The function of KGF in morphogenesis of epithelium and reepithelialization of wounds

Epidermal differentiation and basement membrane formation by HaCaT cells in surface transplants

The immortal human keratinocyte line HaCaT has been employed in many studies as paradigm for epidermal keratinocytes. In order to demonstrate its potential to form stable epidermal structures in response to connective tissue, this was challenged in surface transplants on nude mice, where normal keratinocytes rebuild a typical epidermis within two weeks. During the initial regeneration phase (day 1-4) multilayered but poorly organized epithelia formed with proliferating cells in all layers in analogy to normal keratinocytes. Similarly, with tissue consolidation (around day 7) proliferation was reduced and restricted to cells in basal position marked by keratin K14 and beta1-integrin immunostaining. The strong suprabasal reaction for K1 and K10, the appearance of the late markers K2e, filaggrin and loricrin as well as the polarized distribution of alpha2beta1 and alpha3beta1 indicated advancing tissue normalization (day 14). Keratinization further improved at around three weeks switching from the initial parakeratotic to the regular orthokeratotic type which was prominent at six weeks. Accordingly, most ultrastructural features typical for epidermis or normal keratinocyte grafts were detectable including a complete basement membrane (BM) with regular attachment structures. Matrix- and BM-components appeared sequentially with marked linear deposition of laminin-5 (day 4) followed by accumulation of collagen-IV and 'classical' BM-laminin between one and two weeks. With the general codistribution of integrin alpha6beta4 and BM-molecules (day 14) collagen-VII lining of BM became prominent, while epithelium and host connective tissue were still separated by the collagen matrix. In accordance with the delayed orthokeratinization, wound-matrix molecules (fibronectin, tenascin) persisted longer than in normal keratinocyte transplants. Finally, grafts of long-term passaged (no. 310) cells demonstrated a remarkable stability in the expression of epidermal markers. Thus, the immortalized HaCaT cells reveal a generally high competence to realize an epidermal phenotype in a natural environment and appear therefore qualified for in vitro studies on structural and regulatory aspects of keratinocyte physiology and pathology.

Keratinocyte growth factor mRNA expression in periodontal ligament fibroblasts

Keratinocyte growth factor (KGF) is a fibroblast growth factor which mediates epithelial growth and differentiation. KGF is expressed in subepithelial fibroblasts, but generally not in fibroblasts of deep connective tissue, such as fascia and ligaments. Here we demonstrate that KGF mRNA is expressed in periodontal ligament fibroblasts, and that the expression is increased upon serum stimulation. Fibroblasts from human periodontal ligament, from buccal mucosa, from gingiva, and from skin were established from explants. Alkaline phosphatase activity was used as an indicator of the periodontal nature of fibroblasts. Cells were first cultured in DMEM with 0.5% fetal calf serum (FCS) and then incubated for 8 h, and 72 h in fresh DMEM with 10% FCS. Total RNA was isolated and used for Northern blotting with a P32-labeled KGF cDNA probe. Total RNA from cultured keratinocytes was used as negative controls. KGF mRNA was found in all cultured fibroblasts. Upon addition of 10% FCS to the cell cultures, an increase in KGF mRNA levels was noticed especially after 72 h. Furthermore, RT-PCR analysis of material scraped from the tooth root surface indicated the presence of KGF mRNA even in noncultured periodontal ligament cells.

A role for fibroblast growth factor signaling in the lobuloalveolar development of the mammary gland

The inappropriate expression of growth factors, or activating mutations of their receptors, have been implicated as causative factors in mouse and human mammary cancer. For example, it has been known for some time that three members of the fibroblast growth factor (FGF) family behave like oncogenes in virally induced mammary cancer of mice. In normal circumstances, signaling via FGF receptors is known to mediate growth, differentiation, and patterning, during embryogenesis and fetal development. A powerful approach to dissecting the roles for these signaling pathways is to determine the developmental consequences of abrogating their function in transgenic mice. In this review, we describe the use of dominant negative FGF receptors to evaluate the contribution of specific FGF signals in normal mammary gland development. These studies have revealed that normal lobuloalveolar development requires FGF signaling to the mammary epithelium, a function that is presumably usurped by MMTV in mouse mammary tumorigenesis.

Structure and expression of human fibroblast growth factor-10

We isolated the cDNA encoding a novel member of the human fibroblast growth factor (FGF) family from the lung. The cDNA encodes a protein of 208 amino acids with high sequence homology (95.6%) to rat FGF-10, indicating that the protein is human FGF-10. Human FGF-10 as well as rat FGF-10 has a hydrophobic amino terminus ( approximately 40 amino acids), which may serve as a signal sequence. The apparent evolutionary relationships of human FGFs indicate that FGF-10 is closest to FGF-7. Chromosomal localization of the human FGF-10 gene was examined by in situ hybridization. The gene was found to map to the 5p12-p13 region. Human FGF-10 (amino acids 40 to 208 with a methionine residue at the amino terminus) was produced in Escherichia coli and purified from the cell lysate. Recombinant human FGF-10 (approximately 19 kDa) showed mitogenic activity for fetal rat keratinizing epidermal cells, but essentially no activity for NIH/3T3 cells, fibroblasts. The specificity of mitogenic activity of FGF-10 is similar to that of FGF-7 but distinct from that of bFGF. In structure and biological activity, FGF-10 is similar to FGF-7.

