Báo cáo khoa học: BRCA1 16 years later: nuclear import and export processes doc

Breast cancer susceptibility gene 1 protein product BRCA1 is involved in multiple nuclear located func-tions.. In this Keywords BRCA1-associated RING domain protein 1 BARD1; breast cance

BRCA1 16 years later: nuclear import and export processes Marilyn E Thompson

Department of Pharmaceutical Sciences, School of Pharmacy, Belmont University, Nashville, TN, USA

Introduction

The importance of regulated nuclear transport

pro-cesses in modulating protein function has become

increasingly evident over the past several years

Energy-dependent, receptor-mediated mechanisms are

required for proteins in excess of 40 kDa to cross the

nuclear envelope [1] This suggests that in order for

many proteins to reside in their compartment of

func-tion, the nucleus, they must first be recognized by

import receptors, termed importins or karyopherins,

which effectuate the translocation of the cargo protein

across the membrane Likewise, removal of the protein

from the nuclear compartment involves its binding to nuclear export receptors (exportins) This translocation event would preclude the functional activity of the nuclear protein

Breast cancer susceptibility gene 1 protein product (BRCA1) is involved in multiple nuclear located func-tions At an apparent relative molecular mass of approximately 220 kDa, it can only traverse the nuclear envelope by an active process However, in more recent years, it has been realized that BRCA1 can also be exported from the nucleus as well In this

Keywords

BRCA1-associated RING domain protein 1

(BARD1); breast cancer susceptibility gene 1

(BRCA1); nuclear export; nuclear export

sequence; nuclear import; nuclear

localization signal; RING finger

Correspondence

M E Thompson, Department of

Pharmaceutical Sciences, School of

Pharmacy, Belmont University, 1900

Belmont Blvd., Nashville, TN 37212, USA

Fax: +615 460 6537

Tel: +615 460 8121

E-mail: marilyn.odom@belmont.edu

Website: http://www.belmont.edu/

pharmacy/

(Received 7 July 2009, revised 27 April

2010, accepted 26 May 2010)

doi:10.1111/j.1742-4658.2010.07733.x

Over the past several years, the importance of regulated nuclear transport processes for tumor suppressors has become evident Proteins with a molecular mass greater than 40 kDa can enter the nucleus only by active transport across the nuclear membrane The most common pathway by which this occurs is via the importin alpha⁄ beta pathway, whereby the cargo protein binds importin alpha This heterodimer binds importin beta and the heterotrimer passes through nuclear pores at the expense of GTP Breast cancer susceptibility gene 1 (BRCA1) is one such protein As a mediator of transcription and DNA repair, two exclusively nuclear func-tions, BRCA1, at 220 kDa, can enter the nucleus only via active transport mechanisms In addition to the classical importin alpha⁄ beta pathway, BRCA1 can also enter the nucleus in a piggyback mechanism with BRCA1-associated RING domain protein 1 (BARD1) The interaction between BRCA1 and BARD1 is also important in the retention of BRCA1

in the nucleus This is important because BRCA1 also undergoes active nuclear export BRCA1 is also involved in apoptotic processes Whether this occurs within the nucleus or cytoplasm is still unclear; thus, the conse-quences of BRCA1 nuclear export have not been clearly elucidated This review will discuss the literature regarding the subcellular localization

of BRCA1, with particular emphasis on its nuclear import and export processes

Abbreviations

BARD1, BRCA1-associated RING domain protein 1; BRCA1, breast cancer susceptibility gene 1; NLS, nuclear localization signal; TRAIL, tumor necrosis factor apoptosis inducing ligand.

