Directional specificity of prostate stromal to epithelial cell communication via FGF7/FGFR2 is set by cell- and FGFR2 isoform-specific heparan sulfate

Type 2 Fibroblast Growth Factor Receptor Signaling Preserves Stemness and Prevents Differentiation of Prostate Stem Cells from the Basal Compartment

Prostate stem cells (P-SCs) are capable of giving rise to all three lineages of prostate epithelial cells, which include basal, luminal, and neuroendocrine cells. Two types of P-SCs have been identified in both human and mouse adult prostates based on prostasphere or organoid cultures, cell lineage tracing, renal capsule implantation, and expression of luminal- and basal-specific proteins. The sphere-forming P-SCs are from the basal cell compartment that express P63, and are therefore designated as basal P-SCs (P-bSCs). Luminal P-SCs (P-lSCs) express luminal cytokeratins and Nkx3.1. Herein, we report that the type 2 FGF receptor (FGFR2) signaling axis is crucial for preserving stemness and preventing differentiation of P-bSCs. FGFR2 signaling mediated by FGFR substrate 2α (FRS2α) is indispensable for formation and maintenance of prostaspheres derived from P63(+) P-bSCs. Ablation of Fgfr2 in P63(+) cells in vitro causes the disintegration of prostaspheres. Ablation of Fgfr2 in vivo reduces the number of P63-expressing basal cells and enriches luminal cells. This suggests a basal stem cell-to-luminal cell differentiation. In addition, ablation of Fgfr2 in P63(+) cells causes defective postnatal development of the prostate. Therefore, the data indicate that FGFR2 signaling is critical for preserving stemness and preventing differentiation of P-bSCs.

Restoration of fibroblast growth factor receptor 2IIIb enhances the chemosensitivity of human prostate cancer cells

Fibroblast growth factor receptor 2 (FGFR2) is thought to mediate an important signaling pathway between prostate epithelial cells and stromal cells for maintenance of homeostasis in normal prostate tissue. Abnormalities of FGFR2 have been shown in advanced prostate cancer or prostate cancer cell lines, and we previously demonstrated the tumor-suppressive effects of the restoration of FGFR2IIIb in prostate cancer cells. The aim of the present study was to determine whether FGFR2IIIb plays a role in the chemosensitivity of castration-resistant prostate cancer cells. A clonal line of PC-3 cells expressing FGFR2IIIb (PC-3R2IIIb) was established by transfection with an IRESneo2-expressing vector bearing FGFR2IIIb cDNA. The effects of chemotherapeutic agents (docetaxel, cisplatin, 5-fluorouracil and zoledronic acid) on cell viability and apoptosis were examined by MTT assay and western blot analysis, respectively. Expression levels of molecules that were markers of epithelial-to-mesenchymal transition and chemosensitivity-related proteins were assessed by western blot analysis. Viability of the PC-3R2IIIb cells was significantly lower than that of the control PC-3 cells transfected with the vector alone (PC-3neo), and viability was further suppressed by treatment with chemotherapeutic agents, particularly docetaxel. Induced expression of caspase-3 was evident in the PC-3R2IIIb cells and was further enhanced by treatment with docetaxel. Expression of N-cadherin, vimentin, survivin and XIAP was lower in the PC-3R2IIIb cells than that in the PC-3neo cells. In contrast, expression of p21 was higher in the PC-3R2IIIb cells than that in the control PC-3neo cells. These data indicate that restoration of FGFR2IIIb in castration-resistant prostate cancer cells may reverse some of the epithelial-to-mesenchymal cell properties characteristic of tumor cells and induce in part mesenchymal-to-epithelial transition properties. This together with enhancement of apoptotic pathways involving caspase-3 may enhance chemosensitivity particularly to docetaxel which is widely used in the treatment of castration-resistant prostate cancer.