A novel type of glutathione peroxidase: expression and regulation during wound repair

We have previously identified and cloned a novel keratinocyte growth factor (KGF)-regulated gene in human keratinocytes that encodes the human homologue of a bovine non-selenium glutathione peroxidase (GPx). To gain insight into the regulation of this gene in vivo, we isolated the murine homologue from a mouse skin cDNA library. In vitro transcription/translation demonstrated that the cDNA encodes a 27 kDa protein. Furthermore, we amplified by PCR a partial cDNA that most likely corresponds to a related gene. RNase protection analysis revealed tissue-specific expression of both genes and the occurrence of alternative splicing or RNA editing of at least one of the primary transcripts. Similar to that of KGF, expression of GPx was strongly induced after cutaneous injury, and each isoform displayed unique kinetics of expression during the repair process. In situ hybridization studies demonstrated high levels of GPx mRNA in keratinocytes of the hyperproliferative epithelium at the wound edge. Since these cells express functional KGF receptors, induction of GPx expression by KGF might also occur in vivo. These data suggest a role for GPx in the protection of epithelial cells against oxidative stress, particularly during the inflammatory phase of wound repair.

Heparin-binding ligands mediate autocrine epidermal growth factor receptor activation In skin organ culture

Exogenous EGF and TGF-alpha accelerate wound healing, but treatment effects are often modest. Using short-term human skin organ culture, we found that autocrine EGF receptor activation could account for this observation. Amphiregulin and heparin-binding EGF-like growth factor (HB-EGF) transcripts were rapidly and markedly induced, whereas EGF and TGF-alpha mRNAs were undetectable or only slightly increased. Vascular permeability factor and keratin 6 transcripts were also strongly induced, albeit with a >/= 3 h delay relative to HB-EGF and amphiregulin. All four transcripts were upregulated in actual healing skin wounds, HB-EGF and keratin 6 being the most prominent. The highly EGF receptor-specific tyrosine kinase inhibitor PD153035 strongly inhibited induction of all four transcripts in organ culture, as well as release of immunoreactive HB-EGF into the medium. These effects were confirmed using the anti-EGF receptor mAb 225 IgG. Neither PD153035 nor 225 IgG was toxic to keratinocytes, as judged by calcein-AM uptake. PD153035 completely abrogated the proliferative phase of keratinocyte outgrowth in skin explant cultures, whereas it had no effect on the antecedent migratory phase. Based on these results, we conclude that EGF receptor activation by highly inducible, keratinocyte-derived heparin-binding ligands is an important mechanism for amplification and transmission of the cutaneous wound healing signal.

Differential regulation of keratinocyte growth factor and hepatocyte growth factor/scatter factor by different cytokines in human corneal and limbal fibroblasts

Corneal epithelial stem cells and transient amplifying cells are located in the limbal and corneal regions, respectively. In a serum-free medium with or without different cytokines, limbal fibroblasts consistently produced greater levels of keratinocyte growth factor (KGF) transcript and protein than corneal fibroblasts, whereas corneal fibroblasts produced greater levels of hepatocyte growth factor/ scatter factor (HGF/SF) transcript and protein than limbal fibroblasts. Expression of HGF/SF transcript and protein was up-regulated mildly by epidermal growth factor (EGF), transforming growth factor-alpha (TGF-alpha), or platelet-derived growth factor B (PDGF-BB) but markedly by interleukin-1 beta (IL-1 beta) and was more pronounced in limbal than in corneal fibroblasts. Expression of KGF transcript was down-regulated by EGF, TGF-alpha, and PDGF-BB, was markedly up-regulated by IL-1 beta, and was more pronounced in limbal than in corneal fibroblasts. Expression of KGF protein was up-regulated markedly by IL-1 beta and moderately by PDGF-BB, especially in limbal fibroblasts. TGF-beta 1 uniquely turned off transcript and protein expression of HGF/SF and KGF in corneal fibroblasts. Although its transcript levels were similarly down-regulated in limbal fibroblasts, KGF protein levels were paradoxically up-regulated by TGF-beta 1 when added alone or with TGF-alpha or IL-1 beta. These data indicate that KGF and HGF/SF, two fibroblast-derived epithelial mitogens, are expressed differentially by limbal and corneal fibroblasts and are modulated by cytokines activated during epithelial-mesenchymal interactions, suggesting that they may play a different role in modulating corneal epithelial stem cells and transient amplifying cells.

Cell type-specific inhibition of keratinocyte collagenase-1 expression by basic fibroblast growth factor and keratinocyte growth factor. A common receptor pathway

Collagenase-1 is invariantly expressed by migrating basal keratinocytes in all forms of human skin wounds, and its expression is induced by contact with native type I collagen. However, net differences in enzyme production between acute and chronic wounds may be modulated by soluble factors present within the tissue environment. Basic fibroblast growth factor (bFGF, FGF-2) and keratinocyte growth factor (KGF, FGF-9), which are produced during wound healing, inhibited collagenase-1 expression by keratinocytes in a dose-dependent manner. However, KGF was >100-fold more effective than bFGF at inhibiting collagenase-1 expression, suggesting that this differential signaling is transduced via an FGF receptor that binds these ligands with different affinities. Reverse transcriptase-polymerase chain reaction analysis of human keratinocyte mRNA for fibroblast growth factor receptors (FGFRs) revealed expression of only FGFR-2 IIIb, the KGF-specific receptor, which also binds bFGF with low affinity, and FGFR-3 IIIb, which does not bind bFGF or KGF. FGFRs that bind bFGF with high affinity were not detected. Our results suggest that bFGF and KGF inhibit collagenase-1 expression through the KGF cell-surface receptor (FGFR-2 IIIb). Because bFGF induces collagenase-1 in most cell types, cell-specific expression of FGFR family members may dictate the regulation of matrix metalloproteinases in a tissue-specific manner.

Activity of keratinocyte growth factor-like substance extracted from rabbit liver on renal tubular cell growth during compensatory renal hyperplasia

BACKGROUND

In response to unilateral nephrectomy, the rabbit liver transiently produces 2 growth regulators for cultured renal cortical tubular cells: a tubular cell growth factor and a growth inhibitor. We report on the effects of the tubular cell growth factor on a variety of cell lines.

