My laboratory studies how nuclear transport and signal transduction control the compartmentalization and activity of transcription factors, particularly in the context of prostate cancer. A major focus of these studies is the androgen receptor (AR), a steroid hormone receptor critical for prostate cell growth. AR must undergo nuclear import in order to function as a transcription factor, a process that is normally regulated by androgen. There is compelling evidence, however, that AR acquires the ability to undergo androgen-independent import in advanced prostate cancer. Defining the underlying transport mechanism might reveal new strategies to inhibit AR activity in proliferating prostate cells. AR undergoes cycles of nuclear import and export (nucleocytoplasmic shuttling). While nuclear export would be expected to provide an effective mechanism for terminating a transcriptional response to androgen, we have recently found that AR translocation to the cytoplasm is important for its activity in the nucleus. This apparent paradox may reflect an undefined step in AR maturation, or crosstalk between AR and signal transduction pathways in the cytoplasm. My laboratory is interested in the mechanisms responsible for protein and RNA transport between the nucleus and cytoplasm. Transport between these compartments is mediated by the nuclear pore complex, a supramolecular assembly of proteins organized into an elaborate channel that spans the double membrane system of the nuclear envelope. The nuclear pore complex mediates bi-directional movement of proteins and RNA by facilitated transport pathways. Both import and export pathways involve shuttling receptors that recognize specific targeting signals contained within the protein or RNA cargo. Receptor-cargo complexes are rapidly translocated through the central gated channel of the nuclear pore complex over a distance of more than 100 nanometers. Receptors are then recycled to the original compartment for a new round of transport. Interactions between receptors, cargo, and the nuclear pore complex are controlled by several types of regulators, including the Ran GTPase and its effector proteins.

AR is the target of multiple kinases, and we have generated phosphosite antibodies to study the pathways and functions of phospho-regulation. In the course of these studies we discovered a novel mechanism for loading protein phosphatase 2A (PP2A) onto AR. The loading mechanism requires small t antigen, a product encoded by SV40 that binds and alters the structure of a PP2A subunit. Current experiments are aimed at defining the structural basis of the PP2A loading reaction, as well as determining the cellular factors that mediate PP2A loading onto AR in untransformed cells.

Translocation of AR through the NPC relies on three features common to most nuclear transport pathways: (i) the use of nuclear import or export signals; (ii) the recognition of these signals by receptors that mediate translocation through the NPC; and (iii) RanGTPase-dependent assembly and disassembly of transport complexes. The signals that specify nuclear import and export of AR, the receptors that mediate AR translocation, and the role of the RanGTPase in AR transport are all under investigation in my laboratory. This analysis includes a Mog1, a nuclear protein that regulates the GTPase cycle of Ran.

Selected References

Black BE, Vitto MJ, Gioeli D, Spencer A, Afshar N, Conaway MR, Weber MJ, PaschalBM. (2004) "Transient, ligand-dependent arrest of the androgen receptor in subnuclear foci alters phosphorylation and coactivator interactions." Mol Endocrinol. 18:834-50. Epub 2003 Dec 18. [PubMed]

Black BE, Paschal BM. (2004) "Intranuclear organization and function of the androgen receptor." Trends Endocrinol Metab. 15:411-7. [PubMed]