super>CEG), was examined. This work was significant in part because the response to specific types of DNA damage could be studied. The DNA mismatch repair (MMR) pathway was known to mediate the cytotoxicity of methylating agents, and it was found here that MutSα, which is responsible for recognizing single base mismatches and initiating MMR, is absolutely required for signaling the initiation of apoptosis in response to <italic>O</italic><super> 6</super>MeGs, and partially required for signaling apoptosis in response to <italic>O</italic><super>6</super>CEGs. However, MutSα, does not mediate apoptosis in response to all DNA lesions, including another methylated base, 3-methyladenine. <italic>O</italic><super>6</super>MeG lesions signal the stabilization of the p53 tumor suppressor and such signaling is also MutSα-dependent. Despite this, MutSα-dependent apoptosis can be executed in a p53-independent manner, albeit slightly delayed. This suggests the importance of a p53-independent pathway, and a potential role for the p53 family member, p73, in this pathway will be discussed.
<italic>O</italic><super>6</super>MeG triggers activation of caspases, cysteine proteases that effect the apoptotic program, and this is also MutSα-dependent. However, the caspases are only partially required for the apoptotic response, indicating the importance of caspase-independent mechanisms. The mitochondrial pathway regulates the majority of <italic>O</italic><super>6</super>MeG-induced apoptosis since expression of Bcl-2 or Bcl-x_L, which can each block this pathway, dramatically diminishes the response. On the other hand, preventing death receptor signaling had no effect on <italic>O</italic><super>6</super>MeG-triggered apoptosis. Surprisingly, the anti-apoptotic Bcl-2 and Bcl-x_L proteins do not prevent overall cell death triggered by <italic>O</italic><super>6 </super>MeG, suggesting that apoptosis is not always required for the cytotoxicity of DNA damage. (Abstract shortened by UMI.).