Macroautophagy-aided elimination of chromatin: sorting of waste, sorting of fate?

Abstract

How tumor cells process damaged or unwanted DNA is a matter of much interest. Recently, Rello-Varona et al. (Cell Cycle 2012; 11:170–76) reported the involvement of macroautophagy (hereon autophagy) in the elimination of micronuclei (MN) from osteosarcoma cells. Prior to that, diminution of whole nuclei from multinucleated TP53-mutant tumor cells was described. Here, we discuss these two kinds of chromatin autophagy evoked after genotoxic stress in the context of the various biological processes involved: (1) endopolyploidy and the ploidy cycle; (2) the timing of DNA synthesis; (3) DNA repair; (4) chromatin:nuclear envelope interactions; and (5) cytoplasmic autophagy. We suggest that whereas some MN can be reunited with the main nucleus (through interactions with envelope-limited chromatin sheets) and participate in DNA repair, failure of repair serves as a signal for the chromatin autophagy of MN. In turn, autophagy of whole sub-nuclei in multi-nucleated cells appears to favor de-polyploidization, mitigation of aneuploidy with its adverse effects, thereby promoting the survival fitness of descendents and treatment resistance. Thus, both kinds of chromatin autophagy provide tumor cells with the opportunity to repair DNA, sort and resort chromatin, reduce DNA content, and enhance survival.

Figure 1. (A and B) Characterization of the association between MN, homologous recombination repair and DNA integrity in TP53 mutant lymphoma cells after γ-irradiation insult. (A) A polyploid nucleus undergoing intensive DNA repair by HR (testified by multiple repair foci positive for Rad51 and γH2AFX/γH2AX). A MN (arrow) is being released which involves a large repair focus. The corresponding DAPI gray-scale image (insert below) shows no loss of DNA content in this MN. (B) A polyploid lymphoma cell undergoing both DNA repair by HR (as testified by the presence of multiple Rad52-positive foci) and simultaneous release of two MN (arrows) containing large aggregates of Rad52 and degrading DNA, as seen by the reduced DAPI content (shown in the insert to the left); (C and D) release of whole sub-nuclei from polyploid lymphoma cells showing (C) selective chromatin degradation of sub-nuclei seen using the acridine orange in situ denaturation test [where red fluorescence indicates degraded DNA (arrowed) and green fluorescence indicates intact DNA] and on (D) sequestration of large amounts of DNA (DAPI stained blue) in a perinuclear vacuole accumulating CTSB/Cathepsin B, indicative of autophagy (red, arrowed). Bars: 20 μm. Figures are republished with new annotation: (A) from reference and (C) from reference , with permission of Elsevier; (B and D) were originally published in reference , copyright holder Portland Press.

Figure 2. The relationship between MN, NE, ELCS, DNA repair and chromatin autophagy in endopolyploid cells undergoing genotoxic stress. (A) A MN with normal chromatin structure is linked to the nucleus (NU) by ELCS. (B) In the nuclear pocket (NP) the dark organelles with convoluted membranes represent the residual body (RB), which usually results from autophagic lysosomal activity. The sequestration by a double membrane and the presence of a multivesicular body (MvB) nearby are also indicative of cytoplasmic autophagy within the NP; Nu, nucleus; Cy, cytoplasm. (C) Extrusion of a large membrane enclosed MN containing degraded chromatin via the nuclear pore. Note the sequestration of the cytoplasmic territory around it by a double membrane (arrows) and assembly of activated mitochondria nearby (asterisk). Bars: 1 μm. Figures are republished with new annotation: (A and B) from reference , with permission of Springer and (C) from reference , with permission of Elsevier. (D) A schematic showing a cross-section of a nucleus undergoing micronucleation and chromatin autophagy. Interphase chromosomes are joined to the NE by heterochromatin rows attached to the LBR of the inner nuclear membrane, which form the chromatin band of the ELCS. The left micronucleus (MN) is connected to the nucleus by ELCS and is not autophagic; it can be reunited with the nuclear DNA or alternatively form an ELCS nuclear pocket bridging to the nucleus at another site. The process may favor the search for homology for recombination repair of double-strand breaks in the foci (HRR). The cytoplasmic content of the nuclear pocket often undergoes autophagy (Au-NP) (designated by sequestration of the double membrane fusing with a lysosome). If DNA repair has failed (and DNA remains fragmented), a signal for budding of autophagic MN (Au-MN) may be obtained from unbound repair factors (illustrated as free rings OOO) for execution of selective autophagy (designated by sequestration of the double membrane fusing with a lysosome).