© The Author 2006. Published by Oxford University Press.
EDITORIAL |
Pak up Your Breast Tumor—and Grow!
Correspondence to: V. Craig Jordan, OBE, PhD, DSc, Vice President and Research Director of Medical Sciences, Fox Chase Cancer Center, 333 Cottman Ave., Philadelphia, PA 19111 (e-mail: v.craig.jordan{at}fccc.edu).
The estrogen receptor (ER) has proved to be an extremely important target for the treatment of breast cancer (1). Tamoxifen, a nonsteroidal antiestrogen, blocks estrogen-stimulated breast cancer growth by binding to ER. However, years of adjuvant tamoxifen therapy are required to "smother" estrogen-sensitive micrometastases. Thus, two principles for optimal molecular therapeutics in breast cancer have emerged: tumor targeting and the appropriate duration of treatment that creates optimal survival advantages for patients (2). Although these principles have taken two decades to translate from the laboratory (3) to changes in healthcare (4), the widespread use of tamoxifen is credited with contributing substantially to the decline in death rates from breast cancer in the United States (5). Most importantly, in these days of meteoric rises in health care costs for targeted therapies, tamoxifen is, by contrast, a cheap and affordable drug that is known to save lives. However, because not all ER-positive breast cancers respond to tamoxifen treatment, the development of ways to improve targeting tamoxifen to treat the "right" tumors would be of value throughout the world.
It is well recognized that antihormone action in breast cancer is subverted by increases in breast cancer signal transduction that are mediated by phosphorylation cascades initiated by cell surface receptors (Fig. 1). Members of the p21-activated kinase (Pak) family of kinases (6) perform a variety of survival functions by activating angiogenesis or gene expression; the first member of this family to be identified, Pak1, is associated with mammary hyperplasia and malignancy (7).
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In this issue of the Journal, Holm et al. (8) report an association between Pak1 expression and resistance to tamoxifen. They draw on several layers of laboratory evidence backed by evidence from a clinical trial (9) to conclude that the presence and subcellular localization of Pak1 may be responsible for tamoxifen resistance in a substantial subset (
20%) of ER-positive tumors. Research over the past two decades has identified two forms of resistance to tamoxifen therapy: intrinsic (de novo) resistance, when ER-positive tumors do not respond to tamoxifen at the outset of treatment, and acquired resistance, when ER-positive tumors that initially respond to tamoxifen subsequently exploit the tamoxifen–ER complexes as a stimulatory growth signal. Tamoxifen-stimulated growth is a unique form of drug resistance (10). Holm et al. (8), however, focus their research entirely on intrinsic resistance; i.e., ER is present but tamoxifen cannot override the strong apparently receptor-independent growth signal.
The clinical correlations reported by Holm et al. (8) are based on an analysis of tumor blocks from 564 premenopausal breast cancer patients who participated in a randomized clinical trial (9) of 2 years of adjuvant tamoxifen therapy (n = 276) versus no treatment (n = 288). Patients from the two recruitment sites received either 40 mg or 20 mg of tamoxifen. Although the authors of the original trial (9) correctly make the point that no differences in response rates have been noted for postmenopausal patients who take these different doses of tamoxifen, the question of the optimal dose for premenopausal patients is less clear. Adjuvant tamoxifen monotherapy dramatically increases circulating estrogen levels (11), and such increases can undermine the action of a competitive inhibitor, such as tamoxifen, at the level of the tumor ER (12). Because the coadministration of a luteinizing hormone releasing hormone superagonist with adjuvant tamoxifen can improve response rates (13), the variables of tamoxifen dose and unpredictable elevations in individual patient's estrogen levels could potentially confound interpretation of the response rates in that trial.
The clinical trial (9) that was the source of the tumor blocks also used a suboptimal duration of tamoxifen therapy, i.e., 2 years rather than 5 years. Five years of tamoxifen is essential to achieve optimal disease-free and overall survival in premenopausal patients (4) because the excess circulating estrogen in these patients can undoubtedly increase tumor cell survival once tamoxifen is stopped at 2 years. No overall survival advantage was noted in the Swedish trial (9). Although it is unclear how these treatment issues of tamoxifen dose and duration will ultimately affect correlations with biochemical parameters in the tumors, it is fair to say that the response rates in the clinical trial (9) would have been better had a longer duration of tamoxifen therapy been used. Estrogen is known to regulate Pak1 activity and estrogen-mediated survival functions through phosphorylation of forkhead transcription factors (14). Holm et al. (8) confined their analysis of Pak1 to 285 ER-positive patients for whom tumors blocks were available (151 patients received no treatment and 134 patients received adjuvant tamoxifen). The authors used six graded subjective classification categories to evaluate the impact of tamoxifen on Pak1 expression; however, the small number of tissue samples in some categories necessitated pooling of samples for statistical purposes. Although such nonquantitative evaluations are, more often than not, imprecise, the authors adequately demonstrated that Pak1 level was not associated with disease recurrence of patients in the control (no treatment) group but that high Pak1 levels was associated with recurrence despite tamoxifen treatment.
