more sensitive than columnar primitive
cells to the noxious effects resulting
from the gastroesophageal reflux andchronic inflammatory condition, which
likely trigger Barrett’s esophagus (Herfset al., 2009)?
If correct, the model presented by
Wang and colleagues would change theview of metaplasia, shifting the focus ofattention from ‘‘after’’ to ‘‘before thefact.’’ For further ‘‘conjectures and refutations,’’ we refer to the authors’ interesting
discussion of the ‘‘competitive or opportunistic cell replacement mechanism,’’
which may be at the basis of Barrett’sesophagus and other metaplastic conditions with increased cancer risk.

Koster, M.I., and Roop, D.R. (2007). Annu. Rev.
Cell Dev. Biol. 23, 93–113.

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Stress Signaling Etches
Heritable Marks on Chromatin
Haruhiko Siomi1,* and Mikiko C. Siomi1
1Department of Molecular Biology, Keio University School of Medicine, 35 Shinanomachi, Shinjuku-ku, Tokyo 160-8582, Japan*Correspondence: awa403@z2.keio.jp
DOI 10.1016/j.cell.2011.06.009

Transgenerational inheritance of epigenetic states allows organisms to pass on adaptive responses
to the environment to their offspring. Seong et al. (2011) now reveal how stress-induced signaling
through dATF-2 disrupts heterochromatin and leaves heritable marks that influence patterns ofgene expression in subsequent generations.
The packing of DNA into condensed andinaccessible heterochromatin exerts a
powerful influence on the genome, notonly by suppressing transcription but
also by governing proper segregation ofchromosomes during cell division,
silencing transposons and suppressing
illegitimate recombination (Grewal and Elgin, 2007; Moazed, 2009). Heterochromatin nucleation involves the parallel
actions of both the RNA interference(RNAi) machinery and sequence-specificDNA-binding proteins, and changes in
the activity of either of these pathways
provide a window of opportunity for chromatin states to change without corresponding alterations in DNA sequence.
Such epigenetic changes, if passed to
the next generation, may add an almost
intangible layer of phenotypic variation atan organismal level. Although much attention has focused on how environmental
stresses initiate ‘‘a highly programmedsequence of events within the cell thatserve to cushion the effects of the shock’’(McClintock, 1984), we are only now
beginning to learn about how stressmodifies chromatin structure, andwhether such stress-induced chromatin
modification is heritable. In this issue,
Seong et al. (2011) show that stressinduced phosphorylation of the transcription factor dATF-2 triggers the loss ofheterochromatin structures at several
regions of the genome in Drosophila, andthat the disrupted heterochromatin stateis transmitted to the next generation ina non-Mendelian fashion.
Previous work in the fission yeast
Schizosaccharomyces pombe revealed

that the RNAi pathway targets histone H3lysine 9 (H3K9) methylation and the Swi6
protein (a homolog of animal HP1 proteins)to repetitive DNA sequences for heterochromatin assembly at the centromeres(Volpe et al., 2002). However, in RNAimutants, heterochromatin assembly canstill occur at low efficiency. The transcription factors Atf1 and Pcr1, which belong to
the activating transcription factor/cAMP
response element-binding protein (ATF/CREB) family of proteins, bind to theirrecognition sequences and cooperatewith chromatin-modifying complexes
such as SHREC (SNF2- and histone deacetylase-containing repressor complex)
to nucleate and spread heterochromatin
assembly independently at the centromeres (Jia et al., 2004). Notably, Atf1
activity is regulated by stress-activated