METHODS

The tubular cell growth factor activity was partially purified from the rabbit liver by using gel filtration and ion-exchange chromatography. The activity was monitored by the incorporation of iododeoxyuridine into DNA of cultured cells. Expression of the fibroblast growth factor receptor-2 in the rabbit kidney was determined by the immunoblot analysis.

RESULTS

This growth factor stimulated the DNA synthesis in LLC-PK1 cells, LLC-RK1 cells, and human keratinocytes. It did not affect the growth of BS-C-1 cells, MDCK cells, BALB/c 3T3 fibroblasts, or rat parenchymal hepatocytes. The additive effect of this factor on the DNA synthesis of cultured tubular cells maximally was stimulated by insulin-like growth factor-I, basic fibroblast growth factor, and epidermal growth factor, but was not stimulated by keratinocyte growth factor. The amount of this activity also increased in the liver after sham operation. In the days after surgery, expression of fibroblast growth factor receptor-2, which includes the keratinocyte growth factor receptor, was down-regulated in the kidneys of both uninephrectomized and sham-operated rabbits.

CONCLUSION

These findings indicate that tubular cell growth factor in the liver seems to be a keratinocyte growth factor, and acts in an endocrine manner in renal tubular hyperplasia.

Fibroblast growth factor receptor signalling has a role in lobuloalveolar development of the mammary gland

We have used the mouse mammary tumor virus promoter to express two dominant negative (DN) fibroblast growth factor receptor (FGFR) isoforms in the mammary epithelium of transgenic mice. While expression of DN-FGFR1(IIIc) showed no discernible phenotype, a similar kinase negative form of FGFR2(IIIb) caused a marked impairment of lobuloalveolar development. The growth retardation was apparent by mid-pregnancy and persisted in the post-partum glands. Despite the substantial underdevelopment of the mammary gland there was a measurable lactational response, but it was insufficient to properly sustain the new-born pups. These findings demonstrate that fibroblast growth factor signalling is necessary for pregnancy dependent lobuloalveolar development of the mammary gland.

Wound healing--aiming for perfect skin regeneration

The healing of an adult skin wound is a complex process requiring the collaborative efforts of many different tissues and cell lineages. The behavior of each of the contributing cell types during the phases of proliferation, migration, matrix synthesis, and contraction, as well as the growth factor and matrix signals present at a wound site, are now roughly understood. Details of how these signals control wound cell activities are beginning to emerge, and studies of healing in embryos have begun to show how the normal adult repair process might be readjusted to make it less like patching up and more like regeneration.

Regulation of the expression of stromelysin-2 by growth factors in keratinocytes: implications for normal and impaired wound healing

Keratinocyte growth factor (KGF) has been implicated in wound re-epithelialization and branching morphogenesis of several organs. To determine whether KGF induces these effects via induction of matrix metalloproteinase expression we have analysed the effect of KGF on the expression of stromelysin-2 in cultured HaCaT keratinocytes. Here we show a strong induction of stromelysin-2 mRNA within 5-8 h of stimulation of these cells with KGF. The degree of induction was similar to that achieved by treatment with epidermal growth factor or tumour necrosis factor alpha, whereas the stimulatory effect of transforming growth factor beta 1 was even stronger. To determine whether the induction of stromelysin-2 expression by growth factors and cytokines might be important for wound healing, we analysed the expression of this gene during the healing process of full-thickness excisional wounds in mice. Whereas stromelysin-2 mRNA could hardly be detected in unwounded skin, a biphasic induction was seen after injury and highest levels were found at days 1 and 5 after wounding. Hybridization in situ revealed the presence of stromelysin-2 mRNA in basal keratinocytes at the wound edge but not in the underlying mesenchymal tissue. During impaired wound healing as seen in glucocorticoid-treated mice, stromelysin-2 expression was significantly increased compared with untreated control mice. Taken together, these results suggest that correct regulation of this broad-spectrum metalloproteinase might be important for normal repair.

Spatial and temporal relationships between Shh, Fgf4, and Fgf8 gene expression at diverse signalling centers during mouse development

During limb outgrowth, Shh, Fgf4, and Fgf8 act as signals controlling limb growth and patterning. Because these genes are expressed in the limb bud and other known signalling centers, we have explored the relationships between the expression of these during mouse development using double in situ hybridization. Within the node and limb bud the expression domains of these genes contact each other, whereas in the floor plate the Fgf8 expression domain does not contact that of Shh. The relative temporal order of gene expression varies in different centers. The spatial and temporal expression of Fgf4, Fgf8, and Shh suggests that conservation of molecular mechanisms in different organizing centers as well as differences between them.

Growth factors and cytokines in hair follicle development and cycling: recent insights from animal models and the potentials for clinical therapy

Hair growth disorders, particularly those that lead to hair loss (alopecia), are common and frequently cause significant mental anguish in affected individuals. The mechanisms underlying the majority of these disorders are unknown. However, insights into the specific molecular mechanisms of hair follicle development and cycling have recently been made using animal models, particularly mice that over- or underexpress a specific gene for a growth factor or cytokine. Other animal models have demonstrated that certain growth factors and cytokines can prevent much of the alopecia caused by cancer chemotherapeutic agents. These animal models have confirmed the importance of growth factors and cytokines in hair follicle development and cycling, and have formed the foundation for potential clinical therapy of hair growth disorders, particularly alopecia. Nevertheless, important questions concerning their efficacy, safety and delivery will need to be answered before successful clinical therapy of any hair growth disorder becomes a reality.

