The Prostate 42:150–160 (2000) Review Article Dysregulated Expression of Growth Factors and Their Receptors in the Development of Prostate Cancer Daniel Djakiew* Department of Cell Biology, Division of Urology of the Department of Surgery, and the Vincent T. Lombardi Cancer Center, Georgetown University Medical Center, Washington, DC KEY WORDS: prostate cancer; growth factors; receptors; review INTRODUCTION Progression of prostate cancer is accompanied by modifications in the expression of growth factors and their receptors, many of which are the products of protooncogenes or tumor suppressor genes. These alterations may take the form of up- or downregulation of growth factors and/or their receptors as well as changes from paracrine to autocrine mediation of growth. A large body of literature implicating specific growth factors and their receptors in the development of prostate cancer has been derived from animal models and cell lines. However, man is unique in that no other species develops prostate cancer, except under extreme circumstances when induced with potent carcinogens. Hence, elucidation of growth factors and their receptors implicated in the progression of prostate cancer in animal models and cell lines requires verification with the etiology of human prostate cancer. Even then, the altered expression of some growth factors and their receptors in prostate cancer may be collateral rather than causal of the malignant transformation of human prostate epithelial cells. Although many protooncogenes and tumor suppressor genes that are not growth factors or growth factor receptors play just as significant a role in the progression of prostate cancer, the recent publication of an extensive review on that topic  has ameliorated the necessity for their inclusion herein. On the other hand, the recent rapid growth of research on growth factors and their receptors in the prostate has provided a timely opportunity to assess their contribution to the development of prostate cancer. Consequently, this review focuses specifically on those growth factors and their receptors, some of which function as protooncogenes © 2000 Wiley-Liss, Inc. and tumor suppressor genes, that appear to be risk factors in the development of human prostate cancer. EPIDERMAL GROWTH FACTOR FAMILY OF PEPTIDES AND RECEPTORS Malignant epithelial cells of the human prostate exhibit an enhanced capacity for autocrine expression of epidermal growth factor (EGF) and the related family member, transforming growth factor-␣ (TGF-␣) that appears to circumvent a paracrine dependence on stromal-cell derived EGF. Both EGF and heparinbinding EGF are secreted by smooth muscle stromal cells [2,3]. EGF is also secreted by human epithelial cancer cell lines [4,5]. Moreover, EGF and TGF-␣, both signaling through the EGF receptor (EGFR), vary in expression during malignant transformation . Evidence that EGF signaling through the EGFR is important in prostate cancer cell proliferation is that 1) human prostate cancer cell lines produce EGF [4,5], 2) human prostate cancer cells express EGFR in vitro  and in vivo , 3) addition of EGF to cultures of prostate cancer cells stimulates growth , 4) addition of anti-EGFR antibody inhibits the proliferation of prostate cancer cells in vitro  and in xenografted mice in vivo , 5) EGFR blockade inhibits EGF- and insulin-like growth factor-I (IGF-I)-mediated transducGrant sponsor: Concern Foundation; Grant sponsor: NIH; Grant number: R01 DK52626. *Correspondence to: Daniel Djakiew, Ph.D., Department of Cell Biology, School of Medicine, Georgetown University, 3900 Reservoir Rd., NW, Washington, DC 20007. E-mail: email@example.com Received 28 May 1999; Accepted 18 August 1999 Dysregulated Growth Factors and Receptors tion of convergent mitogenic signaling pathways , and 6) levels of EGFR may be higher in prostate cancer than benign prostate , although several studies have observed the opposite relationship [14,15]. Taken together, it appears that the autocrine expression of EGF and TGF␣ signaling through EGFR may contribute to the autonomous growth of human prostate cancer [6,16]. A second major function of EGF is the stimulation of invasiveness of prostate cancer [2,17,18]. This has been demonstrated in Boyden chamber assays where 1) EGF promotes the chemomigration of human prostate cancer cells [2,17], 2) antagonism of EGFR function prevents EGF-stimulated chemomigratory activity of human cancer cells , and 3) clones of transfected cells expressing elevated levels of EGFR invade across membranes to a greater extent than parental cells . Hence, these observations suggest that EGF/TGF␣ contributes to the autonomous growth of cancer cells and that EGF has a pleiotropic effect on human prostate cancer cells, promoting both growth and invasiveness. The EGFR family-related oncogenes HER-2/neu, HER-3, and HER-4 are also differentially expressed in the prostate . The HER-2/neu gene product, p185erbB-2, is not expressed in normal secretory epithelial cells , and may be expressed in basal cells  but not luminal cells of benign prostatic hyperplasia (BPH) tissues [21–23]. However, p185erbB-2 is expressed in the majority of epithelial cells in PIN and human cancer cells [22,23]. Moreover, HER-2/neu gene amplification and, to a lesser extent, p185erbB-2 protein expression correlate with increasing cancer grade [24,25]. Experimental overexpression of p185erbB-2 in normal epithelial cells produces a phenotype with an increased rate of proliferation and enhanced capacity for metastasis . A similar pattern of expression has been observed for the HER-3 gene product, p160erbB-3, in BPH, prostatic intraepithelial neoplasia (PIN), and cancer . Interestingly, interleukin (IL)-6 induces tyrosine phosphorylation of both p185erbB-2 and p160erbB-3, but not EGFR . HER-4 receptor protein is strongly expressed in normal epithelial cells but not in prostate cancer . The heregulin ligand (neu differentiation factor), which preferentially binds HER-3 and HER-4 gene products, is expressed in all stromal cells and basal cells, and in approximately half of the luminal epithelial cells in normal and BPH tissue, and is largely absent in cancer . Hence, heregulin may be a paracrine factor which stimulates growth in vitro . It then appears that enhanced expression of p185erbB-2 and p160erbB-3 occurs with progression and that these oncogenes infer a phenotype with an increased capacity for proliferation and metastasis similar to that exhibited by EGFR and its ligands EGF and TGF␣. 151 TRANSFORMING GROWTH FACTOR-BETA FAMILY OF PEPTIDES AND RECEPTORS The transforming growth factor (TGF) family consists of 1) TGF-␤ isoforms; and more distantly related 2) bone morphogenetic proteins; 3) the activins and inhibins, all of which are differentially expressed in the adult prostate; and 4) the developmentally expressed Mullerian inhibitory substance. In vitro, mammalian TGF-␤ isoforms 1–3 inhibit the proliferation of normal human epithelial cells  and human cancer cells , whereas, in vivo, TGF-␤1 enhances cancer growth and metastasis . This paradoxical role of TGF-␤ in the regulation of cancer growth results from modified expression of TGF-␤ receptors and the response of the host to TGF-␤. Normal prostate epithelial cells express TGF-␤1–3 to differing degrees , whereas TGF-␤1–2 are overexpressed in human cancer . As a consequence, both urinary TGF-␤1 and plasma TGF-␤2 levels are elevated in cancer patients . TGF-␤ can recruit two distinct receptors, designated type I (RI) and type II (RII) receptors. This signaling pathway appears to be downregulated in prostate cancer. In this context, the TGF-␤ RI and RII proteins, which are abundantly expressed in normal prostate epithelial cells, exhibit progressive reduction of expression in primary cancer, and lymph node metastases . Hence, even though cancer cells exhibit an upregulation of TGF-␤1–2 expression , the downregulation of TGF-␤ RI and RII expression  appears to ameliorate the autocrine growth-inhibitory effects of the TGF-␤s. This is further supported by the observation that restoration of TGF-␤ RII expression in a human prostate tumor cell line inhibits the growth of xenograft tumors by induction of apoptosis . Hence, human cancer cells that exhibit upregulation of the TGF-␤s and downregulation of their receptors appear to exhibit host effects that facilitate cancer growth. These cancer cells