Susan K. Logan

Src and Pyk2 Mediate G-protein-coupled Receptor Activation of Epidermal Growth Factor Receptor (EGFR) but Are Not Required for Coupling to the Mitogen-activated Protein (MAP) Kinase Signaling Cascade

The epidermal growth factor receptor (EGFR) and the non-receptor protein tyrosine kinases Src and Pyk2 have been implicated in linking a variety of G-protein-coupled receptors (GPCR) to the mitogen-activated protein (MAP) kinase signaling cascade. In this report we apply a genetic strategy using cells isolated from Src-, Pyk2-, or EGFR-deficient mice to explore the roles played by these protein tyrosine kinases in GPCR-induced activation of EGFR, Pyk2, and MAP kinase. We show that Src kinases are critical for activation of Pyk2 in response to GPCR-stimulation and that Pyk2 and Src are essential for GPCR-induced tyrosine phosphorylation of EGFR. By contrast, Pyk2, Src, and EGFR are dispensable for GPCR-induced activation of MAP kinase. Moreover, GPCR-induced MAP kinase activation is normal in fibroblasts deficient in both Src and Pyk2 (Src−/−Pyk2−/− cells) as well as in fibroblasts deficient in all three Src kinases expressed in these cells (Src−/−Yes−/−Fyn−/− cells). Finally, experiments are presented demonstrating that, upon stimulation of GPCR, activated Pyk2 forms a complex with Src, which in turn phosphorylates EGFR directly. These experiments reveal a role for Src kinases in Pyk2 activation and a role for Pyk2 and Src in tyrosine phosphorylation of EGFR following GPCR stimulation. In addition, EGFR, Src family kinases, and Pyk2 are not required for linking GPCRs with the MAP kinase signaling cascade.

Phosphatidylinositol 3-kinase mediates epidermal growth factor-induced activation of the c-Jun N-terminal kinase signaling pathway.

The signaling events which mediate activation of c-Jun N-terminal kinase (JNK) are not yet well characterized. To broaden our understanding of upstream mediators which link extracellular signals to the JNK pathway, we investigated the role of phosphatidylinositol (PI) 3-kinase in epidermal growth factor (EGF)-mediated JNK activation. In this report we demonstrate that a dominant negative form of PI 3-kinase as well as the inhibitor wortmannin blocks EGF-induced JNK activation dramatically. However, wortmannin does not have an effect on JNK activation induced by UV irradiation or osmotic shock. In addition, a membrane-targeted, constitutively active PI 3-kinase (p110beta) was shown to produce in vivo products and to activate JNK, while a kinase-mutated form of this protein showed no activation. On the basis of these experiments, we propose that PI 3-kinase activity plays a role in EGF-induced JNK activation in these cells.

Cell-specific Regulation of Androgen Receptor Phosphorylation in Vivo

The biological ramifications of phosphorylation of the androgen receptor (AR) are largely unknown. To examine the phosphorylation of AR at serine 213, a putative substrate for Akt, a phosphorylation site-specific antibody was generated. The use of this antibody indicated that AR Ser-213 is phosphorylated in vivo and that phosphorylation is tightly regulated in a cell type-specific manner. Furthermore, Ser-213 phosphorylation took place with rapid kinetics and was inhibited by the phosphatidylinositol 3-kinase inhibitor LY294002. Phosphorylation occurred in response to R1881 and dihydrotestosterone but weakly if at all in response to testosterone. It did not occur in response to AR antagonists or growth factor stimulation in the absence of an AR agonist. Transcription assays using an AR-responsive reporter gene construct showed that activated phosphatidylinositol 3-kinase inhibited transcription mediated by wild type AR but not that of a mutant AR variant (S213A), which could not be phosphorylated at Ser-213. By immunohistochemistry, the AR Ser(P)-213 antigen was detected in prostate epithelial but not stromal cells despite the fact that an antibody recognizing both phosphorylated and non-phosphorylated forms of AR demonstrates that AR is present in both cell types as expected. In fetal tissue the AR-Ser(P)-213 antigen was present in epithelial cells of the urogenital sinus when endogenous androgen levels were high and activated Akt was prevalent, but absent at a later stage of development when endogenous androgen levels were low and Akt activation was minimal. Immunoreactivity was evident in differentiated cells lining the lumen of the urogenital sinus but not in rapidly dividing, Ki67 positive cells within the developing prostate or stromal tissue, suggesting that site-specific phosphorylation of AR Ser-213 by cellular kinases occurs in a non-proliferating cellular milieu.