An estrogen receptor pathway regulates the telogen-anagen hair follicle transition and influences epidermal cell proliferation

The hair follicle is a cyclic, self renewing epidermal structure which is thought to be controlled by signals from the dermal papilla, a specialized cluster of mesenchymal cells within the dermis. Topical treatments with 17-beta-estradiol to the clipped dorsal skin of mice arrested hair follicles in telogen and produced a profound and prolonged inhibition of hair growth while treatment with the biologically inactive stereoisomer, 17-alpha-estradiol, did not inhibit hair growth. Topical treatments with ICI 182,780, a pure estrogen receptor antagonist, caused the hair follicles to exit telogen and enter anagen, thereby initiating hair growth. Immunohistochemical staining for the estrogen receptor in skin revealed intense and specific staining of the nuclei of the cells of the dermal papilla. The expression of the estrogen receptor in the dermal papilla was hair cycle-dependent with the highest levels of expression associated with the telogen follicle. 17-beta-Estradiol-treated epidermis demonstrated a similar number of 5-bromo-2'-deoxyuridine (BrdUrd) S-phase cells as the control epidermis above telogen follicles; however, the number of BrdUrd S-phase basal cells in the control epidermis varied according to the phase of the cycle of the underlying hair follicles and ranged from 2.6% above telogen follicles to 7.0% above early anagen follicles. These findings indicate an estrogen receptor pathway within the dermal papilla regulates the telogen-anagen follicle transition and suggest that diffusible factors associated with the anagen follicle influence cell proliferation in the epidermis.

The human homologue of the yeast CHL1 gene is a novel keratinocyte growth factor-regulated gene

Keratinocyte growth factor (KGF) is a potent and specific mitogen for different types of epithelial cells, including keratinocytes of the skin. To gain insight into the mechanisms of KGF action in this tissue, we attempted to identify genes that are regulated by KGF in keratinocytes. Using the differential display reverse transcription polymerase chain reaction technology, a gene was identified which was strongly induced in these cells by treatment with KGF but not with serum growth factors or pro-inflammatory cytokines. This gene seems to be part of a multigene family as assessed by Southern blot analysis. Molecular cloning and sequencing of the full-length cDNA revealed a strong homology with the yeast CHL1 gene. The latter encodes a putative helicase, which is involved in correct chromosome transmission and cell cycle progression. Furthermore, the CHL1 gene product and the protein encoded by the novel KGF-regulated gene were identical in size, indicating that we had cloned the human CHL1 homologue. This finding suggests a novel and specific role of KGF in correct chromosome segregation and/or cell cycle progression.

Hair follicle growth controls

Research in hair biology has embarked in the pursuit for molecules that control hair growth. Many molecules already have been associated with the controls of hair patterning, hair maturation, and hair cycling and differentiation. Knowing how these molecules work gives us the tools for understanding and treating patients with hair disorders.

Expression of keratinocyte growth factor in periapical lesions

The epithelial proliferation associated with inflammatory periapical lesions and with periapical cyst formation represents an interesting but poorly understood pathological change. Keratinocyte growth factor (KGF) is a recently identified growth factor that is produced by stromal fibroblasts and acts specifically to stimulate epithelial growth and differentiation. To investigate its possible role in the activation of the normally quiescent rests of Malassez, we examined the expression of KGF by in situ hybridization of sections of normal periodontal ligament (PDL) and of 12 periapical granulomas or cysts. Normal PDL and periapical granulomas with scant inflammatory infiltration showed few cells expressing message for KGF. However, KGF-expressing cells were found in the connective tissue stroma close to dense foci of inflammatory cells and to proliferating epithelial elements and cystic epithelial linings. Examination of tissues by the reverse-transcription polymerase chain reaction (RT-PCR) showed KGF expression in 4 specimens of periapical lesions but low or undetectable levels in normal PDL. These observations suggest that the induction of KGF expression in the stromal cells of periapical lesions may play an important role in stimulating the epithelial proliferation associated with cyst formation.

Fibroblast growth factor receptor 2 mutations in Beare-Stevenson cutis gyrata syndrome

Beare-Stevenson cutis gyrata syndrome (MIM 123790) is an autosomal dominant condition characterized by the furrowed skin disorder of cutis gyrata, acanthosis nigricans, craniosynostosis, craniofacial dysmorphism, digital anomalies, umbilical and anogenital abnormalities and early death. Many of these features are characteristic of some of the autosomal dominant craniosynostotic syndromes. Mutations in Crouzon, Jackson-Weiss, Pfeiffer and Apert syndromes have been reported in the FGFR2 extracellular domain. In Crouzon syndrome patients with acanthosis nigricans, a recurrent mutation occurs in the transmembrane domain of FGFR3. We now describe the detection of FGFR2 mutations in the Beare-Stevenson cutis gyrata syndrome. In three sporatic cases, a novel missense mutation was found causing an amino acid to be replaced by a cysteine; two had the identical Ty375Cys mutation in the transmembrane domain and one had a Ser372Cys mutation in the carboxyl-terminal end of the linker region between the immunoglobulin III-like (Iglll) and transmembrane domains. In two patients, neither of these mutations were found suggesting further genetic heterogeneity.

Keratinocyte growth factor attenuates lung leak induced by alpha-naphthylthiourea in rats

OBJECTIVE

To investigate the effect of pretreatment with keratinocyte growth factor on acute permeability pulmonary edema.

DESIGN

Prospective, randomized, controlled animal study.

SETTING

University research laboratory.

SUBJECTS

Specific pathogen-free Sprague-Dawley rats.

INTERVENTION

Acute permeability pulmonary edema was induced with an injection of alpha-naphthylthiourea, and lung leak was assessed in an isolated perfused lung model over 180 mins. Leak was confirmed with wet/dry lung weight ratios, and the alveolar fluid protein concentration was measured after bronchoalveolar lavage. The effect of pretreatment with keratinocyte growth factor (injected intratracheally 48 hrs before the experiment) on alpha-naphthylthiourea-induced pulmonary edema was assessed (keratinocyte growth factor/alpha-naphthylthiourea group). Control groups (Control and keratinocyte growth factor/Control) were also studied. Histopathology was performed for each of the four groups.