exhibit an immunosuppressive effect on lymphocyte action , and promote angiogenesis, extracellular matrix deposition, and metastases . The family of bone morphogenetic proteins (BMPs) induces bone morphogenesis in vivo and has been implicated in skeletal metastases of advanced prostate cancer. BMP-2, -3, and -4 have been identified in normal epithelial cells and cancer in vitro . However, in vivo, BMP-6 expression is higher in organ-confined cancer than in adjacent normal benign epithelial cells . BMP-6 expression is correlated with Gleason score , pathologic stage [39,40], and bony metastases from prostate carcinoma . The receptors BMPR-IA, BMPR-IB, and BMPR-II are present on prostatic epithelial cells [42,43]. BMPR-IB, but neither BMPR-IA nor BMPR-II, is upregulated by androgens 152 Djakiew in the LNCaP cell line [42,43]. Moreover, BMPR-IA stimulates growth, whereas BMPR-IB inhibits growth in response to BMP-2 . The activin ␤A and ␤B subunits are expressed in normal human epithelial cells and cancer , whereas the inhibin ␣ subunit appears to be absent in both human and rodent prostate epithelial cells [45– 47]. Since the inhibin ␣ subunit may function as a tumor suppressor, its absence may be involved with the development of prostate cancer, since activin action cannot be opposed by inhibins [46,47]. However, activins are expressed in human prostate cancer , and inhibit prostate cancer growth [48,49] by induction of apoptosis . Moreover, the activin-binding protein, follistatin, inhibits activin action . In the normal prostate, follistatin is expressed by the stromal cells and basal cells . However, cancer acquires expression of follistatin [45,48,50]. Hence, based on the in vivo colocalization of follistatin and activins in human cancer, a resistance to the growth-inhibitory effects of activin was suggested to be conferred by follistatin . This was confirmed in primary cultures of human prostate carcinoma where treatment with activin was shown to inhibit epithelial cell proliferation; conversely, treatment with follistatin enhanced epithelial cell proliferation . FIBROBLAST GROWTH FACTOR FAMILY OF PEPTIDES AND RECEPTORS The fibroblast growth factor (FGF) family of peptides currently consists of at least 19 members, many of which are expressed in the prostate to varying degrees. In the adult human prostate, FGF1 (acidic FGF) is expressed at low levels or is undetectable . Conversely, FGF-2 (basic FGF) is abundantly expressed . FGF-2 is produced by stromal cells  and acts as a weak mitogen on the normal epithelium . However, human cancer cells acquire autocrine expression of FGF-2 , which may further stimulate cancer cell proliferation [57,58] and elevates the titer of FGF-2 in the serum of prostate cancer patients [56,58]. In addition to the mitogenic action of autocrine FGF-2 on cancer cells, FGF-2 also enhances cell motility , which may reflect an acquired capacity for metastasis. The capacity of cancer cells to invade and metastasize is a reflection of the ability of FGF-2 to regulate the turnover of extracellular matrix by modulating the expression of proteases and promoting the synthesis of collagen, fibronectin, and proteoglycans . Autocrine FGF-2 expression by cancer also contributes to the angiogenesis of primary  and metastatic cancers , thereby circumventing growth limitations of diffusion in the supply of nutrients and removal of waste products. FGF-3, -4, -5, and -6 are oncogene products. FGF-3 (int-2 gene product) is probably not involved in localized prostate cancer . This is supported by studies of FGF-3 transgenic mice, which exhibit morphologically normal prostates . Conversely, expression of FGF-3 and FGF-5 has been reported to correlate with progression to malignancy in the Dunning rat model of prostate cancer . Hence, the role of these oncogene products in human prostate cancer remains unclear. FGF-7, also known as keratinocyte growth factor (KGF), has been extensively investigated in rodent models, with limited confirmatory studies in the human prostate. FGF-7 and its homologue FGF-10 are androgen-regulated peptides [65,66], which are expressed predominantly in the stromal cells of the rat [65,66] and human [67,68] prostate. Hence, FGF-7 and FGF-10 were proposed as candidate andromedins [65,66]. However, subsequent studies showed FGF-7 not to be androgen-regulated in vivo, thereby diminishing its status as a true andromedin . FGF-7 has also been localized to epithelial cells in the human fetal prostate, normal adult prostate, and prostatic cancer . FGF-7 stimulates proliferation of prostate epithelial cells  but not the LNCaP cancer cell line . In the rodent prostate, FGF-7 also stimulates proliferation of epithelial cells [65,71]. However, rodent Dunning cancer cells exhibit a reduced response to FGF-7 , attributed to exon switching in FGF receptor (FGFR) isoforms. In this context, epithelial cells of the rat Dunning model appear to normally express the FGFR2 (IIIb) isoform , which can bind FGF-7 and FGF-10 as a mitogen [66,72]. However, cancer cells express the alternatively spliced FGFR2 (IIIc) isoform , which preferentially binds FGF-2 over FGF-7. Both of these FGFR2 isoforms are present in human prostate cancer , and even though androgeninsensitive human prostate cancer cell lines exhibit a loss of FGFR2 (IIIb) in vitro , progression in exon switching has not been observed in pathologic tissue specimens , suggesting that the rodent model may not be fully applicable. Nevertheless, the ability of autocrine FGF-2 to substitute for paracrine FGF-7 in cancer cells that exhibit the FGFR (IIIc) isoform may further facilitate proliferation of the cancer cells. FGF-8, also known as androgen-induced growth factor (AIGF), occurs in the epithelial cells of prostate cancer as multiple isoforms . It is expressed in LNCaP [75,76], DU-145, and PC-3 cancer cell lines [75,76], and enhances the growth of the LNCaP cells . FGF-8 is largely absent from the epithelial cells of normal prostate and BPH tissues, whereas it is overexpressed in cancer, and the level of expression appears to be related to progression [77,78]. Dysregulated Growth Factors and Receptors INSULIN-LIKE GROWTH FACTOR FAMILY OF PEPTIDES, RECEPTORS, AND BINDING PROTEINS The insulin-like growth factor (IGF) system is characterized by complex interactions between the IGFs, their receptors, high-affinity binding proteins, receptors for these binding proteins, and proteases. Epithelial cells cultured from normal prostate, BPH, and cancer have been reported not to secrete significant amounts of IGF-I or IGF-II . In contrast, IGF-II mRNA and protein has been localized to human adenocarcinoma tissue of the prostate . Several reports on human prostate cancer cell lines derived from metastases (DU-145, PC3, LNCaP) indicate that these cells do not secrete IGF-I  or IGF-II . In contrast, the same cell lines have been reported to secrete substantial amounts of IGF-I . Prostatic stromal cells secrete IGF-II , and possibly express IGF-I . Irrespective of paracrine or autocrine origin, both IGF-I and IGF-II stimulate the growth of epithelial cells derived from primary cultures  and human cancer cell lines . Furthermore, IGFs appear to stimulate the EGFR signal transduction cascade [12,86]. Hence, elevated levels of serum IGF-I appear to be a risk factor for the development of human prostate cancer [87–89]. The type 1 IGF receptor (IGF1R), which preferentially binds IGF-I, is expressed in epithelial cells cultured from normal prostate, BPH, and cancer  as well as human prostate cancer cell lines derived from metastases [81,86] and stromal cells . Antagonism of IGF1R function with IGF-I analogs inhibits the growth of prostate cancer cell lines . IGF1R mediates signal transduction upon binding either IGF-I or IGF-II through its intrinsic tyrosine kinase activity . IGF2R does not contain intrinsic kinase activity but does contain the mannose 6-phosphate recognition marker  which, upon binding IGF-II, directs the IGF2R protein to lysosomes for degradation . Hence, IGF2R may function as a negative growthregulatory molecule by depletion of ligand. The ability of IGFs to mediate a growth response via their receptors can be altered by interactions with a variety of insulin-like growth factor-binding proteins (IGFBPs). Although receptors for some of the IGFBPs have been described, their roles in the prostate remain poorly