Many pieces of the drug-resistance puzzle have fallen into place during the past 20 years. It is perhaps appropriate to integrate the findings reported by Holm et al. (8) for Pak1 into a general model of intrinsic resistance (Fig. 1). Early studies that directly addressed the mechanism responsible for ER-positive tumors cells' being refractory to tamoxifen focused on the potential influence of growth factor signaling pathways to subvert antiestrogen action. Epidermal growth factor (EGF) (15) and growth factors secreted from ER-negative tumor cells (16) both cause ER-positive breast cancer cell replication that cannot be controlled by antiestrogens. Indeed, in human breast cancer cells, EGF has been shown to cause a reduction in ER levels and impaired expression of progesterone receptor (PgR) (17). These laboratory findings fit nicely with the recent observation that the presence of epidermal growth factor receptor (EGFR) and the more powerful family member HER2/neu are associated with ER-positive, PgR-negative breast tumors, which are intrinsically resistant to adjuvant tamoxifen (18). Also, the presence of AIB1 along with EGFR and HER2/neu in ER-positive breast cancers predicts intrinsic tamoxifen resistance (19). Experimental studies have shown that overexpression of HER2/neu in ER-positive breast cancer cells creates rapid resistance to tamoxifen (20).
Holm et al. (8) report that increased levels of Pak1, specifically nuclear localization of Pak1, is linked to intrinsic tamoxifen resistance of breast cancer cells. Pak1 is one of many kinases that are phosphorylated by Rac and AKT within the breast cancer cell, but this phosphorylation cascade does not necessarily start inside the cell (21). The phosphorylation cascade begins at the growth factor receptors on the cell surface (Fig. 1). For example, the kinase cascade that originates either at Her2/neu (22) or at Shc and the insulin-like growth factor 1 receptor (23) results in the redeployment of ER from the nucleus to the cell membrane. Indeed, phosphorylation within the ER-positive hormone-dependent cell could result in hypersensitivity to estrogen and antiestrogens (24) through phosphorylation of serine 118 by MAPK (25) and serine 167 by AKT (26), respectively, of the activation function 1 domain of the ER and through phosphorylation of the coactivator AIB1 (27). Similarly, the elevation of PKA and Pak1 (8) results in the phosphorylation of serine 305 (28,29), which can enhance transactivation of cyclin D—thereby increasing cell cycle activity. Indeed, recent evidence (30) demonstrates that phosphorylation of serine 305 in the ER can, in fact, enhance serine 118 phosphorylation, thereby increasing the probability of tamoxifen resistance. If that was not enough, it seems that src (the first oncogenic tyrosine kinase to be identified) is making a comeback as a drug target (31) and is also connected to the ER in that phosphorylation of tyrosine 537 of the tumor ER by p60 src enhances ligand binding and ER dimerization (32). This amino acid is located at a strategic site in helix 12 of the ER that is known to play a critical role in estrogen and antiestrogen action. Indeed, it is already known that p60 src plays a part in drug resistance to tamoxifen through phosphorylation of a serine 118–independent pathway (33).
Overall, the environment within breast cancer cells creates a strong and complex network of survival signaling. As a result, tamoxifen is poorly equipped to block the cell cycle simply by occupying the ER in cells that are resistant (Fig. 1).
If the goal of studying drug resistance is either to improve overall response rates in ER-positive patients by blocking pathways that subvert antiestrogen action or to target tamoxifen to specific tumors to provide optimal benefit, then the first goal seems remote. Even if we could knock out Pak1 activity completely with a kinase inhibitor, the opportunity for the intrinsically resistant cell to survive, through the phosphorylation network, seems very high. A combined attack at multiple upstream targets would be more likely to succeed. The second approach, selecting tumors to be targeted with tamoxifen, has immediate application. Tumors that express low levels of ER; no PgR; and high levels of AIB1, Her2/neu, EGFR, and phosphoproteins such as Pak1 and src are unlikely to respond to long-term adjuvant tamoxifen, and patients who have such tumors should probably not receive tamoxifen. Protein and phosphoprotein assays of markers known to undermine tamoxifen action would have been a valuable comparator to Pak1 expression alone in the dataset analyzed by Holm et al. (8).