MEASUREMENTS AND MAIN RESULTS

The alpha-naphthylthiourea produced an acute permeability pulmonary edema detected by lung leak over the 180-min ex vivo period of monitoring the isolated perfused lung (leak = 8+/-mL; wet/dry weight ratio 14.7+/-2; lavage protein 3.1+/-1 mg/mL). Pretreatment with keratinocyte growth factor significantly attenuated these parameters (leak = 2.3+/-0.4 mL; wet/dry weight ratio 7.1 +/- 0.5; lavage protein 0.28 +/-0.03 mg/mL), which were not significantly different from the control group and the keratinocyte growth factor/control group. Histopathology showed abundant type II pneumocyte hyperplasia in the lungs of animals pretreated with keratinocyte growth factor, and marked pulmonary edema in animals pretreated with alpha-naphthylthiourea. Less edema was apparent in the keratinocyte growth factor/alpha-naphthylthiourea group. All data are expressed as mean +/- SEM.

CONCLUSIONS

Pretreatment with keratinocyte growth factor significantly attenuates pulmonary edema induced by alpha-naphthylthiourea. The mechanisms of this protection are likely related to type II pneumocyte hyperplasia, but remain to be specifically elucidated.

Motogenic and morphogenic activity of epithelial receptor tyrosine kinases

Receptor tyrosine kinases play essential roles in morphogenesis and differentiation of epithelia. Here we examined various tyrosine kinase receptors, which are preferentially expressed in epithelia (c-met, c-ros, c-neu, and the keratin growth factor [KGF] receptor), for their capacity to induce cell motility and branching morphogenesis of epithelial cells. We exchanged the ligand-binding domain of these receptors by the ectodomain of trkA and could thus control signaling by the new ligand, NGF. We demonstrate here that the tyrosine kinases of c-met, c-ros, c-neu, the KGF receptor, and trkA, but not the insulin receptor, induced scattering and increased motility of kidney epithelial cells in tissue culture. Mutational analysis suggests that SHC binding is essential for scattering and increased cell motility induced by trkA. The induction of motility in epithelial cells is thus an important feature of various receptor tyrosine kinases, which in vivo play a role in embryogenesis and metastasis. In contrast, only the c-met receptor promoted branching morphogenesis of kidney epithelial cells in three-dimensional matrices, which resemble the formation of tubular epithelia in development. Interestingly, the ability of c-met to induce morphogenesis could be transferred to trkA, when in a novel receptor hybrid COOH-terminal sequences of c-met (including Y14 to Y16) were fused to the trkA kinase domain. These data demonstrate that tubulogenesis of epithelia is a restricted activity of tyrosine kinases, as yet only demonstrated for the c-met receptor. We predict the existence of specific substrates that mediate this morphogenesis signal.

Keratinocyte growth factor inhibits cross-linked envelope formation and nucleosomal fragmentation in cultured human keratinocytes

Keratinocyte growth factor (KGF) exhibits paracrine action on numerous epithelia, including skin. We have found that cultures of normal human keratinocytes must attain confluence before KGF promotes an increase in cell number relative to untreated controls. In postconfluent cultures, treatment with KGF promoted tight packing of keratinocytes with a small basal cell morphology. Based on these observations, we hypothesized that KGF increased cell number in postconfluent cultures by affecting the ability of normal keratinocytes to undergo terminal differentiation and/or programmed cell death. In support of this hypothesis, keratinocytes treated with KGF produced fewer cross-linked envelopes and exhibited reduced membrane-associated transglutaminase activity relative to cells treated with epidermal growth factor or untreated controls. We also found that nucleosomal fragmentation was reduced in postconfluent cultures of KGF-treated keratinocytes. Furthermore, KGF-treated keratinocytes were more resistant to suspension-induced nucleosomal fragmentation than control or epidermal growth factor-treated cultures. Therefore, it appears that KGF modulates aspects of keratinocyte terminal differentiation which share features with programmed cell death. We propose that stromally-derived KGF may act as a paracrine survival factor in skin and perhaps other renewal tissues.

The opioid growth factor, [Met5]-enkephalin, and the zeta opioid receptor are present in human and mouse skin and tonically act to inhibit DNA synthesis in the epidermis

Opioid peptides serve as tonically active negative growth factors in neural and non-neural cells, in addition to being neuromodulators. To investigate the involvement of opioids in homeostatic renewal of epithelial cells in the epidermis, mice were given systemic injections of the potent opioid antagonist, naltrexone (NTX) (20 mg/kg). Disruption of opioid-receptor interaction by NTX resulted in an elevation of 42 and 72% in DNA synthesis in skin from the dorsum and plantar surface of the hindfoot, respectively, within 2 h; response to NTX was dependent on the circadian rhythm in each region examined. Injection of the naturally occurring and potent opioid growth factor (OGF), [Met5]-enkephalin, at 1 mg/kg depressed DNA synthesis in the dorsum and plantar surface by 42 and 19%, respectively, within 2 h; the effects of OGF complied with the pattern of circadian rhythm in each area of skin. The decreases in labeling index evoked by OGF were blocked by concomitant administration of the opioid antagonist, naloxone (10 mg/kg); naloxone alone at the dosage utilized had no influence on cell replicative processes. In tissue culture studies, OGF and NTX respectively depressed and elevated DNA synthesis. Both OGF and its receptor, zeta, were detected in all but the cornified layer of the epidermis in murine skin from the dorsum, plantar surface, pinnae, and tail. In addition, both peptide and receptor were observed in basal and suprabasal cells of the human epidermis. These results lead to the suggestion that an endogenous opioid peptide and its receptor are present and govern cellular renewal processes in the skin in a direct manner, regulating DNA synthesis in a tonically inhibitory, circadian rhythm-dependent fashion.

