FUNCTIONAL ACTIVITIES OF THE A-FORM AND B-FORM OF THE HUMAN ANDROGEN RECEPTOR IN RESPONSE TO ANDROGEN RECEPTOR AGONISTS AND ANTAGONISTS

The androgen receptor (AR) is present in many cells in two forms. The B form migrates with an apparent mass of 110 kDa and constitutes more than 80% of the immunoreactive receptor in most cell types. The A form of the AR migrates with an apparent mass of 87 kDa, appears to derive from internal translation initiation at methionine-188 in the AR open -reading frame, and usually constitutes 20% or less of the immunoreact ive AR present. Previous experiments designed to examine the functiona l capacity of the A and B forms of the AR have been hampered by marked differences in the expression levels of the two isoforms, as the nucl eotide sequence surrounding the codon encoding methionine-188 causes i t to be used inefficiently as a translation initiation site. To circum vent this, we altered the nucleotide sequence surrounding methionine-1 88 to render it more similar to that surrounding the codon encoding me thionine-1. Transfection of a cDNA containing these changes resulted i n similar levels of expression of A and a forms of the AR as assessed by immunoblot assays using antibodies directed at an epitope preserved in both. Functional activities of these cDNAs were assessed using cot ransfection assays that employed two model androgen-responsive genes ( MMTV-luciferase and PRE2-tk-luciferase) in response to mibolerone, a p otent androgen agonist, in three different cell lines. These studies d emonstrated subtle differences in the activities of the A and B isofor ms, which depended on the promoter and cell context. Additional studie s failed to reveal any major differences in the responses of the AR-A and AR-B isoforms to a variety of androgen agonists and antagonists, s uggesting that the previously reported functional defect of the AR-A i s due principally to its level of expression. When assays of AR functi on are performed under conditions in which levels of expression of the two isoforms are equivalent, the AR-A and AR-B possess similar functi onal activities.