Inhibition of androgen receptor and β-catenin activity in prostate cancer

Androgen receptor (AR) is the major therapeutic target in aggressive prostate cancer. However, targeting AR alone can result in drug resistance and disease recurrence. Therefore, simultaneous targeting of multiple pathways could in principle be an effective approach to treating prostate cancer. Here we provide proof-of-concept that a small-molecule inhibitor of nuclear β-catenin activity (called C3) can inhibit both the AR and β-catenin–signaling pathways that are often misregulated in prostate cancer. Treatment with C3 ablated prostate cancer cell growth by disruption of both β-catenin/T-cell factor and β-catenin/AR protein interaction, reflecting the fact that T-cell factor and AR have overlapping binding sites on β-catenin. Given that AR interacts with, and is transcriptionally regulated by β-catenin, C3 treatment also resulted in decreased occupancy of β-catenin on the AR promoter and diminished AR and AR/β-catenin target gene expression. Interestingly, C3 treatment resulted in decreased AR binding to target genes accompanied by decreased recruitment of an AR and β-catenin cofactor, coactivator-associated arginine methyltransferase 1 (CARM1), providing insight into the unrecognized function of β-catenin in prostate cancer. Importantly, C3 inhibited tumor growth in an in vivo xenograft model and blocked renewal of bicalutamide-resistant sphere-forming cells, indicating the therapeutic potential of this approach.

Androgen receptor levels are upregulated by Akt in prostate cancer.

Multiple lines of evidence suggest a functional link between the androgen receptor (AR) and the serine/threonine kinase Akt in the development and progression of prostate cancer. To investigate the impact of Akt activity on AR homeostasis, we treated androgen-dependent LNCaP and LAPC-4 prostate cancer cells with Akt inhibitor. Akt inhibition decreased AR expression, suggesting that Akt activity was required for regulation of AR protein levels. However, while androgen-independent LNCaP-abl cells also showed diminished AR protein levels in response to Akt inhibition, treatment of androgen-independent LNCaP-AI cells failed to alter AR protein levels upon similar treatment, suggesting that AR protein levels in these androgen-independent prostate cells were regulated by mechanisms independent of Akt activation. Regulation of AR, downstream of activated Akt, also was observed in vivo when examining transgenic mice that overexpress constitutively active mutant myristoylated (myr)-Akt1 in the prostate. Transgenic mice expressing activated myr-Akt1 exhibited higher levels of AR mRNA and protein. Expression of activated myr-Akt1 did not alter prostate cell growth and no significant size differences between prostate tissues derived from transgenic animals were observed when comparing transgenic mice with wild-type mice. Still, transgenic mice overexpressing Akt exhibited higher levels of γH2AX and phosphorylated Chk2 in prostate tissue. These changes in markers associated with oncogene-induced senescence confirmed significant altered signaling in the transgenic mouse model. Overall, results presented here suggest that AR levels are regulated by the Akt pathway.

Down-regulation of p57Kip2 Induces Prostate Cancer in the Mouse

p57(Kip2) has been considered a candidate tumor suppressor gene because of its location in the genome, biochemical activities, and imprinting status. However, little is known about the role of p57(Kip2) in tumorigenesis and cancer progression. Here, we show that the expression of p57(Kip2) is significantly decreased in human prostate cancer, and the overexpression of p57(Kip2) in prostate cancer cells significantly suppressed cell proliferation and reduced invasive ability. In addition, overexpression of p57(Kip2) in LNCaP cells inhibited tumor formation in nude mice, resulting in well-differentiated squamous tumors rather than adenocarcinoma. Furthermore, the prostates of p57(Kip2) knockout mice developed prostatic intraepithelial neoplasia and adenocarcinoma. Remarkably, this mouse prostate cancer is pathologically identical to human prostate adenocarcinoma. Therefore, these results strongly suggest that p57(Kip2) is an important gene in prostate cancer tumorigenesis, and the p57(Kip2) pathway may be a potential target for prostate cancer prevention and therapy.

Phosphorylation of the androgen receptor by PIM1 in hormone refractory prostate cancer

Integration of cellular signaling pathways with androgen receptor (AR) signaling can be achieved through phosphorylation of AR by cellular kinases. However, the kinases responsible for phosphorylating the AR at numerous sites and the functional consequences of AR phosphorylation are only partially understood. Bioinformatic analysis revealed AR serine 213 (S213) as a putative substrate for PIM1, a kinase overexpressed in prostate cancer. Therefore, phosphorylation of AR serine 213 by PIM1 was examined using a phosphorylation site-specific antibody. Wild-type PIM1, but not catalytically inactive PIM1, specifically phosphorylated AR but not an AR serine-to-alanine mutant (S213A). In vitro kinase assays confirmed that PIM1 can phosphorylate AR S213 in a ligand-independent manner and cell type-specific phosphorylation was observed in prostate cancer cell lines. Upon PIM1 overexpression, AR phosphorylation was observed in the absence of hormone and was further increased in the presence of hormone in LNCaP, LNCaP-abl and VCaP cells. Moreover, phosphorylation of AR was reduced in the presence of PIM kinase inhibitors. An examination of AR-mediated transcription showed that reporter gene activity was reduced in the presence of PIM1 and wild-type AR, but not S213A mutant AR. Androgen-mediated transcription of endogenous PSA, Nkx3.1 and IGFBP5 was also decreased in the presence of PIM1, whereas IL6, cyclin A1 and caveolin 2 were increased. Immunohistochemical analysis of prostate cancer tissue microarrays showed significant P-AR S213 expression that was associated with hormone refractory prostate cancers, likely identifying cells with catalytically active PIM1. In addition, prostate cancers expressing a high level of P-AR S213 were twice as likely to be from biochemically recurrent cancers. Thus, AR phosphorylation by PIM1 at S213 impacts gene transcription and is highly prevalent in aggressive prostate cancer.