Immunity at the surface: homeostatic mechanisms of the skin immune system

The coordinated function of multiple epidermal and dermal cell populations allows the skin immune system to respond rapidly and effectively to a wide variety of insults occurring at the interface of the organism and its environment. Keratinocytes are the first line of defense in the skin immune system, and keratinocyte-derived cytokines are pivotal in mobilizing leukocytes from blood and signaling other cutaneous cells. Cytokine-mediated cellular communication also enables dermal fibroblasts and endothelial cells lining the cutaneous vasculature to participate in immune and inflammatory responses. Skin is an important site for antigen presentation, and both epidermal Langerhans cells and dermal dendritic cells play pivotal roles in T cell-mediated immune responses to antigens encountered in skin. Proinflammatory signaling pathways are necessarily balanced by a variety of regulatory pathways that help maintain the homeostatic functioning of the skin immune system.

Keratinocyte growth factor is required for hair development but not for wound healing

Keratinocyte growth factor (KGF), also known as fibroblast growth factor 7 (FGF7), is synthesized by skin fibroblasts. However, its mitogenic activity is on skin keratinocytes, where it is the most potent growth factor identified thus far. To explore KGF's function in vivo, we used embryonic stem cell technology to generate mice lacking KGF. Over time, their fur developed a matted appearance, very similar to that of the rough mouse, whose recessive mutation maps at or near the KGF locus on mouse chromosome 2. In contrast to the recently reported transforming growth factor-alpha (TGF-alpha) and FGF5 knockouts, which showed defects in the follicle outer-root sheath and the hair growth cycle, respectively, the hair defect in the KGF knockout seemed to be restricted to the cells giving rise to the hair shaft. Thus, we have uncovered a third, and at least partially nonoverlapping, growth factor pathway involved in orchestrating hair follicle growth and/or differentiation. Surprisingly, the absence of KGF resulted in no abnormalities in epidermal growth or wound healing. This was true even when we engineered double knockout mice, null for both KGF and TGF-alpha, two factors that are increased dramatically in the normal wound-healing process. Whereas we found no evidence of compensatory changes at the mRNA level of wounded knockout mice, these data imply that the regulation of epidermal growth is complex and involves a number of growth stimulatory factors that go beyond what are thought to be the major paracrine and autocrine growth factors. We suggest that the redundancy in epidermal growth and wound healing is likely to stem from the vitality of these functions to the organism, a feature that is not a consideration for the hair follicle.

Regulation and clinical implications of corneal epithelial stem cells

The corneal epithelium is known to have a rapid self-renewing capacity. The major advance in the field of corneal epithelial cell biology in the last decade is the establishment of the location of corneal epithelial stem cells at the limbus, i.e., the junctional zone between the cornea and the conjunctiva. This concept has helped explain several experimental and clinical paradoxes, produced a number of important clinical applications, and spawned many other research studies. This unique enrichment of epithelial stem cells at a site anatomically separated from their transient amplifying cells makes the ocular surface an ideal model to study the regulation of epithelial stem cells. The present review includes data from more recent studies and lays out other areas for future investigation, especially with respect to the role of apoptosis and cytokine dialogue between limbal epithelial stem cells and their stromal microenvironment.

Regulation of keratinocyte growth factor gene expression in human skin fibroblasts

Human keratinocyte growth factor (KGF) is a recently identified mitogen for epithelial cells produced by normal stromal fibroblasts. KGF has been shown to stimulate keratinocyte migration and promote re-epithelialization of skin suggesting a critical role for KGF in wound healing. To understand how KGF might be regulated during wound healing, we examined the ability of the pro-inflammatory cytokines interleukin-1 alpha (IL-1 alpha), interleukin-1 beta (IL-1 beta) interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-alpha) transforming growth factor-beta 1 (TGF-beta 1) and interferon-gamma (IFN-gamma) to modulate KGF gene expression in cultured human fibroblasts, using northern blot analysis. Exposure to IL-1 alpha (20 units/ml) or IL-1 beta (100 units/ml) for 24 h increased KGF mRNA expression by 352% and 504%, respectively, with early induction seen at 2 h and maximal induction seen at 8 h. TNF-alpha (30 ng/ml) increased KGF mRNA expression by 535% at 24 h, with induction first seen at 8 h. The maximal induction of KGF mRNA was observed when IL-1 alpha, IL-1 beta and TNF-alpha were used at 100 units/ml, and 3 ng/ml, respectively, although concentrations 100-500-fold lower (IL-1 alpha, 0.02 units/ml; IL-beta, 0.02 units/ml; and TNF-alpha, 0.03 ng/ml) were nearly as stimulatory, increasing KGF mRNA expression by 175%, 254% and 322%, respectively. IL-6 (200 units/ml), TGF-beta 1 (5 ng/ml) and IFN-gamma (200 units/ml) did not change the level of KGF mRNA at 24 h in human fibroblasts under the same conditions. The protein synthesis inhibitor cycloheximide abrogated the effects of IL-1 alpha, IL-1 beta and TNF-alpha on KGF gene induction, indicating that new protein synthesis is required in the process. Dexamethasone (10(-7) M), known to inhibit inflammatory reactions and retard wound healing, also inhibited the induction of KGF mRNA expression by IL-1 alpha, IL-1 beta and TNF-alpha. Individual variation in KGF mRNA expression was see when fibroblasts from different aged donors were analysed, but no consistent age-associated change was observed. These results suggest that IL-1 alpha, IL-1 beta and TNF-alpha up-regulate KGF gene expression in fibroblasts and might be responsible for its induction following skin wounding or other injury.

