Attenuation of androgen receptor-dependent transcription by the serine/threonine kinase Pim-1

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 androgen receptor 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.

PIM-1 kinase interacts with the DNA binding domain of the vitamin D receptor: a further kinase implicated in 1,25-(OH)2D3 signaling

BACKGROUND

The vitamin D3 receptor (VDR) is responsible for mediating the pleiotropic and, in part, cell-type-specific effects of 1,25-dihydroxyvitamin D3 (calcitriol) on the cardiovascular and the muscle system, on the bone development and maintenance, mineral homeostasis, cell proliferation, cell differentiation, vitamin D metabolism, and immune response modulation.

RESULTS

Based on data obtained from genome-wide yeast two-hybrid screenings, domain mapping studies, intracellular co-localization approaches as well as reporter transcription assay measurements, we show here that the C-terminus of human PIM-1 kinase isoform2 (amino acid residues 135-313), a serine/threonine kinase of the calcium/calmodulin-regulated kinase family, directly interacts with VDR through the receptor's DNA-binding domain. We further demonstrate that PIM-1 modulates calcitriol signaling in HaCaT keratinocytes by enhancing both endogenous calcitriol response gene transcription (osteopontin) and an extrachromosomal DR3 reporter response.

CONCLUSION

These results, taken together with previous reports of involvement of kinase pathways in VDR transactivation, underscore the biological relevance of this novel protein-protein interaction.

Why target PIM1 for cancer diagnosis and treatment?

The highly conserved proto-oncogenic protein PIM1 is an unusual serine or threonine kinase, in part because it is constitutively active. Overexpression of PIM1 experimentally leads to tumor formation in mice, while complete knockout of the protein has no observable phenotype. It appears to contribute to cancer development in three major ways when it is overexpressed; by inhibiting apoptosis, by promoting cell proliferation and by promoting genomic instability. Expression in normal tissues is nearly undetectable. However, in hematopoietic malignancies and in a variety of solid tumors, increased PIM1 expression has been shown to correlate with the stage of disease. This characteristic suggests it can serve as a useful biomarker for cancer diagnosis and prognosis. Several specific and potent inhibitors of PIM1’s kinase activity have also been shown to induce apoptotic death of cancer cells, to sensitize cancer cells to chemotherapy and to synergize with other anti-tumor agents, thus making it an attractive therapeutic target.

Pim1 promotes human prostate cancer cell tumorigenicity and c-MYC transcriptional activity

Background

The serine/threonine kinase PIM1 has been implicated as an oncogene in various human cancers including lymphomas, gastric, colorectal and prostate carcinomas. In mouse models, Pim1 is known to cooperate with c-Myc to promote tumorigenicity. However, there has been limited analysis of the tumorigenic potential of Pim1 overexpression in benign and malignant human prostate cancer cells in vivo.

Methods

We overexpressed Pim1 in three human prostate cell lines representing different disease stages including benign (RWPE1), androgen-dependent cancer (LNCaP) and androgen-independent cancer (DU145). We then analyzed in vitro and in vivo tumorigenicity as well as the effect of Pim1 overexpression on c-MYC transcriptional activity by reporter assays and gene expression profiling using an inducible MYC-ER system. To validate that Pim1 induces tumorigenicity and target gene expression by modulating c-MYC transcriptional activity, we inhibited c-MYC using a small molecule inhibitor (10058-F4) or RNA interference.

Results

Overexpression of Pim1 alone was not sufficient to convert the benign RWPE1 cell to malignancy although it enhanced their proliferation rates when grown as xenografts in vivo. However, Pim1 expression enhanced the in vitro and in vivo tumorigenic potentials of the human prostate cancer cell lines LNCaP and DU145. Reporter assays revealed increased c-MYC transcriptional activity in Pim1-expressing cells and mRNA expression profiling demonstrated that a large fraction of c-MYC target genes were also regulated by Pim1 expression. The c-MYC inhibitor 10058-F4 suppressed the tumorigenicity of Pim1-expressing prostate cancer cells. Interestingly, 10058-F4 treatment also led to a reduction of Pim1 protein but not mRNA. Knocking-down c-MYC using short hairpin RNA reversed the effects of Pim1 on Pim1/MYC target genes.

