The ErbB Receptor-Ligand Axis in the Human Prostate
August 1, 1999
The ErbB Receptor-Ligand Axis in the Human Prostate
By Rosalyn M. Adam, PhD, and Michael R. Freeman, PhD
Stromal-epithelial interactions are critical for the development, growth, and maintenance of many organs in the body, and alterations in communication between the stromal and epithelial compartments have been proposed to contribute to proliferative pathologies such as cancer. The concept of a reactive stroma is now well established and defines the critical role played by stromal cells, typically smooth muscle cells and fibroblasts, in regulating the behavior of the epithelial cells with which they interact.1
Stromal-epithelial interactions in the prostate have been widely studied and are believed to be mediated by diffusible peptide growth factors, elaborated by the stromal compartment, which bind to and activate the cognate receptors expressed by epithelial cells.2 Central among these peptide mediators are members of the EGF-like growth factor family, which are activating ligands for the epidermal growth factor receptor tyrosine kinase (EGFR)/ErbB1.3 ErbB receptors and several activating ligands have been shown to be overexpressed in prostate cancer. Therefore, a thorough understanding of ErbB-ligand interactions may lead to improvements in therapeutic intervention in the treatment of prostatic disease.
ErbB Receptor Signaling
The ErbB receptor family currently comprises four structurally related, single membrane-spanning glycoproteins: the epidermal growth factor receptor (EGFR)/ErbB1, ErbB2, ErbB3 and ErbB4. There are six known activating ligands for ErbB1, namely epidermal growth factor (EGF), transforming growth factor-alpha (TGF-a), heparin-binding EGF-like growth factor (HB-EGF), amphiregulin (ARG), betacellulin (BTC), and epiregulin. ErbB3 and ErbB4 activation is mediated by a distinct family of ligands, the heregulins (or neuregulins); however, no ligand has been identified for ErbB2.
Binding of ligand to the extracellular domain of the receptor leads to receptor dimerization and subsequent activation of the intrinsic tyrosine kinase activity, which in turn promotes phosphorylation of receptor cytoplasmic domains on tyrosine residues. The resulting phosphotyrosine motifs are recognized by intracellular signaling molecules that are recruited to the receptor and thereby enable transduction of signals into the cell. Specificity of signal generation is achieved by 1) the tendency for different ligands to promote preferential formation of specific ErbB receptor homo- or heterodimers; 2) the generation of phosphotyrosine motifs that are specific for a given receptor subtype; and 3) specific recognition of the phosphotyrosine-containing motifs by signaling molecules. Together, these features lead to recruitment of different combinations of signaling molecules to the receptor cytoplasmic domain such that signals of differing quality and intensity are generated in response to ligand-induced receptor activation, and signal specificity is achieved.
The ErbB Receptor-Ligand Axis in the Human Prostate
A functional role for the ErbB receptor family in the human prostate was suggested by early studies in which EGF was demonstrated to be an important mitogen for proliferation of prostate epithelial cells in culture.4 The EGFR/ErbB1 was subsequently detected in prostate tissue, and several studies have described localization of the EGFR/ErbB1 to the basal epithelial cells of normal prostatic ducts.5 Epithelial cells have also been demonstrated to express the ligands EGF and TGF-a, which suggested that epithelial cells may undergo ErbB1-dependent autocrine stimulation in response to these ligands. Consistent with their identity as mitogens for many other cell types, the EGF-like growth factors are important growth regulatory molecules in the prostate, and are also believed to mediate some of the effects of androgens, which are known to be critical for normal development and function of the prostate and other male accessory sex organs. Furthermore, the androgen-independent proliferation of prostate carcinoma cells, that typically occurs in patients who have undergone androgen ablation therapy to eliminate a primary prostate tumor, is thought to arise, in part, from activation of the ErbB-ligand axis.6 This confers on carcinoma cells the ability to undergo autonomous proliferation in the absence of circulating androgens, thereby promoting disease progression.
Stromal-Epithelial Interactions in the Prostate
In the prostate, the epithelium-lined ducts ramify throughout the fibromuscular stroma, such that stromal smooth muscle cells are in intimate contact with the basal epithelial cell layer. This close anatomical relationship arises as a consequence of inductive interactions in development between the instructive urogenital sinus mesenchyme and the permissive undifferentiated epithelium. Subsequent differentiation of the adjacent epithelium in response to signals from the mesenchyme promotes differentiation of the mesenchyme into smooth muscle cells (SMC).7 Bidirectional signaling between the epithelial and SMC compartments persists through development and into adulthood, and has been proposed to mediate cellular homeostasis, thereby maintaining the integrity of the organ.8 Androgens are believed to exert their effects on the developing and adult prostate by binding to androgen receptors expressed in the epithelial and stromal SMC compartments.9 In the epithelial cells, androgens promote and maintain the secretory phenotype, and modulate expression of secretory products, such as prostate-specific antigen (PSA). In the stroma, however, androgens are proposed to affect the expression of genes encoding for peptide growth factors, such as the EGF-like growth factors, through direct binding of androgen-receptor dimers to the promoter region of these androgen-responsive genes.10 The expressed growth factors can subsequently diffuse from the stromal compartment and elicit receptor-dependent growth modulation in the epithelial compartment. The identification of androgen-stimulated growth factor activity has led to the description of such growth factors as "andromedins."