Metabolic regulator betaKlotho interacts with fibroblast growth factor receptor 4 (FGFR4) to induce apoptosis and inhibit tumor cell proliferation

In organs involved in metabolic homeostasis, transmembrane α and βklothos direct FGFR signaling to control of metabolic pathways. Coordinate expression of βklotho and FGFR4 is a property of mature hepatocytes. Genetic deletion of FGFR4 or βklotho in mice disrupts hepatic cholesterol/bile acid and lipid metabolism. The deletion of FGFR4 has no effect on the proliferative response of hepatocytes after liver injury. However, its absence results in accelerated progression of dimethynitrosamine-initiated hepatocellular carcinomas, indicating that FGFR4 suppresses hepatoma proliferation. The mechanism underlying the FGFR4-mediated hepatoma suppression has not been addressed. Here we show that βklotho expression is more consistently down-regulated in human and mouse hepatomas than FGFR4. Co-expression and activation by either endocrine FGF19 or cellular FGF1 of the FGFR4 kinase in a complex with βklotho restricts cell population growth through induction of apoptotic cell death in both hepatic and nonhepatic cells. The βklotho-FGFR4 partnership caused a depression of activated AKT and mammalian target of rapamycin while activating ERK1/2 that may underlie the pro-apoptotic effect. Our results show that βklotho not only interacts with heparan sulfate-FGFR4 to form a complex with high affinity for endocrine FGF19 but also impacts the quality of downstream signaling and biological end points activated by either FGF19 or canonical FGF1. Thus the same βklotho-heparan sulfate-FGFR4 partnership that mediates endocrine control of hepatic metabolism plays a role in cellular homeostasis and hepatoma suppression through negative control of cell population growth mediated by pro-apoptotic signaling.

Structural specificity in a FGF7-affinity purified heparin octasaccharide required for formation of a complex with FGF7 and FGFR2IIIb

Variations in sulfation of heparan sulfate (HS) affect interaction with FGF, FGFR, and FGF-HS-FGFR signaling complexes. Whether structurally distinct HS motifs are at play is unclear. Here we used stabilized recombinant FGF7 as a bioaffinity matrix to purify size-defined heparin oligosaccharides. We show that only 0.2%-4% of 6 to 14 unit oligosaccharides, respectively, have high affinity for FGF7 based on resistance to salt above 0.6M NaCl. The high affinity fractions exhibit highest specific activity for interaction with FGFR2IIIb and formation of complexes of FGF7-HS-FGFR2IIIb. The majority fractions with moderate (0.30-0.6M NaCl), low (0.14-0.30M NaCl) or no affinity at 0.14M NaCl for FGF7 supported no complex formation. The high affinity octasaccharide mixture exhibited predominantly 7- and 8-sulfated components (7,8-S-OctaF7) and formed FGF7-HS-FGFR2IIIb complexes with highest specific activity. Deduced disaccharide analysis indicated that 7,8-S-OctaF7 comprised of DeltaHexA2SGlcN6S in a 2:1 ratio to a trisulfated and a variable unsulfated or monosulfated disaccharide. The inactive octasaccharides with moderate affinity for FGF7 were much more heterogenous and highly sulfated with major components containing 11 or 12 sulfates comprised of predominantly trisulfated disaccharides. This suggests that a rare undersulfated motif in which sulfate groups are specifically distributed has highest affinity for FGF7. The same motif also exhibits structural requirements for high affinity binding to dimers of FGFR2IIIb prior to binding FGF7 to form FGF7-HS-FGFR2IIIb complexes. In contrast, the majority of more highly sulfated HS motifs likely play FGFR-independent roles in stability and control of access of FGF7 to FGFR2IIIb in the tissue matrix.

Exogenous Fibroblast Growth Factors Maintain Viability, Promote Proliferation, and Suppress GADD45α and GAS6 Transcript Content of Prostate Cancer Cells Genetically Modified to Lack Endogenous FGF-2

