Nuclear localization of Beclin 1 promotes radiation-induced DNA damage repair independent of autophagy

Abstract

Beclin 1 is a well-established core mammalian autophagy protein that is embryonically indispensable and has been presumed to suppress oncogenesis via an autophagy-mediated mechanism. Here, we show that Beclin 1 is a prenatal primary cytoplasmic protein but rapidly relocated into the nucleus during postnatal development in mice. Surprisingly, deletion of beclin1 in in vitro human cells did not block an autophagy response, but attenuated the expression of several DNA double-strand break (DSB) repair proteins and formation of repair complexes, and reduced an ability to repair DNA in the cells exposed to ionizing radiation (IR). Overexpressing Beclin 1 improved the repair of IR-induced DSB, but did not restore an autophagy response in cells lacking autophagy gene Atg7, suggesting that Beclin 1 may regulate DSB repair independent of autophagy in the cells exposed to IR. Indeed, we found that Beclin 1 could directly interact with DNA topoisomerase IIβ and was recruited to the DSB sites by the interaction. These findings reveal a novel function of Beclin 1 in regulation of DNA damage repair independent of its role in autophagy particularly when the cells are under radiation insult.

Autophagy is required for lung development and morphogenesis

“…Ventilation (73) and oxygen (74) have been shown to induce autophagy results in decreased NKX2-1 expression. Nuclear translocation of BECN1 indicates a role for BECN1 in regulating DNA damage repair (70) and autophagic growth control (71); however, an interaction between BECN1 and NKX2-1 during lung development remains to be explored. Many of the NKX2-1 + cells lining the primitive and dilated air sacs in E18.5 Becn1 Epi -KO mice were bipotential alveolar progenitor cells, based on double-positive SFTPC + RAGE + and triple-positive HOPX + SFTPC + PNPD + staining.…”

Monitoring Autophagy Immunohistochemically and Ultrastructurally during Human Head and Neck Carcinogenesis. Relationship with the DNA Damage Response Pathway

“…Furthermore, as stated earlier, nuclear p62 (Nuc-p62) normally participates in the recruitment of polyubiquitinated nuclear proteins or protein aggregates to PML bodies for proteasomal degradation [ 21 ]. All other expression patterns of the triad Beclin-1/LC3B/p62, cytoplasmic or nuclear, were regarded abnormal ( Figure 3 and Table 1 ), most likely reflecting dysfunctional autophagic flux or implying non-autophagic activity, such as that of nuclear Beclin-1 regulating DSB repair via topoisomerase-IIβ under conditions of cellular stress [ 27 ] (see also Introduction).…”

“…Although Beclin-1, LC3 and p62 are considered to be principally cytosolic proteins, they are also localized in the nucleus [ 20 , 21 , 22 ] ( Figure 1 and Figure 2 ). In both cellular compartments these factors have been shown to exert autophagy, as well as non-autophagy related functions [ 23 , 24 , 25 , 26 , 27 ]. Specifically, the presence of Beclin-1 in the nucleus and its transport to the cytoplasm is tightly linked to its autophagic function and tumor-suppressive role.…”

“…Prerequisite for the nucleocytoplasmic shuttling of Beclin-1 is the presence of an intact and functional nuclear export signal (NES) [ 22 ]. Moreover, it has been recently shown that the nuclear localization of Beclin-1 in stressed cells with functional autophagy is associated with the regulation of DNA double-strand break (DSB) repair, irrespectively of its autophagic function, by interacting with DNA topoisomerase IIβ and by recruiting it to DSB sites, ensuring genomic integrity [ 27 ]. On the other hand, the nuclear pool of LC3 serves as a principal source of membrane-conjugated LC3 on autophagosomes, after its translocation to the cytoplasm.…”

A novel transcriptional signalling pathway mediated by the trafficking protein Ambra1 via scaffolding Atf2 complexes

“…Like Ambra1, other proteins involved in autophagy have been reported in the nucleus, e.g. LC3B binds to Lamin B1, mediating the degradation of the nuclear lamina and Beclin 1, promoting autophagy-independent DNA damage repair (46,47). No typical nuclear localisation sequence is evident for Ambra1, and hence the mechanism of nuclear translocation is unknown; however, as nuclear Ambra1 interacts with components of nuclear pore complexes and importins ( Figure 1C), it is perhaps likely that nuclear import of Ambra1 occurs via binding to these in some way.…”