Conclusion

Our results suggest an in vivo role of Pim1 in promoting prostate tumorigenesis although it displayed distinct oncogenic activities depending on the disease stage of the cell line. Pim1 promotes tumorigenicity at least in part by enhancing c-MYC transcriptional activity. We also made the novel discovery that treatment of cells with the c-MYC inhibitor 10058-F4 leads to a reduction in Pim1 protein levels.

Pharmacologic inhibition of Pim kinases alters prostate cancer cell growth and resensitizes chemoresistant cells to taxanes

The serine/threonine family of Pim kinases function as oncogenes and have been implicated in prostate cancer progression, particularly in hormone-refractory prostate disease, as a result of their antiapoptotic function. In this study, we used a pharmacologic inhibitor targeting the Pim family members, SGI-1776, to determine whether modulation of Pim kinase activity could alter prostate cancer cell survival and modulate chemotherapy resistance. Extensive biochemical characterization of SGI-1776 confirmed its specificity for the three isoforms of the Pim family. Treatment of prostate cancer cells with SGI-1776 resulted in a dose-dependent reduction in phosphorylation of known Pim kinase substrates that are involved in cell cycle progression and apoptosis (p21(Cip1/WAF1) and Bad). Consequently, SGI-1776 compromised overall cell viability by inducing G(1) cell cycle arrest and triggering apoptosis. Overexpression of recombinant Pim-1 markedly increased sensitivity of SGI-1776-mediated prostate cancer cell apoptosis and p21(Cip1/WAF1) phosphorylation inhibition, reinforcing the specificity of SGI-1776. An additional cytotoxic effect was observed when SGI-1776 was combined with taxane-based chemotherapy agents. SGI-1776 was able to reduce cell viability in a multidrug resistance 1 protein-based taxane-refractory prostate cancer cell line. In addition, SGI-1776 treatment was able to resensitize chemoresistant cells to taxane-based therapies by inhibiting multidrug resistance 1 activity and inducing apoptosis. These findings support the idea that inhibiting Pim kinases, in combination with a chemotherapeutic agent, could play an important role in prostate cancer treatment by targeting the clinical problem of chemoresistance.

Potential roles for the PIM1 kinase in human cancer - a molecular and therapeutic appraisal

In vitro experiments have shown the PIM1 kinase to have diverse biological roles in cell survival, proliferation and differentiation. In humans, PIM1 is often expressed in both normal and transformed cells. The PIM1 kinase is a true oncogene implicated in early transformation and tumour progression in haematopoietic malignancies and prostate carcinomas. It is associated with aggressive subgroups of lymphoma, is a marker of poor prognosis in prostate carcinomas and has been suggested to have a role in hormone insensitivity of prostate malignancies. PIM1 has a possible role in other carcinomas with 6p21 genomic alterations. On one hand, PIM1 (due to its role in malignancy) appears to be a promising target for drug development programmes but, on the other hand, the complexity of its molecular structure has posed challenges in the development of PIM1 inhibitors. In this review we discuss PIM1 expression in human tissues (including some new data from our laboratory), its role in human malignancies, as well as the possibilities and challenges in the development of target therapy for PIM1.