Understanding processes regulating prostate cancer cell survival is critical to management of advanced disease. We used prostate cancer cell transfectants genetically modified to be deficient in either endogenous fibroblast growth factor (FGF-1) or endogenous FGF-2 to examine FGF maintenance of transfectant survival and proliferation and FGF-2-regulated expression of transfectant growth arrest DNA damage (GADD) and growth arrest sequences (GAS) family genes (known modulators of cell cycle progression and survival) and the AS3 gene (an androgen-modulated effector of prostate cell proliferation). When propagated in the absence of exogenous FGFs, FGF-2-deficient transfectants undergo exponential death, whereas FGF-1-deficient transfectants proliferate. Exogenous FGF-1, FGF-2, FGF-7, or FGF-8 promote survival and proliferation of FGF-2-deficient transfectants and enhance FGF-1-deficient transfectant proliferation. Transfectants express FGF receptor FGFR1, FGFR2(IIIb), FGFR2(IIIc), and FGFR3 transcripts, findings consistent with the effects of exogenous FGFs. FGF-2-deficient transfectants express high levels of AS3, GADD45α, GADD45γ, GAS8, and GAS11 transcripts and moderate levels of GADD153, GAS2, GAS3, and GAS6 transcripts and lack demonstrable GAS1 or GAS5 transcripts. FGF withdrawal-mediated death of FGF-2-deficient transfectants did not significantly affect cell AS3, GADD153, GADD45γ, GAS2, GAS3, GAS7, GAS8, or GAS11 transcript content, whereas GADD45α and GAS6 transcript content was elevated. These studies establish that endogenous FGF-2 dominantly regulates prostate cancer cell survival and proliferation and that exogenous FGFs may assume this function in the absence of endogenous FGF-2. Additionally, we provide the first evidence that FGFs regulate prostate GADD45α and GAS6 transcript content. The latter observations suggest that GADD45α and GAS6 proteins may be effectors of processes that regulate prostate cancer cell survival. Additional studies are required to examine this possibility in detail.

Restoration of fibroblast growth factor receptor2 suppresses growth and tumorigenicity of malignant human prostate carcinoma PC-3 cells

BACKGROUND

Fibroblast growth factors (FGFs) and their receptors (FGFRs) expedite stromal-epithelial communication in development and homeostasis of the human prostate. Loss of resident epithelial cell FGFR2IIIb that responds to stromal FGF7 and FGF10 accompanies malignant progression of both model animal and human prostate tumors.

METHODS

We examined whether restoration of FGFR2IIIb by transfection in the malignant human prostate tumor PC-3 cell line restored cellular properties associated with less malignant tumors. Cell proliferation, apoptosis, and tumor cell implants were used to monitor malignant properties. Activity of FGFR2IIIb was assessed by immunoblot of FRS2 and p44/42 MAP kinase. Immunochemical analysis of pancytokeratin and lactoferrin expression was utilized to assess changes in cellular differentiation.

RESULTS

Expression of FGFR2IIIb in PC-3 cells by transfection resulted in growth suppression in vitro and reduced tumor formation in vivo concurrent with increased cellular differentiation and apoptosis.

CONCLUSIONS

The results indicate that restoration of FGFR2IIIb to the malignant human prostate epithelial cell prototype PC-3 restores properties associated with nonmalignant tumors and normal cells. This further suggests that epithelial cell resident, homeostasis-promoting FGFR2 may be involved in suppression of malignancy and that restoration may be a candidate for gene therapy of hormone-refractory prostate cancer.

Directionally specific paracrine communication mediated by epithelial FGF9 to stromal FGFR3 in two-compartment premalignant prostate tumors

Tissue homeostasis in normal prostate and two-compartment nonmalignant prostate tumors depends on harmonious two-way communications between epithelial and stromal compartments. Within the fibroblast growth factor (FGF) family, signaling to an epithelial cell-specific FGF receptor (FGFR) 2IIIb-heparan sulfate complex from stromal-specific FGF7 and FGF10 delivers directionally specific instruction from stroma to epithelium without autocrine interference. Using a two-compartment transplantable prostate tumor model in which survival of stromal cells in vivo depends on epithelial cells, we show that signaling from epithelial FGF9 to stromal FGFR3 potentially mediates epithelial-to-stromal communication that also is directionally specific. FGF9 mRNA was expressed exclusively in the epithelial cells derived from well-differentiated, two-compartment Dunning R3327 rat prostate tumors. In contrast, FGFR3 was expressed at functionally significant levels only in the derived stromal cells. Competition binding and immunoprecipitation assays revealed that FGF9 only bound to an FGFR on the stromal cells. FGF9 also failed to covalently cross-link to clonal lines of stromal cells devoid of FGFR3 that expressed FGFR1 and FGFR2IIIc. Furthermore, FGF9 specifically stimulated DNA synthesis in stromal cells expressing FGFR3. These results demonstrate a directionally specific paracrine signaling from epithelial FGF9 and stromal FGFR3. Similar to the FGF7/FGF10 to FGFR2IIIb signaling from the stroma to the epithelium, the directional specificity from epithelium to stroma appears set by a combination of cell-specific expression of isoforms and cell-context specificity of FGFR isotypes for FGF.