Regardless, the clinical correlations reported by Holm et al. (8) indicate that packing tumor cell nuclei with Pak can perturb tamoxifen's action. The tumor, once "Paked up," has no alternative but to grow.
NOTES
Supported by SPORE in Breast Cancer CA89018, R01 GM061756, The Avon Foundation, and the Weg Fund of the Fox Chase Cancer Center.
I thank Joan Lewis-Wambi, Ph.D., and Jonathan Chernoff, M.D., Ph.D., for their helpful comments on Paks.
REFERENCES
(1) Jensen EV, Jordan VC. The estrogen receptor: a model for molecular medicine. The Dorothy P. Landon AACR Prize for Translational Research. Clin Cancer Res 2003;9:1980–9.
(2) Jordan VC. Tamoxifen: a most unlikely pioneering medicine. Nat Rev Drug Discov 2003;2:205–13.[CrossRef][ISI][Medline]
(3) Jordan VC, Allen KE, Dix CJ. Pharmacology of tamoxifen in laboratory animals. Cancer Treat Rep 1980;64:745–59.[ISI][Medline]
(4) EBCTCG. Tamoxifen for early breast cancer: an overview of the randomised trials. Lancet 1998;354:1451–67.
(5) EBCTCG. Effects of chemotherapy and hormonal therapy for early breast cancer on recurrence and 15-year survival: an overview of the randomised trials. Lancet 2005;365:1687–717.[CrossRef][ISI][Medline]
(6) Sells MA, Chernoff J. Emerging from the Pak: the p21-activated protein kinase family. Trends Cell Biol 1997;7:162–7.[Medline]
(7) Wang RA, Zhang H, Balasenthil S, Medina D, Kumar R. PAK1 hyperactivation is sufficient for mammary gland tumor formation. Oncogene 2005 Dec 12; [Epub ahead of print]. Available at: http://www.nature.com/onc/journal/vaop/ncurrent/abs/1209309a.html;jsessionid=987FC615C0D9133464C29E64F31E430E.
(8) Holm C, Rayala S, Jirström K, Stål O, Kumar R, Landberg G. Association between Pak1 expression and subcellular localization and tamoxifen resistance in breast cancer patients. J Natl Cancer Inst 2006;98:671–80.
(9) Ryden L, Jonsson PE, Chebil G, Dufmats M, Ferno M, Jirstrom K, et al. Two years of adjuvant tamoxifen in premenopausal patients with breast cancer; a randomised, controlled trial with long-term follow-up. Eur J Cancer 2005;41:256–64.[CrossRef][Medline]
(10) Jordan VC. Selective estrogen receptor modulation: concept and consequences in cancer. Cancer Cell 2004;5:207–13.[CrossRef][ISI][Medline]
(11) Jordan VC, Fritz NF, Langan-Fahey S, Thompson M, Tormey DC. Alteration of endocrine parameters in premenopausal women with breast cancer during long-term adjuvant therapy with tamoxifen as the single agent. J Natl Cancer Inst 1991;83:1488–91.
(12) Iino Y, Wolf DM, Langan-Fahey SM, Johnson DA, Ricchio M, Thompson ME, et al. Reversible control of oestradiol-stimulated growth of MCF-7 tumours by tamoxifen in the athymic mouse. Br J Cancer 1991;64:1019–24.[Medline]
(13) Davidson NE, O'Neill AM, Vukov AM, Osborne CK, Martino S, White DR, et al. Chemoendocrine therapy for premenopausal women with axillary lymph node-positive, steroid hormone receptor-positive breast cancer: results from INT 0101 (E5188). J Clin Oncol 2005;23:5973–82.
(14) Mazumdar A, Kumar R. Estrogen regulation of Pak1 and FKHR pathways in breast cancer cells. FEBS Lett 2003;535:6–10.[CrossRef][Medline]
(15) Cormier EM, Jordan VC. Contrasting ability of antiestrogens to inhibit MCF-7 growth stimulated by estradiol or epidermal growth factor. Eur J Cancer Clin Oncol 1989;25:57–63.[CrossRef][Medline]
(16) Robinson SP, Jordan VC. The paracrine stimulation of MCF-7 cells by MDA-MB-231 cells: possible role in antiestrogen failure. Eur J Cancer Clin Oncol 1989;25:493–7.[CrossRef][ISI][Medline]
(17) Cormier EM, Wolf MF, Jordan VC. Decrease in estradiol-stimulated progesterone receptor production in MCF-7 cells by epidermal growth factor and possible clinical implication for paracrine-regulated breast cancer growth. Cancer Res 1989;49:576–80.
