Dasatinib inhibits site-specific tyrosine phosphorylation of androgen receptor by Ack1 and Src kinases

Activation of androgen receptor (AR) may play a role in the development of castration resistant prostate cancer. Two intracellular tyrosine kinases, Ack1 (activated cdc42-associated kinase) and Src, phosphorylate and enhance AR activity and promote prostate xenograft tumor growth in castrated animals. However, the upstream signals that activate these kinases and lead to AR activation are incompletely characterized. In this study, we investigated AR phosphorylation in response to non-androgen ligand stimulation using phospho-specific antibodies. Treatment of LNCaP and LAPC-4 cells with epidermal growth factor (EGF), heregulin, Gas6 (ligand binding to Mer receptor tyrosine kinase and activating Ack1 downstream), interleukin (IL)-6 or bombesin stimulated cell proliferation in the absence of androgen. Treatment of LNCaP and LAPC-4 cells with EGF, heregulin, or Gas6 induced AR phosphorylation at Tyr-267; IL-6 or bombesin treatment did not. AR phosphorylation at Tyr-534 was induced by treatment with EGF, IL-6 or bombesin, but not by heregulin or Gas6. siRNA-mediated knockdown of Ack1 or Src showed that Ack1 mediates heregulin- and Gas6-induced AR Tyr-267 phosphorylation whereas Src mediates Tyr-534 phosphorylation induced by EGF, IL-6, and bombesin. Dasatinib, a Src inhibitor, blocked EGF-induced Tyr-534 phosphorylation. In addition, we show dasatinib also inhibited Ack1 kinase. Dasatinib inhibited heregulin-induced Ack1 kinase activity and AR Tyr-267 phosphorylation. Dasatinib inhibited heregulin-induced AR-dependent reporter activity. Dasatinib also inhibited heregulin-induced expression of endogenous AR target genes. Dasatinib inhibited Ack1-dependent colony formation and prostate xenograft tumor growth in castrated mice. Interestingly, Ack1 or Src knockdown or dasatinib did not inhibit EGF-induced AR Tyr-267 phosphorylation or EGF-stimulated AR activity, suggesting the existence of an additional tyrosine kinase that phosphorylates AR at Tyr-267. These data suggest that specific tyrosine kinases phosphorylate AR at distinct sites and that dasatinib may exert anti-tumor activity in prostate cancer through inhibition of Ack1.

Restoration of PTEN expression alters the sensitivity of prostate cancer cells to EGFR inhibitors

INTRODUCTION

Prostate cancer (CaP) progression from an androgen-dependent to an androgen-independent state is associated with overexpression of EGFR family members or activation of their downstream signaling pathways, such as PI3K-Akt and MAPK. Although there are data implicating PI3K-Akt or MAPK pathway activation with resistance to EGFR inhibitors in CaP, the potential cross-talk between these pathways in response to EGFR or MAPK inhibitors remains to be examined.

METHODS

Cross-talk between PTEN and MAPK signaling and its effects on CaP cell sensitivity to EGFR or MAPK inhibitors were examined in a PTEN-null C4-2 CaP cell, pTetOn PTEN C4-2, where PTEN expression was restored conditionally.

RESULTS

Expression of PTEN in C4-2 cells exposed to EGF or serum was associated with increased phospho-ERK levels compared to cells without PTEN expression. Similar hypersensitivity of MAPK signaling was observed when cells were treated with a PI3K inhibitor LY294002. This enhanced sensitivity of MAPK signaling in PTEN-expressing cells was associated with a growth stimulatory effect in response to EGF. Furthermore, EGFR inhibitors gefitinib and lapatinib abrogated hypersensitivity of MAPK signaling and cooperated with PTEN expression to inhibit cell growth in both monolayer and anchorage-independent conditions. Similar cooperative growth inhibition was observed when cells were treated with the MEK inhibitor, CI1040, in combination with PTEN expression suggesting that inhibition of MAPK signaling could mediate the cooperation of EGFR inhibitors with PTEN expression.

CONCLUSIONS

Our results suggest that signaling cross-talk between the PI3K-Akt and MAPK pathways occurs in CaP cells, highlighting the potential benefit of targeting both the PI3K-Akt and MAPK pathways in CaP treatment.

Conditional expression of PTEN alters the androgen responsiveness of prostate cancer cells

BACKGROUND

Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) is lost as a function of prostate tumor androgen dependence. While the transcriptional activity of the androgen receptor (AR) is inhibited by PTEN in androgen sensitive prostate cancer (CaP), the role of PTEN in androgen disease is unclear.

METHODS

We developed a system where PTEN can be conditionally re-expressed at physiologic levels into a PTEN null metastatic human CaP cell line, C4-2, and androgen responsiveness examined.

RESULTS

PTEN induction reduces cell growth and blocks the growth effect of synthetic androgen R1881. The anti-androgen Casodex enhances the growth-inhibitory action of PTEN and this effect is independent of Akt phosphorylation. Combined PTEN induction and Casodex, result in a further decrease in prostate specific antigen promoter activity compared to PTEN but not Casodex alone.

CONCLUSIONS

PTEN induction confers androgen independent CaP cells enhanced responsiveness to the anti-proliferative effects of anti-androgens and this action may involve non-AR mediated effects.

