Androgen signalling and prostate cancer

Bevan laboratory 2010 (L-R) Sue Powell, Rachel Culley, Derek Lavery, Charlotte Bevan, Greg Brooke, Claire Fletcher, Alwyn Dart, Laia Querol Cano

The Androgen Receptor

Growth and differentiation of the prostate gland during development is dependent on the androgens testosterone and dihydrotestosterone. After puberty, growth-quiescent maintenance of the organ occurs in the presence of high levels of testosterone. However, in cases of malignant transformation of the prostate, androgen-dependent growth resumes. The cellular effects of androgens are mediated via the androgen receptor, a ligand-activated transcription factor and member of the nuclear receptor superfamily. Hence the androgen receptor is a key protein in prostate cancer progression and the focus of our work.
Liganded androgen receptor binds to specific sequences (androgen response elements) in the regulatory elements of target genes and promotes transcription. To facilitate this, it recruits coactivator proteins, which increase the rate of transcription by promoting histone acetylation and interacting with the basal transcriptional machinery (Bevan & Parker, 1999). The best-characterised coactivators include Steroid Coactivator 1 (SRC1) and Creb-Binding Protein (CBP). Conversely, corepressor proteins have also been identified which interact with nuclear receptors in the absence of ligand or the presence of antagonists, and repress transcription by mechanisms such as histone deacetylation. Their role in androgen receptor signalling in the prostate is not clear. We have identified several novel androgen receptor corepressors, including prohibitin (Gamble et al, 2004 & 2007) and Hey1 (Belandia et al, 2005), and are working to understand their mechanisms of action and role and significance in prostate cancer progression.

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Identifying mechanisms of progression to androgen independence

As prostate cancer growth is initially androgen-dependent, treatment involves removing circulating androgens and opposing their action using antiandrogens. Antiandrogens bind to the androgen receptor but inhibit inhibit androgen-dependent growth via mechanisms which are largely unknown (Whitaker et al, 2004). This treatment initially arrests the disease in the majority of cases. However, symptomatic relapse almost inevitably occurs 2-3 years later. An understanding of the molecular events that characterize this progression to "androgen independence" is important for new targets for treatment of advanced disease to be developed. The term "androgen independent" to describe advanced disease is misleading, since androgen receptor expression is almost never lost and the subsequent transcriptional events following AR stimulation still occur. It appears that in many cases the response to residual levels of androgens or other circulating hormones in the patient could be amplified due to one of several factors including mutation of the androgen receptor and alteration in levels of cofactor proteins. We aim to identify and characterise cofactors involved in androgen/antiandrogen response and ascertain how they interact with wild-type receptor and how these interactions are altered by receptor mutations identified in advanced prostatic tumours. We have characterised the interactions of androgen receptor with the well-known coactivator SRC1 and found important differences between androgen receptor and other nuclear receptors (Bevan et al, 1999, Powell et al, 2004). We have also identified novel androgen receptor corepressors, including (in collaboration with Professor Parker, Institute of Reproductive and Developmental Biology) the Notch target protein Hey1, which could be involved in progression to the hormone-refractory stage of the disease (Belandia et al, 2004).

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Understanding the effects of androgens in the prostate

Little is known about how androgens exert their effect in target tissues. We are using proteomics to identify androgen-regulated target proteins in the prostate. Ultimately these downstream targets may provide novel targets for use in prostate cancer therpay when treatments aimed at inhibiting the androgen receptor itself fail. We found that prohibitin, a putative tumour suppressor protein, is downregulated by androgens (Gamble et al, 2004). We demonstrated that the reduction of prohibitin levels in androgen-stimulated prostate cancer cells was necessary for cell cycle to progress, and that reduction of prohibitin levels caused an increase in cells entering cell cycle. Both of these facts suggest that prohibitin has a vital role to play in the control of androgen response in prostate cancer cells and is a possible target for future therapies.

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Androgen receptor mutations and prostate cancer progression

In approximately 30% of patients, therapy failure is associated with mutation of the androgen receptor. Several receptor variants associated with advanced disease show promiscuous activation by other hormones and antiandrogens. Such loss of specificity could promote receptor activation hence tumour growth in the absence of conventional ligands, explaining therapy failure. We aim to elucidate mechanisms by which alternative ligands promote receptor activation and the extent to which this contributes to failure of current therapies. We have found that the most commonly identified variants in tumours showed differences in coactivator recruitment compared to wild-type receptor, dependent upon ligand and the interaction motif utilized (Brooke et al, 2007). Coexpression and knockdown of coactivators that bind via different motifs, combined with chromatin immunoprecipitation and quantitative PCR, revealed these preferences extend to coactivator recruitment in vivo and affect receptor activity at the transcriptional level, with subsequent effects on target gene regulation. The findings suggest that mutant receptors, activated by alternative ligands, drive growth via different mechanisms to androgen-activated wild-type receptor. Tumours may hence behave differently dependent upon any androgen receptor mutation present and what ligand is driving growth, as distinct subsets of genes may be regulated.

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Markers for prostate cancer

Initial screening for prostate disease involves measurement of PSA (Prostate Specific Antigen). PSA concentration correlates with tumour size in prostatic cancer. Success of treatment is indicated by reduction in blood PSA levels and screening ascertains treatment failure, as indicated by a rise in PSA. The flaw in use of PSA as a marker for prostate cancer is that a high PSA level cannot differentiate between prostate cancer and benign prostatic hyperplasia. This necessitates further tests, which are distressing to the patient and time-consuming. Identification of new diagnostic tests will help detection at an earlier stage of the disease when it is easier to treat. We are collaborating with Professor Nicholson's team in the Department of Biological Chemistry (BMS) to use metabonomic methods to identify new metabolic profiles specific for prostate cancer. It is hoped that, as well as providing a definitive diagnosis of malignant disease, they will facilitate tracking of the disease throughout treatment, acting as a warning of failure of one therapy to allow the implementation of another, thus prolonging the recurrence -free period for the patient.

