DNA repair proteins appear to play a significant role in both the development and progression of renal cell carcinoma, as has been discussed in earlier blog posts from 2010 and 2011. These proteins are responsible for repairing DNA lesions caused by a wide range of agents, such as the alkylating chemical N-Nitroso-N-methylurea (MNU). When damaged DNA cannot be repaired, cells are eliminated by apoptosis to prevent the propagation of potentially deleterious lesions. This process is of particular relevance to BHD syndrome as Komori et al. (2009) found that MAPO1 (which is commonly known as FNIP2) is involved in MNU-induced apoptosis. This finding has now been taken further by work in the same laboratory which suggests that FLCN and AMPK, in addition to FNIP2, are also linked to the induction of apoptosis by MNU (Lim et al., 2011).
Co-immunoprecipitation experiments using mouse YT102 Mgmt-/- cells, which lack the enzyme MGMT that repairs alkylated DNA, confirmed that FNIP2 interacts with both FLCN and AMPK. The authors then showed that siRNA knockdown of FLCN and AMPKα in this cell line suppressed their apoptotic response to MNU when compared to siRNA controls. They subsequently used compound C (a specific inhibitor of AMPK) to show that it also suppressed MNU-induced apoptosis when compared to untreated YT102 cells. AMPK is usually activated after MNU treatment, which is associated with an increased level of AMPKα phosphorylation. However, less of this activation was observed after compound C treatment.
This result was investigated further by treating YT102 cells with MNU, and assessing the levels of phosphorylated AMPKα over 72 hours. Here, Lim et al. (2011) saw that the levels of phosphorylated AMPKα gradually increased over time, but that this increase was abrogated by the siRNA-mediated knockdown of FLCN. In addition, KH101 Mgmt-/- Fnip2+/- cells, which were shown to be resistant to MNU-induced apoptosis by Komori et al. (2009), also showed no increase in phosphorylated AMPKα after MNU treatment.
To confirm that AMPK activation is involved in the induction of apoptosis, AICAR (a specific activator of AMPK) was added to YT102 and KH101 cells. This treatment increased the levels of AMPKα phosphorylation, as well as the amount of cell death detected by trypan blue staining in YT102 cells. However, no such increases were observed in the FNIP2-defective KH101 cells after treatment. To see if this cell death was attributable to apoptosis, mitochondrial membrane depolarisation (which is known to occur during apoptosis) was measured in these cells, and it could be seen that this depolarisation took place in the YT102 cells (but not the KH101 cells) after the addition of AICAR. Similar results were obtained with YT102 cells treated with AICAR and FLCN-siRNA, where less cell death, mitochondrial membrane depolarisation and AMPKα phosphorylation were observed when compared to siRNA controls.
Together, these results suggest that FLCN and FNIP2 play a role in AMPK activation and the induction of apoptosis by MNU. However, the exact mechanism by which this occurs is still unknown, and the elucidation of this process should help us to further understand the pathophysiology of BHD syndrome.
- Komori K, Takagi Y, Sanada M, Lim TH, Nakatsu Y, Tsuzuki T, Sekiguchi M, Hidaka M. A novel protein, MAPO1, that functions in apoptosis triggered by O6-methylguanine mispair in DNA. Oncogene. 2009 Feb 26;28(8):1142-50. PMID: 19137017
- Lim TH, Fujikane R, Sano S, Sakagami R, Nakatsu Y, Tsuzuki T, Sekiguchi M, Hidaka M. Activation of AMP-activated protein kinase by MAPO1 and FLCN induces apoptosis triggered by alkylated base mismatch in DNA. DNA Repair (Amst). 2011 Dec 29 [Epub ahead of print]. PMID: 22209521