review, the literature regarding BRCA1 nuclear import

and export with potential functional implications of

these processes will be discussed

Historical overview of cellular

localization of BRCA1

BRCA1 was first identified and sequenced in 1994 At

that time, the only notable structural feature of the

protein documented was a C3HC4 sequence at its

amino terminus This was recognized as a zinc finger

domain, which is common to many transcription

fac-tors [2] This suggested that BRCA1 served in this

capacity and led to the assumption that BRCA1 was a

nuclear protein In support of this assumption, several

laboratories reported the nuclear localization of

BRCA1 Scully et al [3] indicated that BRCA1 was

present predominantly within the nucleus of a variety

of breast and ovarian tumor-derived cell lines This

countered an earlier report by Chen et al [4], who

sug-gested that BRCA1 was indeed localized to the nucleus

in nonmalignant cells; however, in the tumor cell lines

examined by this group, BRCA1 was concluded to

aberrantly localize within the cytoplasm Many

locali-zation studies [5–8] subsequent to these continued to

implicate BRCA1, as well as its variants, as nuclear

proteins

For example, Thomas et al [6] demonstrated by

both immunofluorescence and biochemical

fraction-ation techniques that in some breast, ovarian and

cer-vical cancer cell lines, BRCA1 was vastly nuclear

Immunoblot analysis of subcellular fractions from two

breast cancer cell lines, MDA-MB-469 and MCF-7,

did indicate the presence of BRCA1 in the cytoplasm

However, the cytoplasmic levels were substantially less

than in the nucleus Whether the presence of BRCA1

in the cytoplasm was attributable to newly translated

protein prior to being transported into the nucleus or

due to another molecular event was not addressed

Rather, this study substantiated the report by Scully

et al [3] that regardless of the malignant state of

the cells, the proportion of BRCA1 localizing to

the nucleus was much higher than that outside the

nucleus

The discrepancies in the literature regarding the

nuclear⁄ non-nuclear distribution of BRCA1 in

malig-nant cells generated much concern and discussion over

the validity of the data Perhaps the most significant

contribution towards resolving this cause ce´le`bre was

provided by Wilson et al [5] In an exhaustive analysis

of 19 different BRCA1 IgGs generated against various

epitopes, this group demonstrated that the presence of

a BRCA1 signal outside the nucleus could be the result

of experimental artifact Analysis of the antibodies for their utility in recognizing BRCA1 via immunoprecipi-tations, immunoblotting, immunohistochemistry and immunocytochemistry revealed that certain variables, such as antibody concentration and method of fixation, influenced the detection of a signal This highly cited paper was instrumental in reconciling that BRCA1 was predominantly localized to the nucleus in both malig-nant and nonmaligmalig-nant cells

In 1997, the Rao laboratory [8] first indicated a reg-ulatory aspect to BRCA1 nuclear⁄ non-nuclear distri-bution Prior to this, Scully et al [9] had proposed that the intranuclear localization of BRCA1 was influ-enced by the cell cycle-dependent phosphorylation state of the protein However, this dictated whether or not BRCA1 localized into nuclear foci or not within the nucleus and did not address the non-nuclear pres-ence of BRCA1 The Rao laboratory suggested that BRCA1 nuclear⁄ non-nuclear distribution could be dictated by the presence of serum growth factors Although they did not attribute this to phosphoryla-tion per se, their report did suggest that BRCA1 did not spontaneously traverse the nuclear envelope

Nuclear import of BRCA1

Although it was appreciated early after the identifica-tion of its gene that BRCA1 was a predominantly nuclear protein, the mechanism of its import into the nucleus was not described until 1997 Two reports doc-umenting BRCA1 nuclear import were published within a short time of each other First, Thakur et al [10] compared the subcellular localization of full-length BRCA1 with that of a BRCA1 variant lacking exon

11, resulting in the expression of a 97 kDa protein This comparison demonstrated the presence of a nuclear localization signal (NLS) within this exon as the variant was unable to localize to the nucleus as the wild-type could Considering potential changes in the secondary and tertiary structures of the protein result-ing from the loss of half of the amino acids, the authors were able to demonstrate it was the loss of a distinct sequence that precluded the variant from entering the nucleus Two NLSs were identified at amino acid residues 501–507 (KCKRKRR; referred to

as NLS1) and 607–614 (KKNRLRRK; referred to as NLS2) Both sequences are located within exon 11, the largest of the exons in BRCA1 (Fig 1) Deletion muta-genesis demonstrated that only the sequence between residues 501 and 507 was vital for BRCA1 nuclear import