Modulation of keratinocyte growth factor and its receptor in reepithelializing human skin

We investigated the expression and distribution of keratinocyte growth factor (KGF) (FGF-7) and its receptor (KGFR) during reepithelialization of human skin. KGF mRNA levels increased rapidly by 8-10-fold and remained elevated for several days. In contrast, KGFR transcript levels decreased early but were significantly elevated by 8-9 d. A KGF-immunoglobulin G fusion protein (KGF-HFc), which specifically and sensitively detects the KGFR, localized the receptor to differentiating keratinocytes of control epidermis, but revealed a striking decrease in receptor protein expression during the intermediate period of reepithelization. Suramin, which blocked KGF binding and stripped already bound KGF from its receptor, failed to unmask KGFRs in tissue sections from the intermediate phase of wound repair. The absence of KGFR protein despite increased KGFR transcript levels implies functional receptor downregulation in the presence of increased KGF. This temporal modulation of KGF and KGFRs provides strong evidence for the functional involvement of KGF in human skin reepithelialization.

Suppression of keratinocyte growth factor expression by glucocorticoids in vitro and during wound healing

We have recently demonstrated an important function of keratinocyte growth factor (KGF) in morphogenesis of epithelium and wound re-epithelialization. Furthermore, abnormalities in KGF expression or responsiveness are associated with wound-healing defects. In this study we have analyzed the regulation of KGF expression during wound repair in glucocorticoid-treated mice that are characterized by severe wound healing abnormalities. Induction of KGF mRNA expression after skin injury was significantly reduced in these mice, whereas KGF receptor mRNA levels were only affected to a minor extent by glucocorticoid treatment. The reduced KGF expression during wound healing in steroid-treated animals is at least partially due to a direct effect of glucocorticoids on the KGF expressing mesenchymal cells, because treatment of cultured fibroblasts with dexamethasone reduced KGF mRNA levels in a time- and concentration-dependent manner. The inhibitory effect of glucocorticoids on KGF expression was compensated for by high levels of serum growth factors or pro-inflammatory cytokines, demonstrating that KGF expression is subject to positive and negative regulation. Thus it seems likely that a fine balance of various KGF-regulating factors is important for normal wound healing.

Intratracheal instillation of keratinocyte growth factor decreases hyperoxia-induced mortality in rats

Alveolar type II cell proliferation occurs after many forms of lung injury and is thought to play a critical role in alveolar epithelial repair. Keratinocyte growth factor/fibroblast growth factor 7 (KGF) has been shown to promote alveolar type II cell growth in primary culture and alveolar epithelial hyperplasia in vivo. In this study, we used immunohistochemical analysis to determine the intrapulmonary distribution and cellular localization of recombinant human KGF (rhKGF) instilled into the trachea of rats. 6 h after administration, immunoreactive KGF was observed within the lung parenchyma and along alveolar epithelial cell membranes. By 18-24 h, KGF was detected intracellularly in alveolar epithelial cells and intraalveolar macrophages. Immunoreactive KGF was not demonstrable 48 h after delivery or in lung sections from PBS-treated animals. Intratracheal instillation of 5 mg/kg rhKGF stimulated a marked, time-dependent increase in the alveolar type II cell specific labeling index to a maximum level of 33 +/- 3% 48 h after rhKGF administration compared with 1.3 +/- 0.3% after PBS instillation. In addition, this increase in type II cell proliferation in vivo was documented by flow cytometric analysis of isolated type II cells which revealed a nearly fivefold increase in the proportion of cells traversing through the S and G2/M phases of the cell cycle. To test the hypothesis that KGFs effects on type II cells in vivo might affect the response to lung injury, rats were treated with rhKGF and exposed to hyperoxia. Animals that received 1 or 5 mg/kg rhKGF exhibited dramatically reduced mortality (P < 0.001, for both doses). Survival for animals treated with 0.1 mg/kg rhKGF was not significantly different from either untreated rats or animals treated with heat-denatured rhKGF. The lungs of rhKGF-treated animals that survived hyperoxia exposure had minimal hemorrhage and no exudate within the intraalveolar space. These experiments established that intratracheal administration of rhKGF stimulated alveolar type II cell proliferation in vivo and reduced hyperoxia-induced lung injury in rats. Directed delivery of KGF to the lungs may provide a therapeutic strategy to preserve or restore the alveolar epithelium during exposure to hyperoxia or other injurious agents.

Fibroblast-growth-factor receptor mutations in human skeletal disorders

Fibroblast-growth-factor receptors (FGFRs), members of the tyrosine-kinase receptor family, play a crucial role in signal transduction and development. Recently, unique mutations in three human FGFR-encoding genes (FGFR1-3) have been identified as the cause of a variety of skeletal disorders. Comparison of these specific mutations with the resulting phenotypes is now providing new insight into the role of these receptors in normal and abnormal bone development.

Regulation of vascular endothelial growth factor expression in cultured keratinocytes. Implications for normal and impaired wound healing

Recent in situ hybridization studies had demonstrated a strong increase in vascular endothelial growth factor (VEGF) mRNA expression in the hyperproliferative epithelium during wound healing. To determine potential mediators of VEGF induction during this process, we analyzed the regulation of VEGF expression in cultured human keratinocytes. We found a large induction of VEGF expression upon treatment of quiescent cells with serum, epidermal growth factor, transforming growth factor-beta 1, keratinocyte growth factor, or the proinflammatory cytokine tumor necrosis factor alpha, respectively. Since all these factors are present at the wound site during the early phase of wound healing, they might also be responsible for VEGF induction after cutaneous injury. To determine the importance of increased VEGF production for wound repair, we compared the time course of VEGF mRNA expression during wound healing of healthy control mice with the kinetics of VEGF expression during skin repair of genetically diabetic db/db mice which are characterized by impaired wound healing. In normal mice we found elevated VEGF mRNA levels during the period when granulation tissue formation occurs. In contrast, VEGF mRNA levels even declined during this period in db/db mice, suggesting that a defect in VEGF regulation might be associated with wound healing disorders.