understood. Epithelial cells cultured from normal prostate, BPH, or cancer express IGFBP2, IGFBP-4 , and possibly IGFBP-3 , but not IGFBP-1 . Overexpression of inhibitory IGFBP-4 delays the onset of tumor formation in vivo . Elevated levels of IGFBP-2  and decreased levels of IGFBP-3  are present in the serum of patients with prostate cancer. Increased levels of IGFBP-2 correlate 153 with increased prostate-specific antigen (PSA) in the serum [94,95] and with cancer burden . Proteolytic cleavage of IGFBPs reduces the affinity for IGFs , facilitating greater interaction of IGF with its receptor and increased mitogenic activity. Indeed, the serine protease, PSA, preferentially cleaves IGFBP-3  and IGFBP-5 , thereby stimulating growth of prostatic epithelial cells  and reducing the levels of IGFBP-3 in the serum . The nerve growth factor (NGF) gamma subunit, which shares homology with PSA, also cleaves IGFBP-3, -4, and -6 . However, NGF gamma is not expressed in the human prostate. Hence, cleavage of IGFBP-3, -4, and -6 by PSA [98–100] probably contributes to the bioavailability of IGF-I in the serum, consistent with the elevated levels of IGF-I in the serum in prostate cancer [87–89]. NERVE GROWTH FACTOR FAMILY OF PEPTIDES AND RECEPTORS Nerve growth factor (NGF) immunoreactive protein has been localized to normal epithelium [101,102] and the stroma of normal, BPH, and cancer specimens of human prostate [101,103,104]. Whether epithelial NGF is synthesized de novo or endocytosed from the stroma remains to be established. In any event, human prostate stromal cells in vitro have been shown to secrete the long (35 kDa) and short (27 kDa) forms of precursor NGF, and the partially processed cleavage product (22 kDa form) of proNGF, whereas the mature (13 kDa) form of NGF␤ is not produced . Stromal cell-derived precursor NGF stimulates the proliferation of the TSU-pr1 human prostate cancer cell line . Furthermore, exogenous NGF␤ stimulates proliferation of the TSU-pr1 , DU-145, PC-3, and LNCaP cell lines  in vitro, and promotes the anchorage-independent growth of the androgenresponsive LNCaP  and androgen-refractory TSUpr1  tumor cell lines. The androgen-refractory cancer cell lines derived from metastases (DU-145, PC3, TSU-pr1) express NGF in an autocrine manner , whereas the androgen-responsive LNCaP cell line does not express the NGF gene . Hence, it appears that the normal prostate expresses stromal cell-derived NGF for the paracrine regulation of epithelial cell growth, and that following human prostate carcinogenesis, the androgen-refractory cancer cells express autocrine NGF. In this manner, the prostate cancer cells that produce autocrine NGF are able to escape a paracrine dependence on stromal cellderived NGF. Since the pathology of malignant cell migration within the human prostate is often by direct extension around prostatic nerves, upregulation of autocrine neurotrophin expression in cancer may be as- 154 Djakiew sociated with invasion along the perineural space and metastasis . NGF is a ligand for the low-affinity p75 neurotrophin receptor (p75NTR), which is expressed to varying degrees in epithelial cells of the human prostate [101,104]. Immunoblotting , immunofluorescent , and immunohistochemical  studies have shown that expression of the p75NTR protein declines in human prostate cancer. Loss of expression of p75NTR is correlated with cancer grade in organconfined disease . Moreover, this protein is absent in four human cancer cell lines derived from metastases . Loss of expression of p75NTR in prostate cancer may be related to its role in the induction of programmed cell death [106,112]. Reexpression of p75NTR by stable and transient transfection showed that p75NTR inhibits growth of prostate epithelium in vitro, at least in part, by induction of programmed cell death . Hence, loss of p75NTR expression in prostate cancer cells appears to eliminate a potential programmed cell death pathway in these cells, thereby facilitating the immortalization of these epithelia during carcinogenesis [111,112]. Consequently, p75NTR has been suggested to be a candidate tumor suppressor gene in the human prostate [111,112]. Since expression of p75NTR is lost in