Although these data indicated that BRCA1 entered the nucleus via a basic arginine-lysine-rich nuclear

localization sequence; they did not clearly demonstrate

interaction with import machinery Chen et al [11]

carried the work to the next step and demonstrated

the interaction of NLS1 (amino acids 501–508) with

importin alpha This clearly marked BRCA1 as a

pro-tein that enters the nucleus via the classical importin

alpha⁄ beta pathway (Fig 2)

An alternative nuclear import pathway for BRCA1

Although BRCA1 does interact with importin alpha to

be actively transported across the nuclear envelope, Wang et al [8] observed that splice variants lacking exon 11, where both NLS are located, could also be

RING finger

domain (aa 1–109)

Transcriptional activation domain (aa 1560–1863)

Nuclear export sequences (aa 22–39 and aa 81–99) Nuclear localization signals (aa 503–508 and aa 607–614) BRCT domains

(aa 1640–1729 and aa 1760–1821)

Fig 1 Schematic of key structural ⁄ functional elements of BRCA1 Nuclear targeting sequences of BRCA1 are shown

in relationship to the RING finger domain, BRCA1 C Terminus (BRCT) and

transcriptional activation domains.

β

BRCA1 α

β

BRCA1 α

β

BRCA1 α

β

BRCA1 α

BRCA1

α

β α

BARD 1

BRCA1 BARD 1

α β

BARD 1

α

BARD 1

α β

BRCA1 BARD 1

α

1

2

3

6

i

ii

iii

iv

v vi

Extracellular

Cytoplasm

Nucleus

Fig 2 Schematic representing the two mechanisms for BRCA1 nuclear import The depiction on the left represents BRCA1 binding directly

to the import machinery (1) BRCA1 binds importin alpha, which subsequently (2) binds importin beta (3) The heterotrimer crosses the nuclear envelope in a GTP-dependent manner (hydrolysis of GTP to GDP not shown) Inside the nucleus, the trimer dissociates (4, 5) and importin alpha and beta are exported from the nucleus (6) to be used again The depiction on the right side represents BRCA1 binding to BARD in order to enter the nucleus The BARD1:BRCA1 complex binds to the import machinery by first binding to importin alpha (i) The BRCA1:BARD1:importin alpha complex binds importin beta (ii) This complex enters the nucleus in an energy-dependent manner (iii) Inside the nucleus, importins beta (iv) and alpha (v) dissociate from the BARD1:BRCA1 complex and are recycled.

found in the nucleus This led to the supposition that

there was an alternative pathway by which BRCA1

entered the nucleus Fabbro et al [12] reported that

the non-NLS-dependent mechanism for BRCA1

locali-zation to the nucleus required its RING finger domain

Furthermore, the extent of nuclear localization of the

variants correlated with the level of BRCA1-associated

RING domain protein 1 (BARD1) expression Given

that BRCA1 and BARD1 bind via their RING finger

domains, it was postulated that BRCA1 can enter the

nucleus in a piggyback mechanism with BARD1

With the demonstration of two independent

mecha-nisms for BRCA1 nuclear import, studies turned to

investigating potential regulation of this translocation

event It had already been proposed that

phosphoryla-tion, potentially due to the presence of serum growth

factors, modulated BRCA1 localization, although no

specific phosphoacceptor sites were initially identified

However, in 1999, Altiok et al [13] published that the

administration of heregulin b1 to T47D human breast

cancer cells resulted in Akt-mediated phosphorylation

of BRCA1 on serine 508, a site adjacent to NLS1

However, no functional consequences of this

phos-phorylation were determined Hinton et al [14]

