Growth Factor Superfamilies and Mammalian Embryogenesis

Development 102. 451-460 (1988) Review Article 451 Printed in Great Britain © The Company of Biologists Limited 1988 Growth factor superfamilies and mammalian embryogenesis MARK MERCOLA and CHARLES D. STILES Department of Microbiology and Molecular Genetics, Harvard Medical School and the Dana-Farber Cancer Institute, Boston, MA 02115, USA Summary With the availability of amino acid and nucleotide unpredicted from the cell biology of most of the sequence information has come the realization that growth factors. Moreover, these actions are reflected growth factors can be clustered into superfamilies. in nonmammalian species where homologues of the Several of these superfamilies contain molecules that mammalian growth factors control crucial steps in the were not initially identified because of growth-promot- choice of developmental fate. This review describes ing activities; rather they were discovered through five growth factor superfamilies and the role these their ability to regulate other processes. Certain molecules may have in controlling proliferation, dif- members of these superfamilies are present during ferentiation, and morphogenesis during mammalian early mammalian embryogenesis. However, until re- development. cently, it has been difficult to manipulate the developing mammalian embryo to observe directly the effects Key words: growth factor, mammal, epidermal growth of inappropriate, excessive, or reduced expression of factor, EGF, insulin-like growth factor, IGF-I, IGF-II, these molecules. Despite this limitation, at least some transforming growth factor-beta, TGF, heparin-binding of these molecules have been implicated in the control growth factor, HBGF, platelet-derived growth factor, of differentiation and morphogenesis, two actions PDGF. Introduction fibroblast growth factor can induce mesoderm differentiation from ectoderm tissue. Thus, growth factors, Recent analysis of early development of C. elegans, or novel growth-factor-like molecules, seem to guide Drosophila, and Xenopus has emphasized the role morphogenesis and differentiation in these species. -that growtH factors, or related molecules, may play as Can the same be said for mammals? An emerging morphogenetic and differentiation signals. Two well- body of literature suggests that polypeptide growth characterized growth factors have sequence similarity factors which are similar, if not identical, to those to pattern-forming genes. Transforming growth fac- isolated from sera and tissues of adult mammals, tor-beta is homologous to predicted proteins of the influence not only growth but differentiation and decapentaplegic complex of Drosophila and a ma- morphogenesis during mammalian development. ternally encoded mRNA in Xenopus embryos This view is furthered by the realization that several (Padgett et al. 1987; Weeks & Melton, 1987). The molecules originally identified because of their devel- gene encoding epidermal growth factor shares opmental roles are structurally similar to growth domains of homology with the homeotic loci lin-Yl in factors. As shown in Table 1, the expanding number C. elegans and Notch in Drosophila (Greenwald et al. of growth factors can be grouped into superfamilies 1985; Wharton et al. 1985; Knust et al. 1987). More- based on nucleotide and amino acid sequence hom- over, growth-factor-like molecules act as mesoderm- ology as well as similar receptor-binding activity. In inducing signals in developing Xenopus blastocysts Table 2 and the paragraphs below, we summarize the (Smith, J. 1987; Slack et al. 1987; Kimelman & chemistry and biology of these superfamilies as well Kizschner, 1987). Slack etal. (1987) and Kimelman & as the data that link them to mammalian develop- Kirschner (1987) showed that purified bovine basic ment. 452 M. Mercola and C. D. Stiles The epidermal growth factor family Sundell et al. 1980). However, the analysis of em- Several structurally related peptides exhibit the ac- bryos and embryonal carcinoma cells indicate that tivity first identified with epidermal growth factor TGF-alpha is the primary component of fetal EGF (EGF). EGF was discovered by Stanley Cohen as a activity. A fetal form of EGF was first suggested by contaminant within certain nerve growth factor prep- Nex0 et al. (1980) who noted high levels of EGF arations which triggered premature eyelid opening activity in fetal tissues using a receptor-binding assay and incisor eruption in neonatal mice (for review see but lower levels using a more specific immunoassay. Carpenter & Cohen, 1979). Using this observation as While the mRNA for salivary gland EGF is not a bioassay, Cohen and his associates purified EGF observed during mouse gestation or in F9 embryonal from mouse salivary glands and, by amino acid carcinoma cells, TGF-alpha protein and mRNA are analysis, showed it to be homologous to the human present in mid to late gestation mouse and rat hormone urogastrone. A structurally and functionally embryos (Table 2). In addition to expression of the similar growth factor, transforming growth factor- growth factor, functional TGF-alpha/EGF receptors alpha (TGF-alpha), is produced by certain tumour have been detected in mouse embryos as early as day and retrovirus-transformed cells (Marquardt et al. 11 (Table 2). 