Analysis of oncogene, tumor suppressor gene, and chromosomal alterations in HeLa x osteosarcoma somatic cell hybrids

Using a series of tumorigenic and non-tumorigenic somatic cell hybrids that resulted from the fusion of the human osteosarcoma cell line OHS50-P16T (P16T) with the HeLa cell line D98OR, we investigated the role that genetic mutations, including alterations of oncogenes, tumor suppressor genes, and chromosomes, play in P16T tumorigenicity. Analysis of a previously identified oncogene mutation, c-myc amplification, in the P16T cell line demonstrated that both the tumorigenic and non-tumorigenic hybrids contained the amplified c-myc gene. Analysis of previously identified P16T tumor suppressor gene alterations, p53 mutation, and loss of RB1 expression demonstrated that the mutated p53 gene was selectively maintained in both the non-tumorigenic and tumorigenic hybrids, whereas loss of RB1 expression was not maintained in either the non-tumorigenic or tumorigenic hybrids. Chromosomes 11, 13, 17, and 22 were analyzed for loss of heterozygosity (LOH) to characterize the status of these previously described chromosomal alterations in the tumorigenic and non-tumorigenic hybrids. Loss of HeLa D98OR chromosome 22, with maintenance of P16T chromosome 22, was observed in the tumorigenic hybrids, a result confirmed by LOH analysis, which demonstrated the specific loss of HeLa chromosome 22 genetic material in the tumorigenic segregants. Together, these results demonstrated that amplified c-myc, mutant p53, and RB1 genes seem to be important in osteosarcoma tumorigenicity and that an additional altered gene or genes on chromosome 22 may play a key role in osteosarcoma tumorigenicity.

Activation of mitogen-activated protein kinase associated with prostate cancer progression

Using an antibody specific for dually phosphorylated extracellular-regulated kinases 1 and 2, we have examined 82 primary and metastatic prostate tumor specimens for the presence of activated mitogen-activated protein (MAP) kinase. Nonneoplastic prostate tissue showed little or no staining with activated MAP kinase antiserum. In prostate tumors, the level of activated MAP kinase increased with increasing Gleason score and tumor stage. In a separate analysis, tumor samples from two patients showed no activation of MAP kinase before androgen ablation therapy; however, following androgen ablation treatment, high levels of activated MAP kinase were detected in the recurrent tumors. Collectively, these data suggest an increase in the activation of the MAP kinase signal transduction pathway as prostate cancer progresses to a more advanced and androgen-independent disease.

Identification of additional complementation groups that regulate genomic instability

By somatic cell hybridization, amplification has been found to be a recessive genetic trait in three tumor cell lines examined. Studies with transgenic mice have shown that amplification frequency can be altered by a lack of wild-type TP53 (p53) activity. Other factors may regulate this phenotype in tumor cell lines possessing both wild-type p53 activity and amplification ability. Complementation analysis of somatic cell hybrids was performed to delineate groups of tumor cell lines that share a common defect that modulates the ability to amplify. The amplification frequencies of three normal fibroblast x tumor hybrids were suppressed 10-100-fold from parental tumor values, extending the observation that amplification is a recessive genetic characteristic in these cell lines. Analysis of tumor x tumor hybrids revealed at least two complementation groups. Defects in these groups differed from TP53 and implicate multiple variables in the regulation of gene amplification.

Localization and characterization of a chromosome 11 tumor suppressor gene using organotypic raft cultures

The development and progression of human cancer often involves the inactivation of tumor suppressor gene function. Alterations in human chromosome 11 during the development of human cutaneous squamous cell carcinoma suggest the presence of a tumor suppressor gene on this chromosome. Moreover, previous studies in our laboratory demonstrated the presence of a functional tumor suppressor gene on chromosome 11 for the human cutaneous squamous cell carcinoma cell line A388.6TG.c2. In this investigation, we have used organotypic culturing of epithelial cells as a novel in vitro assay for tumor suppression. A388.6TG.c2 and control cells form an abnormal stratified epithelium of 8-12 layers when cultivated on organotypic rafts. In contrast, the chromosome 11 microcell hybrids, HMC 100p4B and HMC 100p5A, form an epithelium of only two to three cell layers. This in vitro growth suppression of the chromosome 11 microcell hybrids in the organotypic rafts correlates well with our previous in vivo skin graft experiments. Comparison of the proliferation and apoptotic indices of cell lines grown on the organotypic rafts suggests that the tumor suppressor gene on chromosome 11 has restricted the ability of the microcell hybrids to stratify but has not significantly altered their ability to undergo cell division or programmed cell death. Furthermore, flow cytometric analysis of cells grown on organotypic raft cultures suggests that the chromosome 11 microcell hybrids are actively progressing through the cell cycle rather than arrested in a particular stage. We have used this novel application of organotypic raft cultures to further localize the chromosome 11 tumor suppressor gene. Introduction of a single der(11)t(X;11) chromosome lacking most of the long arm of chromosome 11 into A388.6TG.c2 does not affect growth on organotypic raft cultures. These data suggest the tumor suppressor gene maps to the long arm of chromosome 11 in the region of 11q13-qter.

Identification of Tumor Suppressor Genes by Microcell Hybridization

Somatic cell genetic studies gave the first proof that functional tumor suppressor genes exist in mammalian genomes. Initial studies showed that whole-cell hybrids between tumorigenic mouse or human cell lines and their normal counterparts became nontumorigenic upon inoculation into animals. However, identification of the operative tumor suppressor gene proved difficult due to the presence of the entire chromosomal complement of the normal cell parent. The development of the technique of microcell hybridization has provided a powerful method for overcoming this obstacle. Suppression of transformed properties of a cancer cell line upon transfer of single human chromosomes from normal cells directly maps the location of tumor suppressor activity. One can then use positional cloning techniques or differential expression strategies to isolate the functional tumor suppressor gene. We present a general strategy for the mapping of tumor suppressor genes in mammalian cells. We also outline some of the important control experiments as well as pitfalls encountered in such studies.