Engineering and application of models for in vivo imaging of androgen receptor activity

Androgens are the male sex hormones - the major androgen is testosterone. They are pivotal for male fertility and responsible for masculinisation of genitalia. However, they are also required for female fertility; and in both sexes they are important for bone development and act on the brain, heart, fat and hair follicles. Disorders associated with too much or too little androgen action are common. Severe defects can cause a genetic male to become undervirilised or even female in appearance. Less severe deficiencies, even the fall in testosterone associated with aging, can lead to male infertility, decreased muscle, weight gain and depression. Conversely, excess androgen action has been linked to Polycystic Ovarian Syndrome, female infertility, acne, baldness prostate cancer, and behavioural disorders. Patients are often treated with androgen replacement therapy or antiandrogens (which oppose androgen action). However, due to the number of possible target tissues these almost invariably produce side effects. These can include reduced sperm count, jaundice, changes in libido, hirsutism, acne, osteoporosis and increased risk of certain cancers. New therapies are constantly being developed with the aim of reducing these ?off-target? effects, while male contraceptives and HRT are being developed containing androgens. It is therefore vital we learn more about where and when these act in the body. Knowledge of their sites of action will enable tailoring of such therapies to reduce side-effects.

Androgens act by binding to androgen receptor protein, which In turn binds to specific DNA sequences (androgen response elements) in target genes, increasing the activity of these genes, which then causes growth or function of the relevant tissue. This project will exploit this by linking an androgen response element to a ?reporter? gene, activity of which can be easily detected by scanning. This activation will be detected, accurately located and measured by state-of-the-art 3-dimensional scanners uniquely available at Imperial College. We will use the system to determine where and when natural androgens are active in males and females ? this may highlight previously unknown sites of androgen action. We will also follow activation or inhibition of androgen signalling by known and novel therapeutic agents, including androgens, antiandrogens and novel therapies. As well as increasing knowledge of the role of androgens in development and aging of both sexes, this project will benefit patients with all of the conditions mentioned above since it will allow evaluation of new and more specific therapies.

The many androgen target tissues include male internal and external genitalia, ovary, uterine endometrium and in both sexes bone, skeletal muscle, the CNS, adipose tissue and hair follicles. Disorders associated with excess of or deficiency in androgen action are common, including male and female infertility, pseudohermaphroditism, hypogonadism, benign and malignant prostate conditions, acne, baldness or hirsutism, sexual desire disorders and depression. Patients are often treated with androgens or antiandrogens as appropriate, but due to the number of targets these almost invariably produce side-effects. Androgens can exacerbate prostate conditions, suppress spermatogenesis and cause jaundice, hirsutism and acne. Antiandrogen therapy is associated with hepatic tumours, osteoporosis, loss of libido, impotence and gynaecomastia.

The major circulating androgen is testosterone, secreted by the testes, and in many target tissues this is converted to the more potent dihydrotestosterone. Both of these as well as weak adrenal androgens signal via the androgen receptor a ligand-activated transcription factor. This binds to specific response elements in the promoters of target genes and activates transcription of genes involved in growth and/or differentiation. We aim to exploit this by creating expression cassettes containing (i) the luciferase gene or (ii) the sodium iodide symporter gene under the control of an androgen response element, and create in vivo models expressing these reporters in all tissues. These will be used to visualize where and when the androgen receptor signalling pathway is activated in response to endogenous androgens during development and aging. Exogenous androgens, antiandrogens and selective AR modulators with therapeutic possibilities will also be tested under conditions mimicking those found in patients. Knowledge of their sites of action will predict possible side-effects hence suitability or limitations for therapy. Since imaging will be by non-invasive scanning using (i) luciferase imaging and (ii) more quantitative and accurate Positron Emission Tomography scanning for sodium iodide symporter activity, studies will be longitudinal and in real time. This study will increase knowledge of the role of androgens in differentiation, development and fertility of both sexes. Further, proposed crosses with models deficient in genes involved in androgen signalling will differentiate between the developmental actions of the major androgens testosterone and dihydrotestosterone, and elucidate crosstalk between androgen receptor and other steroid receptors. The model will also lend itself to many downstream applications such as the investigation of effects of hormonal environmental pollutants, androgen action in female fertility, and sexual dichotomy in obesity.