Androgen Receptor Phosphorylation

textabstractMany physiological processes in organisms are regulated by a relatively smallnumber of steroid honnones. Androgens are the so-called male sex steroidhormones which control growth, differentiation and functions of malereproductive and accessory sex tissues. Androgens are mainly produced in thetestis and circulate in the blood. They diffuse in and out of all cells, but areretained with high affinity and specificity in target cells by an intranuclearbinding protein, termed the androgen receptor. Once bound by androgens, theandrogen receptor undergoes a conformational change allowing the receptor tobind with high affinity to DNA and to modulate transcription of certain genes.The androgen receptor appears to be a transcription factor, regulated byandrogenic steroids. Phosphorylation is the predominant cellular mechanism forreversible modification of proteins, and the fact that many transcription factorsare phosphoproteins suggests a regulatory role of phosphorylation. In this thesis,studies on phosphorylation of the androgen receptor in human prostate tumorcells (LNCaP) are described. In LNCaP cells, the androgen receptor protein ispresent as two isofonlls with apparent molecular masses of 110 and 112 kDaduring SDS-PAGE. The 112 kDa isoform reflects the phosphorylated receptor,whereas the 110 kDa isoform is the non-phosphorylated receptor. Both isoformsare able to bind androgens with high affinity and can subsequently betransformed to the DNA binding form. It appears to be unlikely thatphosphorylation is involved in the regulation of steroid- or DNA bindingaffinity. Upon incubation of the prostate tumor cells with androgens, thephosphOlylation degree of the androgen receptor was rapidly increased. Multiplephosphorylation sites on serine residues are located in the N-terminal f/'al/~activationdomain and not in the DNA- and ligand binding domains. Trypticphosphopeptide maps of the androgen receptor show induction of phosphorylation at a novel site(s) by hormone treatment. It is proposed that thisextra phosphorylation in the N-terminal domain causes a conformational change,enabling protein-protein contacts of the trails-activation domain with othertranscription factors or co-activators on a target gene promoter.