1984). Finally, Brown et al. (1985) have described a While studies using embryonic tissue have concen- vaccinia-virus-encoded mitogen which has EGF ac- trated on mid- to late-gestation embryos, exper- tivity. All members of the EGF superfamily compete iments using embryonal carcinoma cells suggest that efficiently with salivary gland EGF for binding TGF-alpha or EGF may function even earlier in 3 to a 170xl0 Mr transmembrane tyrosine-specific development. Retinoic-acid-differentiated F9 and kinase, which is the product of the c-erb B proto- PC13 cells, but not undifferentiated stem cells, se- oncogene (Downward et al. 1984). crete peptides with TGF activity (Rizzino et al. 1983). Early embryological studies have focussed primar- The expression of cell surface TGF-alpha/EGF re- ily on the effects of EGF, administered in utero, on ceptors may reflect the differentiation status of the the developing lung and palate (Catterton et al. 1979; cell. Undifferentiated PC13 and OC15 cells do not express cell surface receptors; however, at least OC15 Table 1. Growth factor families cells contain intracellular receptor molecules (Weller et al. 1987). In contrast, differentiating embryonal carcinoma cells display surface EGF receptors. Thus, Group Members they may be expressed in the embryo as stem cells Epidermal growth factor EGF differentiate. The binding of salivary gland EGF to Transforming growth factor-a\ trophoblast outgrowths of cultured mouse blastocysts TGF-a- supports this view (Adamson & Meek, 1984). Vaccinia growth factor, VGF Whether the cryptic intracellular form of the receptor Insulin-like growth factor IGF-I is expressed in embryonic stem cells, and what IGF-II (Somatomedin-C; function it has, is unknown. multiplication stimulating activity, MSA) EGF-like factors may play a multifunctional role Relaxin during development. Adamson & Meek (1984) demonstrated that, whereas the number of EGF Transforming-growth factor-/? TGF-/8, TGF-ft receptors increases in fetal tissues, the apparent TGF-/5,.2 affinity for EGF declines two- to threefold during Inhibin-A mouse gestation. The authors suggest that these Inhibin B changes correlate with differing roles of the receptor Activin-A as development proceeds from tissue growth to differ- Activin AB Miillerian inhibiting substance entiation. Thus, EGF may stimulate proliferation in stem cells and differentiation or expression of a Heparin-binding growth Acidic HBGF (acidic differentiated phenotype in mature cells. This is factors fibroblast growth factor, aFGF; endothelial cell supported by the range of nonproliferative responses growth factor, ECGF) to EGF seen in differentiated cells (for review see Basic HBGF, bFGF Sporn & Roberts, 1987). The homology between Products of the int-2, hst, and EGF and the homeotic loci Notch and lin-12, men- Kaposi's Sarcoma proto- tioned above, may imply a novel role for EGF and oncogenes TGF-alpha. Like the Notch and lin-12 products, the Platelet-derived growth factor PDGF-A EGF and TGF-alpha precursors are thought to PDGF-B (sis product), encode transmembrane peptides (Rail et al. 1985; PDGF-AB Gentry et al. 1987; Teixido et al. 1987). Thus, these Growth factor superfamilies 453 Table 2. Examples of growth factor expression during embryogenesis Factor Species Material assayed Occurrence Reference TGF-o- Mouse TGF-o- Post-day-7 embryos Proper et al. 1982; Twardzik, 1985 Mouse TGF-o- mRNA Post-day-7 embryos Popliker et al. 1987 Rat TGF-* mRNA Post-day-8 embryos Lee et al. 1985 EGF Receptor Mouse EGF binding Post-day-11 embryos Nex0 et al. 1980; Adamson et al. 1981 Rat Receptor kinase Post-day-10 embryos Hortsch et al. 1983 IGF Rat Serum IGF-II Fetal level > maternal level Moses et al. 1980 Rat Serum IGF-I Maternal level s> fetal level D'Ercole et al. 1980; Sara et al. 1980 Mouse IGF-II Amnion/yolk sac mesoderm Heath & Shi, 1985 Rat IGF-I, II mRNA Post-day-14 embryos/adult Lund et al. 1986 Human IGF-I, II mRNA Predominant expression Han et al. 1987 in connective and mesenchyme -derived tissue IGF Receptor Mouse IGF-I, II receptors Day-9, -12 embryos Smith et al. 1987 Mouse IGF-binding proteins Blastocysts/day-9 embryos Smith et al. 1987 Human IGF-binding Amnion Drop et al. 1984 TGF-jS Mouse TGF-/8 Day-17 embryo Proper et al. 1982 Rat TGF-/3 Day-21 fetal

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