RESEARCH HIGHLIGHTS

These findings are important NUCLEAR RECEPTORS because unlike other treatments reported in the NOD mouse this therapy is effective when adminis- tered relatively late in disease pro- Growing concern gression. Interestingly, NKG2D has been detected on T cells — isolated for tamoxifen from the synovial tissue of rheuma- toid arthritis patients — in the pres- Breast tumours that recur after treatment with the ence of one of its ligands, which anti-oestrogen tamoxifen might result from a raises the posibility that this receptor change in the effect of the drug that turns it from an could play a detrimental role in other antagonist of the oestrogen receptor into an agonist autoimmune diseases. that promotes cancer cell growth.Writing in Melanie Brazil Cancer Cell,Rob Michalides and colleagues report that phosphorylation of oestrogen receptor-α References and links (ER-α) by protein kinase A (PKA) allows tamoxifen ORIGINAL RESEARCH PAPER Ogasawara, K. to bind but obliterates its antagonistic effect. et al. NKG2D blockade prevents autoimmune diabetes in NOD mice. Immunity 20, 757–767 Their findings mean that it will be possible to (2004) screen patients for activated PKA, which will FURTHER READING Raulet, D. Roles of the indicate whether they would be more successfully NKG2D immunoreceptor and its ligands. Nature Rev. Immunol. 3, 781–790 (2003) | Cerwenka, A. treated with tamoxifen or a pure anti-oestrogen & Lanier, L. L. Natural killer cells. Nature Rev. such as ICI-182780 (Fulvestrant/Faslodex). conformational change but had no effect on ICI- Immunol. 1, 41–49 (2001) | Yokoyama, W. M. & Tamoxifen is a very effective treatment for 182780. Even when a tenfold molar excess of Plougastel, B. F. M. Immune functions encoded by the natural killer gene complex. Nature Rev. oestrogen-receptor-positive (ER+) breast cancer ICI-182780 was added after tamoxifen treatment, Immunol. 3, 304–316 (2003) that has few side effects and even beneficial no change in conformation was observed, which properties such as promoting bone growth. demonstrates that tamoxifen still binds to ER-α but However, 50% of recurrent ER+ breast cancers are is unable to induce the inhibitory conformation. resistant to tamoxifen. One of the proposed Mutational analysis of consensus PKA mechanisms for this resistance is phosphorylation phosphorylation sites in ER-α showed that Ser305, of ER-α by PKA.Anti-oestrogens such as tamoxifen which is located in the hinge region, is necessary and ICI-182780 act by binding to ER-α and causing for forskolin to prevent tamoxifen-induced confor- a conformational change that prevents the mational change, and indicates that PKA renders recruitment of co-activators to ER-α. ER-α resistant to antagonism by tamoxifen. A recombinant ER-α was constructed that To further dissect the role of the PKA pathway contained a different variant of green fluorescent in tamoxifen resistance, the authors studied 70 protein at the amino and carboxyl termini to ER+ breast cancers isolated before tamoxifen measure changes in the fluorescence resonance treatment, 20 of which were later classed as energy transfer (FRET) signal that occurs when the tamoxifen-resistant. Gene expression analysis relative position of each fluorophore is altered by a showed that tamoxifen-resistant breast tumours change in receptor conformation. Using this assay, have downregulated PKA-RI-α (the PKA negative the authors studied the effect on receptor confor- regulatory subunit). Correspondingly, RNA mation of several factors that induce resistance to interference knockdown of PKA-RI-α induced tamoxifen (SRC1, cAMP and cyclin D1). ER-α resistance to tamoxifen, but not ICI-182780. FRET analysis showed that cAMP prevented Furthermore, T47D cells, which depend on tamoxifen from causing a conformational change oestrogen for growth, continued to grow in the of ER-α,whereas SRC1 and cyclin D1 had no effect presence of tamoxifen when PKA-RI-α was with or without cAMP.Not only did cAMP prevent downregulated, whereas ICI-182780 retained its tamoxifen antagonist action, it also caused the anti- inhibitory effect on growth. This agonist action of oestrogen to behave as an agonist and upregulated tamoxifen induced by PKA explains the regression ER-α-responsive reporter genes. By contrast, of tumours observed in tamoxifen-resistant cAMP could not overcome ICI-182780 antagonism patients when tamoxifen treatment is withdrawn. of ER-α.This is related to the fact that tamoxifen is Selection of patients for anti-oestrogen treatment a selective oestrogen receptor modulator (SERM) according to PKA activity should improve the that behaves as either an agonist or antagonist success rate of tamoxifen treatment and prevent depending on the cellular context, whereas the further outgrowth of resistant tumour cells. ICI-182780 is a full antagonist. Joanna Owens The authors then investigated the mechanism References and links ORIGINAL RESEARCH PAPER Michalides, R. et al. Tamoxifen of cAMP-induced resistance by studying the FRET resistance by a conformational arrest of the estrogen receptor α after pattern after exposure to forskolin, which stabilizes PKA activation in breast cancer. Cancer Cell 5, 597–605 (2004) WEB SITE the generation of cAMP and therefore the activation Rob Michalides Lab: http://www.nki.nl/nkidep/h4/michalides/ of PKA. Forskolin prevented tamoxifen-induced michalides.htm

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