human cancer, NGF-mediated growth of cancer cells has been shown to occur via the high-affinity Trk receptor [106, 108,110]. The Trk receptor is expressed in PIN, cancer , and human cancer cell lines derived from metastases [106,108,110]. Members of the K252 family of kinase inhibitors, the indolocarbazoles, selectively inhibit activity of the Trk receptors at nanomolar concentrations  and inhibit NGF-stimulated Trk phosphorylation in the TSU-pr1 cancer cell line . Concurrently, Trk-selective indolocarbazoles inhibit growth of cancer cell lines in vitro  and in vivo , further supporting a role of the Trk receptors in NGF-mediated growth of human prostate cancer cells. Although the Trk receptor was originally isolated as a colon cancer oncogene , Trk mutations within the human prostate have not been identified . However, the absence of mutations in otherwise genetically unstable prostate tumor DNA suggests that intact Trk signaling pathways may be important in prostate cancer development . VASCULAR ENDOTHELIAL GROWTH FACTORS AND RECEPTORS Vascular endothelial growth factor (VEGF) promotes angiogenesis in a wide variety of normal and neoplastic tissues. In vivo, contradictory studies have reported a complete lack of VEGF in BPH and epithelial cells , while another study observed two iso- forms (VEGF165, VEGF189) of the protein in stromal cells of BPH . Conversely, a consensus of studies observed VEGF expression in neuroendocrine cells  and more abundantly in epithelial cells of the normal prostate , in organ-confined cancer [118,119], in human cancer cell lines derived from metastases, and in xenografts of prostate tumors . Prostate tumor cell lines express the flk-1 receptor for VEGF . Exogenous VEGF appears to promote the growth of xenograft tumors , and androgen ablation inhibits VEGF expression . Moreover, treatment with anti-VEGF antibody inhibits the growth of xenograft tumors  and their metastatic dissemination to the lungs . Taken together, these observations suggest that human cancer cells express VEGF for the angiogenesis of developing cancer masses, thereby circumventing oxygen diffusion as a ratelimiting step in the growth of prostate cancers. PLATELET-DERIVED GROWTH FACTORS AND RECEPTORS Platelet-derived growth factors (PDGF) exist as dimers formed from A and B chains, which bind with differing affinities to the PDGF␣ receptor and PDGF␤ receptor. PDGFs signal through their receptors to elicit a diversity of cellular responses in vitro, including cell proliferation, survival, transformation, and chemotaxis . In vivo, contradictory studies have reported a complete lack of either form of ligand or receptor in BPH , while others have observed limited expression of the PDGF␤ receptor in BPH . Conversely, epithelial and stromal cells of prostatic intraepithelial neoplasia (PIN) and human cancer express PDGF-A and the PDGF␣ receptor [128,130], while PDGF-B and the PDGF␤ receptor are either absent or weakly expressed [128,130]. Hence, PDGF-A ligand and its PDGF␣ receptor may modulate autocrine growth in BPH , PIN , and human cancer cells , thereby playing a role in malignant transformation of the prostate. CYTOKINES AND RECEPTORS Hepatocyte growth factor (HGF), also called scatter factor, has been grouped with the cytokines based upon the original definition of a cytokine as promoting cell (cyto) motility (kinetics). HGF is expressed in the human prostate exclusively by the stroma [131,132] and stimulates proliferation [57,133] and motility [133,134] of cancer cells. HGF binds to the c-met protooncogene product, which is located exclusively in epithelial cells of PIN , BPH , and human cancer [133,135,136]. The proportion of tissue samples that express c-met progressively increases from BPH Dysregulated Growth Factors and Receptors , to PIN , primary cancer [133,136], and metastases [133,136]. HGF induces motility and scattering of cells from many organs, and the correlation of c-met expression with malignant progression of cancer cells suggests a role of c-met in metastases. Although cytokines exert a variety of effects on cancer cells, there is little evidence to suggest that interleukin (IL)-1, IL-2, or interferon-␣, -␤, and -␥ are expressed