Despite extensive analysis of stromal-epithelial interactions in the prostate through tissue recombination experiments, the molecular identity of the factors mediating such interactions is only beginning to be revealed. In our laboratory, we have recently demonstrated the expression of two structurally related EGF-like factors, HB-EGF11 and amphiregulin (ARG),12 predominantly by the SMC of the human prostate, and have additionally confirmed localization of ErbB1 to the basal epithelial cells. These observations suggest the possibility for paracrine activation of ErbB1, expressed on epithelial cells and mediated by the stroma-derived ligands, HB-EGF and ARG. Consistent with this proposal, we further demonstrated that both HB-EGF and ARG were potent mitogens for prostate epithelial cells in vitro. Furthermore from the mRNA analysis of a large panel of prostate tissue samples, comprising prostate carcinoma and benign prostatic hyperplasia (BPH) specimens, we obtained evidence that the genes encoding HB-EGF and ARG exhibit coordinate expression in vivo, and therefore, may be coordinately regulated. These data provide experimental evidence for the existence of the long-hypothesized directional signaling axis between the stromal SMC and epithelial compartments in the human prostate.
The ErbB Receptor-Ligand Axis in the Human Prostate—Clinical Implications
From our data and those of others, it is evident that signaling through the ErbB receptor-ligand axis profoundly effects both the normal growth and development of the prostate and pathological conditions affecting the prostate—notably cancer and BPH. Activation of ErbB1 on prostate epithelial cells, in response to stromal SMC-derived ligands such as HB-EGF and ARG, further emphasizes the critical influence of the stroma on epithelial cell behavior. These observations suggest that altered signaling along the ErbB-ligand axis may disrupt the cellular homeostasis within the normal prostate and thereby contribute to neoplastic and/or hyperplastic growth. Using pharmacological agents to target either the receptor or ligand components of this signaling, it may be possible to restore normal communication between the stromal and epithelial compartments, and attenuate pathological proliferation of SMC or epithelial cells.
Agents used to target ErbB receptors in other systems include toxin-conjugated ligands, such as HB-EGF or TGF-a conjugated to the Pseudomonas exotoxin A13 and monoclonal antibodies.14 Several antibodies have been evaluated for their efficacy in blocking activation and postreceptor signaling of ErbB1 or ErbB2, both of which are commonly overexpressed in breast carcinoma. For example, the humanized anti-ErbB2 monoclonal antibody, Herceptin, has been demonstrated to inhibit the growth of breast cancer cell lines in vitro and of breast cancer cell xenografts in nude mice, and is currently under evaluation in Phase III clinical trials.15 Although there is no evidence for ErbB1 or ErbB2 overexpression in primary prostate carcinoma, an ErbB3-heregulin-alpha autocrine loop has been described, suggesting the potential for targeting of drugs to the receptor component of the signaling axis.16 Alternatively, therapeutic strategies could be targeted against the activating ligands for ErbB receptors, such that receptor activation is prevented. In this respect, HB-EGF represents an attractive target in view of its identity as the cell-surface receptor for diphtheria toxin (DT) (distinct from its function as an activating ligand for ErbB1).17 A catalytically inactive mutant of DT, termed CRM 197, acts in a similar manner to a neutralizing antibody for HB-EGF and has been demonstrated to downregulate HB-EGF activity in vitro.18,19 Since the mutant possesses none of the toxic activity of DT, such a reagent could be developed for systemic use to downregulate HB-EGF function as a paracrine growth factor in the prostate.
Conclusion
In summary, the ErbB receptor-ligand axis is believed to play a critical role in growth modulation in the human prostate. ErbB1-dependent activation of prostate epithelial cell proliferation in response to stromal-derived ligands exemplifies stromal-epithelial interactions at the molecular level. By increasing our understanding at the molecular level of the mediators involved in maintaining cellular homeostasis in the prostate, and how alterations in their function may lead to prostatic disease, our ability to intervene therapeutically will be significantly improved. (Dr. Adam is Post-Doctoral Fellow and Dr. Freeman is Director of Urology Research at The Urologic Laboratory, Department of Urology, Children’s Hospital and the Department of Surgery, Harvard Medical School, Boston, MA.)
References
1. Chung LWK. Semin Cancer Biol 1993;4:183-192.
2. Hayward SW, Rosen MA, et al. Br J Urol 1997;79:18-26(Suppl. 2).
3. Gullick WJ. Biochem Soc Symp 1998;63:193-198.
4. McKeehan WL, Adams PS, et al. Cancer Res 1984;44:1998-2010.
5. Leav I, McNeal JE, et al. Hum Pathol 1998;29:668-675.
6. Sherwood ER, Lee C. World J Urol 1995;13:290-296.
7. Cunha GR, Donjacour AA. Cancer Treat Res 1989;46:159-175.
8. Cunha GR, Hayward SW, et al. Acta Anatomica 1996;155:63-72.
9. Prins GS, Birch L, et al. Endocrinology 1991;129: 3187-3199.
10. Cunha GR, Foster B, et al. World J Urol 1995;13:264-276.
11. Freeman MR, Paul S, et al. J Cell Biochem 1998;68:328-338.
12. Adam RM, Borer JG, Williams BJ, et al. 1999 (Submitted).
13. Mesri EA, Kreitman RJ, et al. J Biol Chem 1993;268:4853-4862.
14. Modjtahedi H, Affleck K, et al. Int J Oncol 1998;13:335-342.
15. Baselga J, Norton L, et al. Cancer Res 1998;58:2825-2831.
16. Leung HY, Weston J, et al. Br J Urol 1997;79:212-216.
17. Naglich JG, Metherall JE, et al. Cell 1992;69:1051-1061.
18. Higashiyama S, Iwamoto R, et al. J Cell Biol 1995;128:929-938.
19. Freeman MR, Yoo JJ, et al. J Clin Invest 1997;99:1028-1036.
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