Impaired insulin signaling in skeletal muscles from transgenic mice expressing kinase-deficient insulin receptors

Transgenic mice which overexpress kinase-deficient human insulin receptors in muscle were used to study the relationship between insulin receptor tyrosine kinase and the in vivo activation of several downstream signaling pathways. Intravenous insulin stimulated insulin receptor tyrosine kinase activity by 7-fold in control muscle versus < or = 1.5-fold in muscle from transgenic mice. Similarly, insulin failed to stimulate tyrosyl phosphorylation of receptor beta-subunits or insulin receptor substrate 1 (IRS-1) in transgenic muscle. Insulin substantially stimulated IRS-1-associated phosphatidylinositol (PI) 3-kinase in control versus absent stimulation in transgenic muscles. In contrast, insulin-like growth factor 1 modestly stimulated PI 3-kinase in both control and transgenic muscle. The effects of insulin to stimulate p42 mitogen-activated protein kinase and c-fos mRNA expression were also markedly impaired in transgenic muscle. Specific immunoprecipitation of human receptors followed by measurement of residual insulin receptors suggested the presence of hybrid mouse-human heterodimers. In contrast, negligible hybrid formation involving insulin-like growth factor 1 receptors was evident. We conclude that (i) transgenic expression of kinase-defective insulin receptors exerts dominant-negative effects at the level of receptor auto-phosphorylation and kinase activation; (ii) insulin receptor tyrosine kinase activity is required for in vivo insulin-stimulated IRS-1 phosphorylation, IRS-1-associated PI 3-kinase activation, phosphorylation of mitogen-activated protein kinase, and c-fos gene induction in skeletal muscle; (iii) hybrid receptor formation is likely to contribute to the in vivo dominant-negative effects of kinase-defective receptor expression.

Acidic fibroblast growth factor accelerates dermal wound healing in diabetic mice

Acidic fibroblast growth factor (aFGF) is a potent mitogenic and chemotactic agent for vascular endothelial cells, dermal fibroblasts, and epidermal keratinocytes, the principal cellular constituents of skin. To explore its potential to heal chronic dermal wounds, we applied pure recombinant human aFGF topically to full-thickness excisional injuries in healing-impaired genetically diabetic mice. Transformation of the nonlinear percent initial wound areas as a function of time to linear rates of tissue ingrowth from the original wound edges showed that aFGF increased wound closure in a dose-dependent manner. Optimal 3-micrograms/cm2 doses of aFGF nearly tripled the linear rate of healing. The median time to complete closure decreased from 46 d in vehicle-treated wounds to only 16 d in those treated with aFGF. Histomorphometric analyses established that aFGF increased granulation tissue formation and reepithelialization throughout healing. Vehicle- and aFGF-treated wounds appeared to be histologically equivalent by the time of closure. Therefore, aFGF has potential therapeutic applications for promoting healing of dermal ulcers, especially in healing-impaired individuals.

Keratinocyte growth factor as a cytokine that mediates mesenchymal-epithelial interaction

Keratinocyte growth factor (KGF) is a member of the heparin-binding fibroblast growth factor family (FGF-7) with a distinctive pattern of target-cell specificity. Studies performed in cell culture suggested that KGF was mitogenically active only on epithelial cells, though from a variety of tissues. In contrast, KGF was produced solely by cells of mesenchymal origin, leading to the hypothesis that it might function as a paracrine mediator of mesenchymal-epithelial communication. Biochemical analysis and molecular cloning established that the KGF receptor (KGFR) was a tyrosine kinase isoform encoded by the fgfr-2 gene. Many detailed investigations of KGF and KGFR expression in whole tissue and cell lines largely substantiated the pattern initially perceived in vitro of mesenchymal and epithelial distribution, respectively. Moreover, functional assays in organ culture and in vivo and analysis of agents regulating KGF expression reinforced the idea that KGF acts predominantly on epithelial cells. While the data do not implicate a KGF autocrine loop in neoplasia, paracrine sources of factor or ligand-independent signaling by the KGFR might contribute to malignancy. Alternatively, because of its differentiation-promoting effects, KGF may retard processes that culminate in uncontrolled cell growth.

Factors controlling growth, motility, and morphogenesis of normal and malignant epithelial cells

Factors that control epithelial growth, motility, and morphogenesis play important roles in malignancy and in normal development. Here we discuss the molecular nature and the function of two types of molecules that control the development and maintenance of epithelia: Components that regulate epithelial cell adhesion; and soluble factors and their receptors that regulate growth, motility, differentiation, and morphogenesis. In development, the establishment of epithelial cell characteristics and organization is crucially dependent on cell adhesion and the formation of functional adherens junctions. The integrity of adherens junctions is frequently disturbed late in tumor progression, and the resulting loss of epithelial characteristics correlates with the metastatic potential of carcinoma cells. Various soluble factors that induce epithelial growth, motility, or differentiation in cell culture, function via tyrosine kinase receptors. We concentrate here on receptors that are expressed exclusively or predominantly on epithelia, and on ligands that are derived from the mesenchyme. In development, these receptors and their ligands function in mesenchymal-epithelial interactions, which are known to govern growth, morphogenesis, and differentiation of epithelia. During tumor development, mutations or overexpression of the receptors are frequently observed; these alterations contribute to the development and progression of carcinomas.