Small carboxyl-terminal domain phosphatase 2 attenuates androgen-dependent transcription

Small carboxyl-terminal domain (CTD) phosphatase 2 (SCP2) was identified and verified as a protein that interacts with the androgen receptor (AR). Ectopic expression of SCP2 or two other family members, SCP1 and SCP3, attenuated AR transcriptional activity in LNCaP cells and were recruited in an androgen- and AR-dependent fashion onto the prostate-specific antigen (PSA) promoter. Silencing SCP2 and SCP1 by short hairpin RNAs increased androgen-dependent transcription of the PSA gene and augmented AR loading onto the PSA promoter and enhancer. SCP2 also attenuated glucocorticoid receptor (GR) function, and its silencing increased dexamethasone-mediated PSA mRNA accumulation and GR loading onto the PSA enhancer in LNCaP 1F5 cells. SCP2 silencing was accompanied by augmented recruitment and earlier cycling of RNA polymerase II on the promoter. Ser 5 phosphorylation of the RNA polymerase II CTD, a process necessary for initiation of transcription elongation, occurred significantly earlier in SCP2-silenced than parental LNCaP cells. Collectively, our results suggest that SCP2 is involved in promoter clearance during steroid-activated transcription.

Overexpression of Pim-1 during progression of prostatic adenocarcinoma

AIMS

Pim-1 is a serine/threonine kinase that has been shown to play an integral role in the development of a number of human cancers, such as haematolymphoid malignancies. Recently, evidence has shown Pim-1 to be important in prostatic carcinogenesis. In order to further our understanding of its role in prostate cancer, we investigated Pim-1 expression in normal, premalignant, and malignant prostate tissue.

METHODS

Using immunohistochemistry, Pim-1 expression was analysed in prostate tissue from 120 radical prostatectomy specimens. In each case, Pim-1 staining was evaluated in benign prostatic epithelium, high grade prostatic intraepithelial neoplasia (PIN), and prostatic adenocarcinoma. The number of positively staining cells was estimated, and the intensity of staining was scored on a scale of 0 to 3+.

RESULTS

Pim-1 immunoreactivity was identified in 120 cases (100%) of adenocarcinoma, 120 cases (100%) of high grade PIN, and 62 cases (52%) of benign glands. The number of cells staining in benign epithelium (mean 34%) was much lower than that in high grade PIN (mean 80%; p<0.0001) or adenocarcinoma (mean, 84%; p<0.0001). There was no significant difference between high grade PIN and adenocarcinoma in the percentage of cells staining positively for Pim-1 (p = 0.34). The staining intensity for Pim-1 was significantly lower in benign prostatic epithelium than in PIN and adenocarcinoma (p<0.001). There was no statistically significant correlation between the level of Pim-1 expression and Gleason score, patient age, tumour stage, lymph node metastasis, perineural invasion, vascular invasion, surgical margin status, extraprostatic extension, or seminal vesicle invasion.

CONCLUSIONS

Pim-1 expression is elevated in PIN and prostatic adenocarcinoma compared with benign prostatic epithelium. This finding suggests that upregulation of Pim-1 may play a role in prostatic neoplasia.

Cyclin G-associated kinase: A novel androgen receptor-interacting transcriptional coactivator that is overexpressed in hormone refractory prostate cancer

The androgen receptor (AR), a steroid receptor family member, is a ligand-dependent transcription factor that has an integral role in normal prostate development. Alterations in AR-mediated activity can result in abnormal gene expression, dysregulated cell growth and prostate cancer. Coregulator proteins that interact with AR to influence activity and specificity of the AR-response may also have an important role in prostate cancer progression. Since the NH(2)-terminal domain (NTD) of AR encodes the ligand-independent activation function (AF)-1, this domain is incompatible with conventional yeast two-hybrid systems. Therefore, we have used the Tup1 repressed transactivator (RTA) system, which exploits the intrinsic transactivation properties of AR.NTD, for identification of novel AR-interacting proteins. Using this system, cyclin G-associated kinase (GAK) was identified as an AR interacting protein, and GST pull-down assays were used to confirm the interaction. GAK was shown to enhance the AF-1 function of AR activity in a ligand-dependent manner. Additionally, GAK enhanced the AR transcriptional response even at low concentrations of androgens, which is relevant to AR activity in androgen-independent prostate cancer. Finally, neo-adjuvant hormone therapy (NHT) tissue microarray analysis demonstrated that GAK expression increased significantly with prostate cancer progression to androgen independence, which suggests a prognostic role for GAK in advanced disease.