Cell 145, June 24, 2011 ª2011 Elsevier Inc. 1005

protein kinases (SAPKs), andtivelyregulatedATF-2it has been long known that
dependent heterochromatinstress induces defects information.
heterochromatin assembly
The authors found that the(Jia et al., 2004 and refereffects of stress on heterochroences therein).
matic reporter genes can beThe Drosophila homologtransmitted through the germ(dATF-2) of the fission yeastline. However, the phenotypes
Atf1 binds to the cyclic AMP
examined (eye color and wing
response element (CRE) andnotches) persist to a lesser
is phosphorylated by SAPKs
extent in successive generasuch as p38. Seong andtions and disappear unlesscolleagues (2011) now takethe stress is repeated. Intriguadvantage of position effectingly, transgenerational derevariegation (PEV) of whitepression of the white gene
gene silencing as a read-out
occurs even when unstressed
to determine whether dATFanimals harboring the white
2 contributes to heterochrogene are mated with stressedmatin formation in fly. Whenanimals, suggesting that transa gene is juxtaposed closemission of stress-induced
to heterochromatin, the inacdisruption of heterochromatin
tivating influence of theoccurs by transcommunicaheterochromatin can spreadtion between chromosomes.
Figure 1. Stress-Induced Regulation of Heterochromatin Formation
to affect the gene in some,This is reminiscent of paramuby dATF-2
but not all, cells of the sametation,thetemperature(A) In low stress conditions, dATF-2 specifically binds to its recognition
cell type. In the white PEV
sensitive process by which
sequence CRE (cyclic AMP response element) and likely cooperates with HP1
and chromatin-modifying complexes containing a histone methyltransferase
system, variable spread ofhomologous DNA sequences
to nucleate and spread heterochromatin assembly.
pericentric heterochromatin
communicate in trans to(B) Upon heat shock or osmotic stress, dATF-2 is phosphorylated by stressgives rise to mottled patterns
establish meiotically heritable
activated protein kinases including p38, which in turn leads to release ofof eye pigmentation in which
expression states (Chandler,phosphorylated dATF-2 from the heterochromatin region. This results in thedisruption of heterochromatin and then increases the expression of genes
some cells express the white2010). Genetic screens inlocated in the region. This epigenetic change is heritable and occurs byplants for mutants unable togene and are therefore red,transcommunication between chromosomes.
undergo paramutation identiwhereas others do not andare therefore white. The
fied several genes withauthors found that loss of function of reveal that osmotic stress reduces the homology to genes that mediate RNAdATF2 induces derepression of the white levels of H3K9me2 at heterochromatic directed transcriptional silencing, suggestgene in the PEV assay system. Indeed, sites bound to dATF-2 but not those ing that the underlying mechanisms
dATF-2 colocalizes with HP1 on hetero- bound to a mutant dATF-2, in which the regulating paramutation may be sharedchromatin and binds to CREs in hetero- p38 phosphorylation sites (two threo- with those of RNAi.
chromatin. Together, these data show nines) were replaced by alanines, sugFinally, to examine ‘‘natural’’ target
that CRE-dependent association of gesting that disruption of heterochro- genes of heterochromatin that are heritdATF-2 with chromosomes correlates matin by stress is mediated by dATF-2 ably affected by stress, the authors perwith heterochromatin formation.
phosphorylation. The authors also show formed a microarray analysis and foundTo determine whether environmental that heat shock during early develop- that a number of genes involved instresses such as heat shock or osmotic ment (0–3 hr after egg laying), when various biological functions were upregustress could alter the binding of dATF-2 heterochromatin is established, disrupts lated in successive generations of stressto CREs, and whether such alterations heterochromatin formation, which corre- treated flies. We now know that forced
lead to changes in chromatin state, the lates with the release of phosphorylated changes to the transcriptome dramatiauthors examined the phosphorylation dATF-2 from heterochromatic CREs. cally alter cell fate (for example, inof dATF-2 by SAPKs. Heat shock and This disruption is independent of RNAi- induced pluripotent stem cells; Takahashi
osmotic stress induce the phosphoryla- based mechanisms. Based on these and Yamanaka, 2006). It will be intertion of dATF-2, which correlates with data, Seong and colleagues (2011) pro- esting to discover what happens to thea reduction in both the binding of pose a new model for how stress inheritance of the new epigenetic statedATF-2 to CREs located in the pericen- induces disruption of heterochromatin: after multiple stress treatments intromeric heterochromatin and levels of namely, dATF-2 is required for hetero- successive generations. The work byH3K9 dimethylation at these heterochro- chromatin formation, as in the case for Seong and colleagues (2011) opens thematic sites (Figure 1). Chromatin immu- its fission yeast homolog Atf1, and p38 door to studies of how the environment
noprecipitation assays in cultured cells and its upstream kinase Mekk1 nega- could influence germ cells in such a way
1006 Cell 145, June 24, 2011 ª2011 Elsevier Inc.

that epigenetic states and thereby transcriptomes are altered in a directed andheritable way. The work presents aninteresting example of the Lamarckian
paradigm (Martienssen, 2008; Kooninand Wolf, 2009), in which the environment directs evolution, and inducedchanges in the epigenetic state may inturn create variations in gene expression
to be explored, selected for, and coopted through the generations.

REFERENCES

McClintock, B. (1984). Science 226, 792–801.

Chandler, V.L. (2010). Science 330, 628–629.

