Cellular responses to gamma-irradiation exposure are controlled by
phosphatidylinositol 3-kinase-related
kinases (PIKK) in the nucleus, and in addition, cytosolic PIKKs may have a role in such responses. Here, we show that the expression of
tripeptidyl-peptidase II (TPPII), a high molecular weight cytosolic
peptidase, required PIKK signaling and that TPPII was rapidly translocated into the nucleus of gamma-irradiated cells. These events were dependent on
mammalian target of rapamycin, a cytosolic/mitochondrial PIKK that is activated by gamma-irradiation.
Lymphoma cells with inhibited expression of TPPII failed to efficiently stabilize p53 and had reduced ability to arrest proliferation in response to gamma-irradiation. We observed that TPPII contains a BRCA COOH-terminal-like motif, contained within sequences of several
proteins involved in DNA damage signaling pathways, and this motif was important for nuclear translocation of TPPII and stabilization of p53. Novel tripeptide-based inhibitors of TPPII caused complete in vivo
tumor regression in mice in response to relatively low doses of gamma-irradiation (3-4 Gy/wk). This was observed with established mouse and human
tumors of diverse tissue backgrounds, with no
tumor regrowth after cancellation of treatment. These TPPII inhibitors had minor effects on
tumor growth as single agent and had low cellular toxicity. Our data indicated that TPPII connects signaling by cytosolic/mitochondrial and nuclear PIKK-dependent pathways and that TPPII can be targeted for inhibition of
tumor therapy resistance.