during prostatic carcinogenesis. Conversely, IL-6 is secreted in a paracrine manner from human stromal cells  and in an autocrine manner from human cancer cells [138,139]. Contradictory studies have reported that IL-6 signals IL-6 receptors on cancer cells in vitro [139,140] and in tissue , either to inhibit growth [137,141] via p27(Kip1)-mediated G1 arrest  or, conversely, to stimulate growth . IL-6 and, to a greater extent, IL-10 also upregulate expression of the tissue inhibitor of metalloproteinase-1 [144,145]; IL-10 also inhibits matrix metalloproteinase-2 and -9 secretion , consistent with an overall inhibitory effect on cancer cells. Conversely, IL-6 stimulation of growth may be a consequence of a specific tumor cell phenotype that coexpresses p185erbB-2 and/or p160/erbB-3, since IL-6 has been shown to tyrosine phosphorylate p185erbB-2, which forms a complex with the gp130 subunit of the IL-6 receptor, which can then mediate IL-6-dependent growth . This is consistent with reports of elevated serum IL-6 correlating with tumor burden in patients with clinically evident metastases [147,148]. Another cytokine, tumor necrosis factor-alpha (TNF␣), inhibits chemotaxis  and proliferation of human cancer cell lines [141,149]. Moreover, TNF␣ is cytotoxic for cancer cell lines  by inducing bcl-2-mediated  programmed cell death [151,152]. The effect of TNF␣ is mediated by TNF-R1 (55 kDa) and TNF-R2 (75 kDa), both of which are expressed by human prostate cancer cell lines . CONCLUSIONS A distinct subset of growth factors, their receptors, and associated binding proteins participate in the progression of human prostate cancer (Table I). Clearly, the malignant progression of the prostate involves epithelial cell upregulation of autocrine growth factors and their receptors, or autocrine acquisition of stromal cell-derived growth factors by epithelial cells, both of which facilitate the autonomous growth and metastasis of the tumor cells. The growth of tumor foci is facilitated through the action of growth factors that promote the proliferation of tumor cells, angiogenesis growth factors that circumvent limitations of diffusion on tumor volume, and the immunosuppressive TGF␤s which circumvent the host’s defense mechanisms (Table I). In addition, follistatin binding to activin con- 155 TABLE I. Dysregulated Expression of Growth Factors, Their Receptors, Binding Proteins, and Related Protooncogene and Tumor Suppressor Gene Products in the Development of Prostate Cancer in Man 1. Acquisition/upregulation of autocrine growth factors a. Proliferation (EGF, TGF␣, FGF-2, FGF-8, IGF-I, NGF, PDGF-A) b. Immunosuppression (TGF␤1, TGF␤2) c. Angiogenesis (FGF-2, VEGF) d. Metastasis (EGF, FGF-2, BMP-6?) 2. Dysregulation of growth factor-binding proteins Follistatin, IGFBP-2, IGFBP-3 3. Upregulation of growth factor receptor protooncogene products EGFR, p185erB-2, p160erB-3, PDGF␣, c-met 4. Downregulation of tumor suppressor gene products p75NTR, TGF␤ RI, TGF␤ RII, IGFRII? fers resistance to the growth-inhibitory effects of activin. In some instances, the establishment of metastatic foci from primary tumors is regulated by the same growth factors that promote proliferation of tumor cells (Table I). The upregulation of growth factor receptor protooncogene products further facilitates malignant progression of prostate tumor cells by enhancing the capacity of preexisting signal transduction cascades (e.g., EGFR) or through the expression of a transforming phenotype (e.g., p185erbB-2). Interestingly, the least prevalent transformation event appears to be the downregulation of growth factor receptor tumor suppressor gene products (e.g., TGF␤ RI). Clearly, not all of these characteristics of the malignant phenotype occur concurrently within all tumors. Nevertheless, the dysregulated expression of several growth factors, their receptors, binding proteins, protooncogene products, and tumor suppressor gene products (Table I) is associated with the development of the malignant phenotype. When this dysregulated expression occurs in the human prostate it confers an enhanced growth advantage for the development of tumor foci. 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