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p23: its role in androgen signalling and prostate cancer

Prostate cancer is an androgen-dependent disease, hence therapies are aimed at inhibiting androgen signalling. The androgen receptor requires the activity of the cytoplasmic HSP90 heterocomplex to acquire ligand-binding competency. A vital component of this complex is the co-chaperone p23. We have found that p23 appears to have a further role in androgen signalling, since it can increase androgen receptor activity at the transcriptional level. Interestingly, this effect of p23 appears to be steroid receptor-specific, since it has the opposite effect on glucocorticoid receptor activity but a similar effect on oestrogen signalling.
The androgen receptor, like many other HSP90 client proteins, is key in cancer progression since it drives prostate cancer growth. Hence HSP90 has recently been identified as a possible therapeutic target in prostate cancer. We postulate that p23, in isolation or in addition to HSP90, is another such target. We propose to study alterations of p23 expression in prostate cancer and the effects of manipulation of p23 levels and activity on prostate tumour growth. We will also elucidate the mechanism(s) by which p23 affects androgen receptor activity. This will help to determine whether p23 is a useful target for novel prostate cancer therapy.

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Recent publications

Reebye V; Bevan CL; Nohadani M; Hajitou A; Habib NA; Mintz PJ. (2010). Interaction between AR signalling and CRKL bypasses casodex inhibition in prostate cancer. Cell Signal. In press.

Teahan, O., Bevan, C. L., Waxman, J. and Keun, H. C. (2010) Metabolic signatures of malignant progression in prostate epithelial cells. Int. J. Biochem Cell Biol. In press

Villaronga, M.A., Lavery, D. N, Bevan, C. L., Llanos, S. and Belandia, B. (2010) HEY1 Leu94Met gene polymorphism dramatically modifies its biological functions. Oncogene. 29: 411-20.

Dart, D. A., Spencer-Dene, B., Gamble, S. C., Waxman, J. and Bevan, C. L. (2009) Manipulating prohibitin levels provides evidence for an in vivo role in androgen regulation of prostate tumours. Endocr Relat Cancer. 16: 1157-69.

 Brooke, G. N. and Bevan, C. L. (2009). The role of androgen receptor mutations in progression of prostate cancer. Current Genomics. 10: 18-25.

Villaronga MA, Bevan CL, Belandia B. (2008). Notch signalling: a potential therapeutic target in prostate cancer. Curr Cancer Drug Targets. 8: 566-80

Brooke, G. N., Parker, M. G. and Bevan, C. L. (2007). Mechanisms of AR activation in advanced prostate cancer: differential coactivator recruitment and gene expression. Oncogene. 27: 2941-50.

Gamble, S. C., Odontiadis, M., Chotai, D., Dart, D. A., Brooke, G. N., Powell, S. M., Reebye, V., Varela-Carver, A., Kawano, Y., Waxman, J. and Bevan, C. L. (2007) Prohibitin, a protein downregulated by androgens, represses androgen receptor activity. Oncogene. 26:1757-1768

Chang, G. G., Gamble, S. G., Jhama, M., Wait, R., Bevan, C. L., Brinkmann, A. O. (2007) Proteomic analysis of proteins regulated by TRPS1 transcription factor in DU145 prostate cancer cells. Biochim Biophys Acta. 1774:575-82.

Powell, S. M., Gamble, S. C., Whitaker, H. Brooke, G., Reebye, V., Varela-Carver, A., Belandia, B., Garcia-Pedrero, J., Parker, M. G., Wait, R., Chotai, D., Dart, D.A. and Bevan, C. L., (2006)  Mechanisms of action of therapeutic antiandrogens. Biochemical Society Transactions. 34: 1124-7.

Brooke, G. N. and Bevan, C. L. (2006) "The androgen receptor and its role in progression of prostate cancer from androgen dependence to androgen independence". IBScientific Journal of Science. 1: 52-63.

Teahan, O., Gamble, S. G., Holmes, E., Waxman, J., Nicholson, J., Bevan, C. L. and Keun, H. (2006) "Impact of analytical bias in metabonomic studies of human blood serum and plasma". Analytical Chemistry. 78:4307-18.

Bevan, C. L. (2005) "Androgen receptor in prostate cancer: cause or cure?". Trends in Endocrinology and Metabolism 16: 395-7

Belandia, B., Powell, S., Garcia-Pedrero, J., Walker, M. M., Bevan, C.L., and Parker, M.G. (2005) "Hey1, a mediator of Notch signalling, is an androgen receptor corepressor". Molecular and Cellular Biology. 25:1425-36.

Whitaker, H. C., Hanrahan, S., Totty, N. F., Gamble, S.C., Waxman,J., Cato, A.C.B., Hurst, H. C. and Bevan, C. L. (2004) "Androgen receptor is targeted to distinct sub-cellular compartments in response to different therapeutic antiandrogens". Clinical Cancer Research. 10: 7392-401.

Powell SM, Christiaens V, Voulgaraki D, Waxman J, Claessens F, Bevan CL. (2004) "Mechanisms of androgen receptor signalling via steroid receptor coactivator-1 in prostate". Endocr Relat Cancer 11: 117-130

Gamble SC, Odontiadis M, Waxman J, Westbrook JA, Dunn MJ, Wait R, Lam EW, Bevan CL. (2004) "Androgens target prohibitin to regulate proliferation of prostate cancer cells". Oncogene 23: 2996-3004

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