Sin3B Expression Is Required for Cellular Senescence and Is Up-regulated upon Oncogenic Stress

Serial passage of primary mammalian cells or strong mitogenic signals induce a permanent exit from the cell cycle called senescence. A characteristic of senescent cells is the heterochromatinization of loci encoding pro-proliferative genes, leading to their transcriptional silencing. Senescence is thought to represent a defense mechanism against uncontrolled proliferation and cancer. Consequently, genetic alterations that allow senescence bypass are associated with susceptibility to oncogenic transformation. We show that fibroblasts genetically inactivated for the chromatin-associated Sin3B protein are refractory to replicative and oncogene-induced senescence. Conversely, overexpression of Sin3B triggers senescence and the formation of senescence-associated heterochromatic foci. Although Sin3B is strongly up-regulated upon oncogenic stress, decrease in expression of Sin3B is associated with tumor progression in vivo, suggesting that expression of Sin3B may represent a barrier against transformation. Together, these results underscore the contribution of senescence in tumor suppression and suggest that expression of chromatin modifiers is modulated at specific stages of cellular transformation. Consequently, these findings suggest that modulation of Sin3B-associated activities may represent new therapeutic opportunities for treatment of cancers.

Analysis of URI Nuclear Interaction with RPB5 and Components of the R2TP/Prefoldin-Like Complex

Unconventional prefoldin RPB5 Interactor (URI) was identified as a transcriptional repressor that binds RNA polymerase II (pol II) through interaction with the RPB5/POLR2E subunit. Despite the fact that many other proteins involved in transcription regulation have been shown to interact with URI, its nuclear function still remains elusive. Previous mass spectrometry analyses reported that URI is part of a novel protein complex called R2TP/prefoldin-like complex responsible for the cytoplasmic assembly of RNA polymerase II. We performed a mass spectrometry (MS)-based proteomic analysis to identify nuclear proteins interacting with URI in prostate cells. We identified all the components of the R2TP/prefoldin-like complex as nuclear URI interactors and we showed that URI binds and regulates RPB5 protein stability and transcription. Moreover, we validated the interaction of URI to the P53 and DNA damage-Regulated Gene 1 (PDRG1) and show that PDRG1 protein is also stabilized by URI binding. We present data demonstrating that URI nuclear/cytoplasmic shuttling is affected by compounds that stall pol II on the DNA (α-amanitin and actinomycin-D) and by leptomycin B, an inhibitor of the CRM1 exportin that mediates the nuclear export of pol II subunits. These data suggest that URI, and probably the entire R2TP/prefoldin-like complex is exported from the nucleus through CRM1. Finally we identified putative URI sites of phosphorylation and acetylation and confirmed URI sites of post-transcriptional modification identified in previous large-scale analyses the importance of which is largely unknown. However URI post-transcriptional modification was shown to be essential for URI function and therefore characterization of novel sites of URI modification will be important to the understanding of URI function.

Genome-wide impact of Androgen Receptor Trapped Clone-27 Loss on Androgen-regulated Transcription in Prostate Cancer Cells

The androgen receptor (AR) directs diverse biological processes through interaction with coregulators such as AR trapped clone-27 (ART-27). Our results show that ART-27 is recruited to AR-binding sites by chromatin immunoprecipitation analysis. In addition, the effect of ART-27 on genome-wide transcription was examined. The studies indicate that loss of ART-27 enhances expression of many androgen-regulated genes, suggesting that ART-27 inhibits gene expression. Surprisingly, classes of genes that are up-regulated upon ART-27 depletion include regulators of DNA damage checkpoint and cell cycle progression, suggesting that ART-27 functions to keep expression levels of these genes low. Consistent with this idea, stable reduction of ART-27 by short-hairpin RNA enhances LNCaP cell proliferation compared with control cells. The effect of ART-27 loss was also examined in response to the antiandrogen bicalutamide. Unexpectedly, cells treated with ART-27 siRNA no longer exhibited gene repression in response to bicalutamide. To examine ART-27 loss in prostate cancer progression, immunohistochemistry was conducted on a tissue array containing samples from primary tumors of individuals who were clinically followed and later shown to have either recurrent or nonrecurrent disease. Comparison of ART-27 and AR staining indicated that nuclear ART-27 expression was lost in the majority of AR-positive recurrent prostate cancers. Our studies show that reduction of ART-27 protein levels in prostate cancer may facilitate antiandrogen-resistant disease.