subse-quently demonstrated that the exposure of cells to the

growth factor resulted in an increase in nuclear

accu-mulation of BRCA1 The cells were serum starved

prior to the administration of heregulin Interestingly,

these conditions resulted in much of the BRCA1

local-izing outside the nucleus, in contrast to the

observa-tions of Wang et al [8], who reported that in the

absence of serum, NIH3T3 fibroblasts, Hs578BST,

HBL-100, ZR75-1, CAMA breast tumor cells and

NIH OVCAR3 ovarian cancer cells exhibited a mostly

nuclear presence of BRCA1, which redistributed to the

cytoplasm upon serum addition

The far-reaching biological implications of

Akt-med-iated regulation of BRCA1 are not understood The

ability of Akt to enhance the nuclear presence of

BRCA1 superficially seems to be dichotomous

BRCA1 has many tumor suppressive associated

func-tions within the nucleus Thus, it would be

confound-ing for Akt, a cell survival kinase, to facilitate its

nuclear presence However, numerous reports

docu-ment a role for BRCA1 in apoptosis Some of these

data suggest that this function is mediated through

cytoplasmic BRCA1 Indeed, several reports have

doc-umented an association of BRCA1 with the

cytoplas-mic enzyme, acetyl CoA carboxylase [15–17] This

interaction, which is mediated through the BRCA1 C

Terminus domains of BRCA1, regulated lipogenesis in

mammary epithelial cell lines Specifically, BRCA1

interacted with the phosphorylated, inactive form of

acetyl CoA carboxylase and interfered with its phorylation [16] The decrease in functional, dephos-phorylated acetyl CoA carboxylase resulted in limited synthesis of palmitic acid and this led to apoptosis [17,18] Also, noteworthy is the work of Dizin et al [19], who demonstrated that cytoplasmic BRCA1 is proteolytically processed into a 90 kDa fragment that exhibits the apoptotic activity of the protein These results may explain the otherwise confounding scenario

of Akt-mediated nuclear accumulation of BRCA1 Other factors shown to regulate BRCA1 localization include hypoxia and tumor necrosis factor apoptosis inducing ligand (TRAIL) Fitzgerald et al [20] demon-strated that exposure of several malignant mammary cell lines to hypoxia resulted in an enhancement of nuclear BRCA1, with concomitant decreases in cyto-plasmic levels Although earlier reports had suggested that hypoxia decreased BRCA1 expression [21,22], this was not evident under the conditions used by Fitzger-ald et al [20] Additionally, this effect was not observed in nonmalignant cells, but was mimicked by the administration of TRAIL to the cells The expres-sion of BRCA1 facilitated TRAIL-induced apoptosis The idea that TRAIL induction of apoptosis is enhanced by nuclear BRCA1 would suggest that under those conditions it is nuclear BRCA1 that augments apoptosis One plausible mechanism by which this may occur is that BRCA1 could be transactivating genes involved in the apoptotic process However, this hypothesis has yet to be tested

Nuclear export of BRCA1

The nuclear⁄ non-nuclear distribution of BRCA1 has been reported by numerous laboratories This includes several reports of BRCA1 association with microtubules and centrosomes However, the non-nuclear presence of BRCA1 was not specifically addressed until 2000, when Rodriguez & Henderson [23] published that BRCA1 underwent receptor-mediated nuclear export These investigators delineated amino acid residues 81–99 as a Rev-like nuclear export sequence This type sequence, which binds to CRM1 nuclear export receptor to facili-tate translocation of the cargo protein across the nuclear membrane at the expense of GTP, is defined by

a characteristic pattern of hydrophobic amino acids The consensus sequence is L-X1-3-L-X2-4-L-X-L In addition to the presence of this motif at residues 81–99, BRCA1 has three other regions (amino acids 22–30, 591–600, 783–793) that also fit the consensus sequence

Of these, only residues 22–30 have been shown to have functional nuclear export activity [24] The presence of two distinct nuclear export sequences is not novel Other