Moazed, D. (2009). Nature 457, 413–420.

Grewal, S.I.S., and Elgin, S.C. (2007). Nature 447,399–406.

Seong, K.-H., Li, D., Shimizu, H., Nakamura, R.,and Ishii, S. (2011). Cell 145, this issue, 1049–1061.

Jia, S., Noma, K., and Grewal, S.I.S. (2004).
Science 304, 1971–1976.
Koonin, E.V., and Wolf, Y.I. (2009). Biol. Direct 4, 42.
Martienssen, R. (2008). New Physiologist 179,572–574.

Takahashi, K., and Yamanaka, S. (2006). Cell 126,663–676.
Volpe, T.A., Kidner, C., Hall, I.M., Teng, G., Grewal,S.I., and Martienssen, R.A. (2002). Science 297,1833–1837.

Stuck in the Middle: Drugging
the Ubiquitin System at the E2 Step
J. Wade Harper1,* and Randall W. King1,*1Department of Cell Biology, Harvard Medical School, Boston, MA 02115, USA*Correspondence: wade_harper@hms.harvard.edu (J.W.H.), randy_king@hms.harvard.edu (R.W.K.)
DOI 10.1016/j.cell.2011.06.002

The discovery of a small-molecule allosteric inhibitor of the CDC34 ubiquitin-conjugating enzyme(E2) by Ceccarelli et al. raises the possibility that it will be generally feasible to selectively inhibit
ubiquitin transfer at this central step in the ubiquitin pathway.
There is significant interest in targetingthe ubiquitin proteasome system (UPS)with small molecules for treatment ofdiseases such as cancer and neurodegeneration (Bedford et al., 2011). In thispathway, ubiquitin is transferred to substrates through cycles of an E1-activating
enzyme-E2-conjugating enzyme-E3 ubiquitin ligase cascade, leading to production of a polyubiquitin chain covalently
attached to the substrate that targets it
for rapid degradation by the proteasome(Figure 1A). Though molecules targetingthe E1 class of enzymes as well as the proteasome are already in the clinic or in trials,
the development of inhibitors of components in the middle of the pathway have
lagged behind. This is despite the factthat E2s and E3s control the substratespecificity and the type of ubiquitin linkage
formed and therefore potentially represent
more specific targets for small-moleculebased therapy. In this issue of Cell, Ceccarelli et al. (2011) report the identificationof the first inhibitor of an E2 enzyme, in thiscase the human E2 CDC34 (also called
UBE2R1), which plays a critical role in
the elimination of cell-cycle regulatory
proteins by the proteasome (Skaar andPagano, 2009). Unexpectedly, the inhibitor acts allosterically by binding to apocket distant from the active site. Thisstudy indicates that it is possible todevelop a highly selective inhibitor of an
E2 enzyme and raises the possibility oftargeting other E2s in a similar manner.
The success of proteasome inhibitors
clinically has fueled the interest in developing inhibitors that block specific E1E2-E3 pathways (Bedford et al., 2011).
By targeting one or a small number ofE3-dependent processes, the effects on
overall protein homeostasis may be restricted to the precise pathway intended.
CDC34 is one of approximately two dozen
ubiquitin E2s in mammals but is unique inthat it functions solely with one specificclass of E3s: the cullin-RING ligase (CRL).
Cullins employ a RING finger protein(RBX1 or RBX2), together with a conserved

pocket in cullins themselves, to recruit
ubiquitin-charged CDC34 (Deshaies andJoazeiro, 2009; Kleiger et al., 2009). CRL
activity also requires modification of cullinby the ubiquitin-like NEDD8 protein, a process that is inhibited by a small-molecule
inhibitor of the neddylation E1 enzyme(Figure 1B) (Bedford et al., 2011). As CRLs
are responsible for controlling the turnoverof cancer-related genes, including the cellcycle inhibitor p27 (Skaar and Pagano,2009), this class of E3s has received particular attention in cancer drug discovery.
To identify small-molecule inhibitors of
p27 ubiquitination, Ceccarelli et al. employed a fully reconstituted p27 ubiquitination reaction involving E1, CDC34, theE3 CRL1SKP2-CKS1, and p27 in a highthroughput screen, identifying CC0651
as a prime candidate for an inhibitor ofthis pathway. Given the number of components and the complexity of the process
being examined in this assay, CC0651
could block one of numerous steps. Usinga variety of related E3 CRL complexes andCell 145, June 24, 2011 ª2011 Elsevier Inc. 1007