(18) Arpino G, Weiss H, Lee AV, Schiff R, De Placido S, Osborne CK, et al. Estrogen receptor-positive, progesterone receptor-negative breast cancer: association with growth factor receptor expression and tamoxifen resistance. J Natl Cancer Inst 2005;97:1254–61.
(19) Osborne CK, Bardou V, Hopp TA, Chamness GC, Hilsenbeck SG, Fuqua SA, et al. Role of the estrogen receptor coactivator AIB1 (SRC-3) and HER-2/neu in tamoxifen resistance in breast cancer. J Natl Cancer Inst 2003;95:353–61.
(20) Shou J, Massarweh S, Osborne CK, Wakeling A, Ali S, Weiss H, et al. Mechanisms of tamoxifen resistance: increased estrogen receptor-HER2/neu cross-talk in ER/HER2–positive breast cancer. J Natl Cancer Inst 2004;96:926–35.
(21) Mira JP, Benard V, Groffen J, Sanders LC, Knaus UG. Endogenous, hyperactive Rac3 controls proliferation of breast cancer cells by a p21 activated kinase-dependent pathway. Proc Natl Acad Sci U S A 2000;97:185–9.
(22) Yang Z, Barnes CJ, Kumar R. Human epidermal growth factor receptor 2 status modulates subcellular localization of and interaction with estrogen receptor alpha in breast cancer cells. Clin Cancer Res 2004;10:3621–8.
(23) Song RX, Barnes CJ, Zhang Z, Bao Y, Kumar R, Santen RJ. The role of Shc and insulin-like growth factor 1 receptor in mediating the translocation of estrogen receptor alpha to the plasma membrane. Proc Natl Acad Sci U S A 2004;101:2076–81.
(24) Santen RJ, Song RX, Zhang Z, Yue W, Kumar R. Adaptive hypersensitivity to estrogen: mechanism for sequential responses to hormonal therapy in breast cancer. Clin Cancer Res 2004;10:337S–45S.
(25) Kato S, Endoh H, Masuhiro Y, Kitamoto T, Uchiyama S, Sasaki H, et al. Activation of the estrogen receptor through phosphorylation by mitogen-activated protein kinase. Science 1995;270:1491–4.
(26) Campbell RA, Bhat-Nakshatri P, Patel NM, Constantinidou D, Ali S, Nakshatri H. Phosphatidylinositol 3-kinase/AKT-mediated activation of estrogen receptor alpha: a new model for anti-estrogen resistance. J Biol Chem 2001;276:9817–24.
(27) Font de Mora J, Brown M. AIB1 is a conduit for kinase-mediated growth factor signaling to the estrogen receptor. Mol Cell Biol 2000;20:5041–7.
(28) Michalides R, Griekspoor A, Balkenende A, Verwoerd D, Janssen L, Jalink K, et al. Tamoxifen resistance by a conformational arrest of the estrogen receptor alpha after PKA activation in breast cancer. Cancer Cell 2004;5:597–605.[CrossRef][ISI][Medline]
(29) Wang RA, Mazumdar A, Vadlamudi RK, Kumar R. P21-activated kinase-1 phosphorylates and transactivates estrogen receptor-alpha and promotes hyperplasia in mammary epithelium. EMBO J 2002;21:5437–47.[CrossRef][Medline]
(30) Rayala SK, Talukder AH, Balasenthil S, Tharakan R, Barnes CJ, Wang R, et al. P21-activated kinase 1 regulation of estrogen receptor-alpha activation involves serine 305 activation linked with serine 118 phosphorylation. Cancer Res 2006;66:1694–701.
(31) Summy JM, Gallick GE. Treatment for advanced tumors: Src reclaims center stage. Clin Cancer Res 2006;12:1398–402.
(32) Arnold SF, Melamed M, Vorojeikina DP, Notides AC, Sasson S. Estradiol-binding mechanisms and binding capacity of the human estrogen receptor is regulated by tyrosine phosphorylation. Mol Endocrinol 1997;11:48–53.
(33) Feng W, Webb P, Nguyen P, Liu X, Li J, Karin M, et al. Potentiation of estrogen receptor activation function 1 (AF-1) by src/JNK through a serine 118-independent pathway. Mol Endocrinol 2001;15:32–45.
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