tumor suppressors, including p53 [25,26], also have

mul-tiple functional nuclear export sequences

Few reports have documented a regulatory

compo-nent to BRCA1 nuclear export [12,27–30] Fabbro

et al [12] suggested that the interaction of BRCA1

with BARD1 resulted in inhibition of BRCA1 nuclear

export Comparing the localization of wild-type

BRCA1 with BRCA1 mutant construct in which the

amino terminal nuclear export sequence at amino acids

81–99 was transferred to the carboxy terminus, this

group showed that co-expression of BARD1 resulted

in nuclear accumulation of wild-type BRCA1, but had

no inhibitory effect on the nuclear export of the

mutant The authors concluded that BARD1 masks

the amino terminal BRCA1 nuclear export sequence

and, therefore, export could be regulated by BARD1

expression Thus, BARD1 is important for both the

nuclear import and nuclear retention of BRCA1

This laboratory subsequently assigned a

physiologi-cal relevance to BARD1-mediated nuclear retention of

BRCA1 Data support the idea that nuclear retention

of BRCA1 decreases BRCA1-dependent apoptosis

[27] The data indicating that the recovery of BRCA1

nuclear export support those of others who have

sug-gested that the apoptotic function of BRCA1 is due to

its cytoplasmic localization

Another study revealing a novel regulatory event for

BRCA1 nuclear export was reported by Glover-Collins

& Thompson [28] In this report, the authors were able

to demonstrate that nuclear export of BRCA1

occurred in a cell cycle-dependent manner, resulting in

BRCA1 transiently concentrating in a perinuclear

loca-tion during the early part of S phase This relocaloca-tion

was blocked by the introduction of the calcium

chela-tor, BAPTA-AM, and was mimicked by the calcium

ionophore, A23187 The biological relevance of this

calcium-dependent modulation is not yet understood

However, progression of the mammalian cell cycle has

been shown to be dependent on calcium signaling and

a calcium transient occurs at the G1⁄ S boundary [29]

These findings may suggest that the transient nuclear

export of BRCA1 during early S phase is a

conse-quence of the G1⁄ S calcium peak

Conclusion

In this review we have discussed one aspect of BRCA1

regulation – its nuclear to non-nuclear distribution

BRCA1 is one of many tumor suppressors that

undergo active nuclear import and export processes

(see Table 1 for targeting sequences) It would seem

almost wasteful to expend the energy for opposing

transport mechanisms However, the dynamic

equilib-rium between BRCA1 nuclear import and export could provide a fine level of regulation of its nuclear and⁄ or cytoplasmic functions

Tight regulatory control of BRCA1 localization has far-reaching implications Numerous mutations with different functional classifications have been docu-mented in BRCA1 [31] Several reported patient muta-tions probably affect either the nuclear import or export

of BRCA1 and result in a disturbance of the nuclear to non-nuclear distribution and function Pharmacologi-cally, this could potentially be exploited Data suggest that histone deacetylase [32] and poly ADP ribose poly-merase [33,34] inhibitors are more effective in patients who are BRCA1 negative Theoretically, if BRCA1 localization can be pharmacologically manipulated to render it nonfunctional, the use of histone deacetylase and poly ADP ribose polymerase inhibitors could

bene-fit a broader range of patients However, the lack of specificity in manipulating BRCA1 in malignant, but not nonmalignant, cells, would certainly present a chal-lenge for the clinical usefulness of this approach How-ever, with technological advances, such as the development of nanoparticles to deliver drugs specifi-cally to cancer cells [35], future use of this therapeutic regimen does not necessarily have to be ruled out

References

1 Terry LJ, Shows EB & Wente SR (2007) Crossing the nuclear envelope: hierarchical regulation of nucleocyto-plasmic transport Science 318, 1412–1416

2 Miki Y, Swensen J, Shattuck-Eidens D, Futreal PA, Harshman K, Tavtigian S, Liu Q, Cochran C, Bennett

LM, Ding W et al (1994) A strong candidate for the breast and ovarian cancer susceptibility gene BRCA1 Science 266, 66–71

3 Scully R, Ganesan S, Brown M, De Caprio JA, Cannis-tra SA, Feunteun J, Schnitt S & Livingston DM (1996) Location of BRCA1 in human breast and ovarian can-cer cells Science 272, 123–125

4 Chen Y, Chen CF, Riley DJ, Allred DC, Chen PL, Von Hoff D, Osborne CK & Lee WH (1995) Aberrant sub-cellular localization of BRCA1 in breast cancer Science

270, 789–791

Table 1 BRCA1 nuclear targeting sequences.