Chronic activity of ectopic type 1 fibroblast growth factor receptor tyrosine kinase in prostate epithelium results in hyperplasia accompanied by intraepithelial neoplasia

BACKGROUND

Ectopic expression of fibroblast growth factor receptor 1 (FGFR1) tyrosine kinase in epithelial cells is associated with progression of prostate cancer. Ectopic expression by transfection of FGFR1 in premalignant epithelial cells from nonmalignant Dunning tumors accelerated time-dependent progression of epithelial cells to malignancy. This study was designed to test the effect of chronic androgen-dependent ectopic activity of FGFR1 in the normal adult mouse epithelium by gene targeting.

MATERIALS AND METHODS

Constitutively active FGFR1 (caFGFR1) was targeted to prostate epithelial cells using the androgen-dependent probasin (PB) promoter. Prostate tissues of three strains were characterized over a period of 2 years by HE staining, immunohistochemical analyses for cytokeratin and alpha-actin, and rate of androgen-induced regeneration after castration.

RESULTS

Relative to wildtype littermates, transgenic mice showed increased overall size, hyperplasia in epithelial, and, to a lesser extent, stromal compartments and nuclear atypia in epithelial cells of the prostate with increasing age. Androgen-induced regeneration after castration was enhanced at day 3 by two-fold in mice expressing ectopic caFGFR1.

CONCLUSIONS

The ectopic presence and chronic activation of FGFR1 in mouse prostate epithelial cells induces progressive prostate intraepithelial neoplasia. These results confirm results suggested by the transplantable Dunning tumor and cell culture models that, in contrast to homeostasis-promoting resident FGFR2, chronic ectopic FGFR1 kinase activity in the epithelium disrupts homeostasis between stroma and epithelium. Although insufficient alone, it may cooperate with other oncogenic changes to promote epithelial cells down the path to malignancy.

Cell- and receptor isotype-specific phosphorylation of SNT1 by fibroblast growth factor receptor tyrosine kinases

A partnership between the ectodomain of the fibroblast growth factor receptor (FGFR) isotypes and the chains of pericellular matrix heparan sulfate determines the fibroblast growth factor (FGF) and cell-type specificitives of the FGFR signaling complex. The contribution of the FGFR intracellular tyrosine kinase domains to the specificity of FGFR signaling is unclear. This report shows that the quantity and quality of phosphorylation of the FGFR kinase substrate SNT1 (also called FGFR substrate 2, FRS2) is both FGFR isotype and cell-type specific in prostate tumor epithelial cells at different stages of malignancy. Epithelial cell-resident FGFR2 that promotes homeostasis yields a low level of phosphorylated 65-kDa SNT1. Phosphorylation by ectopic FGFR1 that promotes malignancy was much more intense and yielded a phosphorylated 85-kDa SNT1. The amount of the 85-kDa SNT1 increased by 20-fold during proliferative aging of FGFR1-expressing cell populations that is required for FGFR1-stimulated mitogenesis and the malignant phenotype. In addition, the receptor-specific differential phosphorylation of SNT1 by FGFR isotypes, both of which are normally anchored to the cell membrane, occurred only in intact cells. Therefore, similar to kinase subunits within the heparan sulfate-FGFR complex, cell membrane and cytoskeletal context likely determine FGFR isotype- and cell-type-specific conformational relationships between FGFR kinases and external substrates. This determines the quantity and quality of SNT1 phosphorylation and differential signaling.