Filamin-A fragment localizes to the nucleus to regulate androgen receptor and coactivator functions

Abstract

The androgen receptor (AR), a nuclear transcription factor, mediates male sexual differentiation, and its excessive action is associated with prostate cancer. We have characterized a negative regulatory domain in the AR hinge region, which interacted with filamin A (FLNa), an actin-binding cytoskeletal protein. FLNa interfered with AR interdomain interactions and competed with the coactivator transcriptional intermediary factor 2 to specifically down-regulate AR function. Although full-length FLNa was predominantly cytoplasmic, a C-terminal 100-kDa fragment of FLNa colocalized with AR to the nucleus. This naturally occurring FLNa fragment repressed AR transactivation and disrupted AR interdomain interactions and transcriptional intermediary factor 2-activated AR function in a manner reminiscent of full-length FLNa, raising the possibility that the inhibitory effects of cytoplasmic FLNa may be transduced through this fragment, which can localize to the nucleus and form part of the pre-initiation complex. This unanticipated role of FLNa adds to the growing evidence for the involvement of cytoskeletal proteins in transcription regulation. T he androgen receptor (AR), a member of the steroid͞ nuclear receptor superfamily, mediates male morphogenesis in utero, gametogenesis and prostate growth at puberty, and the development of prostatic cancer in older men. The AR has four principal domains: a large N-terminal transactivation domain (ARTAD), a DNA-binding domain (ARDBD), and a hinge domain, followed by the C-terminal ligand-binding domain (ARLBD). In the absence of androgen, it is generally accepted that AR is cytoplasmic. Androgens bind specifically to a ligandbinding pocket in the lower half of the LBD, causing a conformation change (1), the release of heat-shock proteins, and translocation of the ligand-AR complex to the nucleus, where the DBD interacts with specific response elements on the promoters of target genes. Transactivation functions reside mainly in the ARTAD, and very minimal activity can be demonstrated in the ARLBD (2, 3). Unlike other steroid receptors, transactivity depends on ligand-induced interactions between the ARTAD and ARLBD, and mutations that reduce TAD-LBD interactions affect AR activity, causing androgen insensitivity syndromes (4-6). Like other transcription regulators, the promoter-bound AR serves as a nidus to recruit cofactors that up-regulate (coactivators) or down-regulate (corepressors) AR activity. The most clearly defined class of coactivators is the p160͞steroid receptor coactivator (SRC) family. They include SRC1, transcriptional intermediary factor 2 (SRC2͞TIF2), and SRC3͞TRAM1͞A IB1͞pCIP͞ACTR͞ RAC3, who bind hydrophobic grooves in the LBDs of steroid receptors via LXXLL motifs in their nuclear-receptorinteracting boxes, draw in cAMP-response element-binding protein (CBP͞p300), pCAF, and other cofactors to modulate chromatin and initiate transcription by RNA polymerase II (7). Of the p160͞SRC proteins, TIF2 interacts most strongly with the AR (8) and, in concert with CBP͞p30...

Cytoskeletal Filamin A Differentially Modulates RNA Polymerase III Gene Transcription in Transformed Cell Lines

“…In fact, the expression levels of tRNA genes that were insensitive to knockdown of FLNA were normal; therefore, these insensitive tRNA genes cannot be regarded as inactive loci. In addition, tRNAs are essential to the process of translation, and FLNA has been described to regulate RNA polymerase II gene transcription (31)(32)(33). Therefore, it is possible that alteration of FLNA expression leads to a variation of expression for many proteins, which, accordingly, requires the differential expression of tRNA genes in cells.…”

New insights to the MLL recombinome of acute leukemias

“…This proteolytic fragment is able to bind and regulate androgen receptor. 23 Another study has demonstrated colocalization of FLNA and Caveolin 1. 24 Caveolin 1 is downregulated in tumor cells as it inhibits anchorage-independent growth, anoikis and invasiveness.…”

Hypoxia-induced and calpain-dependent cleavage of filamin A regulates the hypoxic response

“…Calpain proteases have been shown to cleave the H1 and H2 sites in vitro, producing a C-terminal 90-kDa fragment (8). Several studies suggest that FLNA, generally regarded as a cytoplasmic architectural molecule, may act as a nuclear transcriptional modulator through this endogenous proteolytic fragment, which is accumulated in the cell nucleus (9,10).…”