NLSs

BRCA1 amino acids 501–508 KLKRKRRP BRCA1 amino acids 607–614 KKNRLRRK Nuclear export sequences

BRCA1 amino acids 22–30 LECPICLEL BRCA1 amino acids 81–99 QLVEELLKIICAFQLDTGL

5 Wilson CA, Ramos L, Villasenor MR, Anders KH,

Press MF, Clarke K, Karlan B, Chen JJ, Scully R,

Livingston D et al (1999) Localization of human

BRCA1 and its loss in high-grade, non-inherited breast

carcinomas Nat Genet 21, 236–240

6 Thomas JE, Smith M, Rubinfeld B, Gutowski M,

Beckmann RP & Polakis P (1996) Subcellular

localiza-tion and analysis of apparent 180-kDa and 220-kDa

proteins of the breast cancer susceptibility gene,

BRCA1 J Biol Chem 271, 28630–28635

7 Maul GG, Jensen DE, Ishov AM, Herlyn M & Rauscher

FJ III (1998) Nuclear redistribution of BRCA1 during

viral infection Cell Growth Differ 9, 743–755

8 Wang H, Shao N, Ding QM, Cui J, Shyam E, Reddy P

& Rao VN (1997) BRCA1 proteins are transported to

the nucleus in the absence of serum and splice variants

BRCA1a, BRCA1b are tyrosine phsophoproteins that

associate with E2F, cyclins and cyclin dependent

kinases Oncogene 15, 143–157

9 Scully R, Chen J, Ochs RL, Keegan K, Hoekstra M,

Feunteun J & Livingston DM (1997) Dynamic

changes of BRCA1 subnuclear location and

phosphor-ylation state are initiated by DNA damage Cell 90,

425–435

10 Thakur S, Zhang HB, Peng Y, Le H, Carroll B, Ward

T, Yao J, Farid LM, Couch FJ, Wilson RB et al

(1997) Localization of BRCA1 and a splice variant

identifies the nuclear localization signal Mol Cell Biol

17, 444–452

11 Chen C-F, Li S, Chen Y, Chen P-L, Sharp ZD & Lee

W-H (1996) The nuclear localization sequences of the

BRCA1 protein interact with the importin-alpha

sub-unit of the nuclear transport signal receptor J Biol

Chem 271, 32863–32868

12 Fabbro M, Rodriguez JA, Baer R & Henderson BR

(2002) BARD1 induces BRCA1 intranuclear foci

forma-tion by increasing RING-dependent BRCA1 nuclear

import and inhibiting BRCA1 nuclear export J Biol

Chem 277, 21315–21324

13 Altiok S, Batt D, Altiok N, Papautsky A, Downward J,

Roberts TM & Avraham H (1999) Heregulin induces

phosphorylation of BRCA1 through

phosphatidylinosi-tol 3-kinase⁄ AKT in breast cancer cells J Biol Chem

274, 32274–32278

14 Hinton CV, Fitzgerald LD & Thompson ME (2007)

Phosphatidylinositol 3-kinase⁄ Akt signaling enhances

nuclear localization and transcriptional activity of

BRCA1 Exp Cell Res 313, 1735–1744

15 Magnard C, Bachelier R, Vincent A, Jaquinod M,

Kieffer S, Lenoir GM & Venezia ND (2002) BRCA1

interacts with acetyl-CoA carboxylase through its

tandem of BRCT domains Oncogene 21, 6729–6739

16 Moreau K, Dizen E, Ray H, Luquain C, Lefai E,

Foufelle F, Billaud M, Lenoir GM & Venezia ND

(2006) BRCA1 affects lipid synthesis through its

interaction with acetyl CoA carboxylase J Biol Chem

281, 3172–3181

17 Chajes V, Cambot M, Moreau K, Lenoir GM & Joulin

V (2006) Acetyl-CoA carboxylase a is essential to breast cancer cell survival Cancer Res 66, 5287–5294

18 Beckers A, Organe S, Timmermans L, Scheys K, Peeters A, Brusselmans K, Verhoeven G & Swinnen JV (2007) Chemical inhibition of acetyl-CoA carboxylase induces growth arrest and cytotoxicity selectively in cancer cells Cancer Res 67, 8180–8187

19 Dizin E, Ray H, Suau F, Voeltzel T & Venezia ND (2008) Caspase-dependent BRCA1 cleavage facilitates chemotherapy-induced apoptosis Apoptosis 12, 237– 246

20 Fitzgerald LD, Bailey CK, Brandt S & Thompson ME (2007) BRCA1 accumulates in the nucleus in response to hypoxia and TRAIL and enhances TRAIL-induced apoptosis in breast cancer cells FEBS J 274, 5137–5146

21 Meng AX, Jalali F, Cuddihy A, Chan N, Bindra RS, Glazer PM & Bristow RG (2005) Hypoxia down-regulates DNA double strand break repair gene expression in prostate cancer cells Radiother Oncol

76, 168–176

22 Bindra RS, Gibson SL, Meng A, Westermark U, Jasin

M, Pierce AJ, Bristow RG, Classon MK & Glazer PM (2005) Hypoxia-induced down-regulation of BRCA1 expression by E2Fs Cancer Res 65, 11597–11604

23 Rodriguez JA & Henderson BR (2000) Identification of

a functional nuclear export sequence in BRCA1 J Biol Chem 275, 38589–38596

24 Thompson ME, Robinson-Benion CL & Holt JT (2005)

An amino-terminal motif functions as a second nuclear export sequence in BRCA1 J Biol Chem 280, 21854– 21857

25 Stommell JM, Marchenko ND, Jimenez GS, Moll UM, Hope TJ & Wahl GM (1999) A leucine-rich nuclear export signal in the p53 tetramerization domain: regula-tion of subcellular localizaregula-tion and p53 activity by NES masking EMBO J 18, 1660–1672

26 Zhang Y & Xiong Y (2001) A p53 amino-terminal nuclear export signal inhibited by DNA damage-induced phosphorylation Science 292, 1910–1915

27 Fabbro M, Schuechner S, Au WWY & Henderson BR (2004) BARD1 regulates BRCA1 apoptotic function by

a mechanism involving nuclear retention Exp Cell Res

298, 661–673

28 Glover-Collins K & Thompson ME (2008) Nuclear export of BRCA1 occurs during early S phase and is calcium-dependent Cell Signal 20, 958–968

29 Kahl CR & Means AR (2003) Regulation of cell cycle progression by calcium⁄ calmodulin-dependent path-ways Endocr Rev 24, 719–736

30 Yang ES & Xia F (2010) BRCA1 16 years later: DNA damage-induced BRCA1 shuttling FEBS J 277, 3079– 3085

31 Linger RJ & Kruk PA (2010) BRCA1 16 years later:

risk-associated BRCA1 mutations and their functional

implications FEBS J 277, 3086–3096

32 Zhang Y, Carr T, Dimtchev A, Zaer N, Dritschilo A &

Jung M (2007) Attenuated DNA damage repair by

tri-chostatin A through BRCA1 suppression Radiat Res

168, 115–124

33 Rottenberg S, Jasper JE, Kersbergen A, van der Burg

E, Nygren AO, Zander SA, Derksen PW, de Bruin M,

Zevenhoven J, Lau A et al (2008) High sensitivity of

BRCA1-deficient mammary tumors to the PARP

inhibitor AZD2281 alone and in combination with platinum drugs Proc Natl Acad Sci USA 105, 17079–17084

34 Fong PC, Boss DS, Yap TA, Tutt A, Wu P, Mergui-Roelvink M, Mortimer P, Swaisland H, Lau A, O’Con-nor MJ et al (2009) Inhibition of poly (ADP-ribose) polymerase in tumors from BRCA1 mutation carriers

N Engl J Med 361, 123–134

35 Xu C, Wang B & Sun S (2009) Dumbbell-like Au-Fe3O4 nanoparticles for target-specific platin delivery

J Am Chem Soc 131, 4216–4217