Heparan sulfate proteoglycan modulates keratinocyte growth factor signaling through interaction with both ligand and receptor

Pleiotropic Effects of Heparins: From Clinical Applications to Molecular Mechanisms in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is a major health problem worldwide and most cases are incurable because of late presentation. It is the most common primary neoplasm of the liver and often arises in the context of a chronic liver disease that impairs coagulation. Portal vein thrombosis (PVT) is a common complication of HCC that is associated with a poor prognosis. Heparin derivatives are widely used in the management of venous thromboembolism (VTE). Among them low molecular weight heparin (LMWH) favorably influences the survival in patients with advanced cancer, including HCC. Due to their pleiotropic function, heparins affect tumorigenesis in many ways and may promote or hamper tumorigenic transformation depending on the cancer type and cancer stage along with their structural properties and concentration. Thus, their application as an antithrombotic along with the conventional therapy regime should be carefully planned to develop the best management strategies. In this review, we first will briefly review clinical applications of heparin derivatives in the management of cancer with a particular focus on HCC. We then summarize the state of knowledge whereby heparin can crosstalk with molecules playing a role in hepatocarcinogenesis. Lastly, we highlight new experimental and clinical research conducted with the aim of moving towards personalized therapy in cancer patients at risk of thromboembolism.

Glycan distribution and density in native skin's stratum corneum

Background

The glycosylation of proteins on the surface of corneocytes is believed to play an important role in cellular adhesion in the stratum corneum (SC) of human skin. Mapping with accuracy the localization of glycans on the surface of corneocytes through traditional methods of immunohistochemistry and electron microscopy remains a challenging task as both approaches lack enough resolution or need to be performed in high vacuum conditions.

Materials and methods

We used an advanced mode of atomic force microscope (AFM), with simultaneous topography and recognition imaging to investigate the distribution of glycans on native (no chemical preparation) stripped samples of human SC. The AFM cantilever tips were functionalized with anti‐heparan sulfate antibody and the lectin wheat germ agglutinin (WGA) which binds specifically to N‐acetyl glucosamine and sialic acid.

Results

From the recognition imaging, we observed the presence of the sulfated glycosaminoglycan, heparan sulfate, and the glycans recognized by WGA on the surface of SC corneocytes in their native state. These glycans were found associated with bead‐like domains which represent corneodesmosomes in the SC layers. Glycan density was calculated to be ~1200 molecules/μm² in lower layers of SC compared to an important decrease, (~106 molecules/μm²) closer to the surface due probably to corneodesmosome degradation.

Conclusion

Glycan spatial distribution and degradation is first observed on the surface of SC in native conditions and at high resolution. The method used can be extended to precisely localize the presence of other macromolecules on the surface of skin or other tissues where the maintenance of its native state is required.

Growth factor-dependent branching of the ureteric bud is modulated by selective 6-O sulfation of heparan sulfate

Heparan sulfate proteoglycans (HSPGs) are found in the basement membrane and at the cell-surface where they modulate the binding and activity of a variety of growth factors and other molecules. Most of the functions of HSPGs are mediated by the variable sulfated glycosaminoglycan (GAG) chains attached to a core protein. Sulfation of the GAG chain is key as evidenced by the renal agenesis phenotype in mice deficient in the HS biosynthetic enzyme, heparan sulfate 2-O sulfotransferase (Hs2st; an enzyme which catalyzes the 2-O-sulfation of uronic acids in heparan sulfate). We have recently demonstrated that this phenotype is likely due to a defect in induction of the metanephric mesenchyme (MM), which along with the ureteric bud (UB), is responsible for the mutually inductive interactions in the developing kidney (Shah et al., 2010). Here, we sought to elucidate the role of variable HS sulfation in UB branching morphogenesis, particularly the role of 6-O sulfation. Endogenous HS was localized along the length of the UB suggesting a role in limiting growth factors and other molecules to specific regions of the UB. Treatment of cultures of whole embryonic kidney with variably desulfated heparin compounds indicated a requirement of 6O-sulfation in the growth and branching of the UB. In support of this notion, branching morphogenesis of the isolated UB was found to be more sensitive to the HS 6-O sulfation modification when compared to the 2-O sulfation modification. In addition, a variety of known UB branching morphogens (i.e., pleiotrophin, heregulin, FGF1 and GDNF) were found to have a higher affinity for 6-O sulfated heparin providing additional support for the notion that this HS modification is important for robust UB branching morphogenesis. Taken together with earlier studies, these findings suggest a general mechanism for spatio-temporal HS regulation of growth factor activity along the branching UB and in the developing MM and support the view that specific growth factor-HSPG interactions establish morphogen gradients and function as developmental switches during the stages of epithelial organogenesis (Shah et al., 2004).

Heparanase 1: a key participant of inner root sheath differentiation program and hair follicle homeostasis

Heparanase is a heparan sulphate endo-glycosidase which was previously detected in the outer root sheath of murine hair follicles. Heparanase overexpression was reported to improve mouse hair (re)growth. In this study, we investigated its involvement in human hair biology. Immunofluorescence detection was used to explore heparanase distribution in both anagen and catagen hair follicles. Heparanase functionality was assessed in in vitro cultured hair follicles, in the presence of a heparanase activity inhibitor. Our results showed that heparanase expression was (i) primarily located in the inner root sheath (IRS) of human hair follicle, and there (ii) restricted to anagen phase. Furthermore, inhibition of heparanase in in vitro cultured hair follicles induced a catagen-like process. Hair shaft retreat upward was accompanied by a decrease in Ki67-positive cells, the formation of an epithelial strand as evidenced by K14 keratin expression, and the loss of IRS as assessed by transglutaminase 1 and desmoglein labelling. IRS distribution of heparanase and the induction of catagen-like involution of hair follicles when a potent heparanase inhibitor is added suggest that heparanase is a key actor of IRS differentiation and hair homeostasis.

Glypican-3-mediated oncogenesis involves the Insulin-like growth factor-signaling pathway

Glypican-3 (gpc3) is the gene responsible for Simpson-Golabi-Behmel overgrowth syndrome. Previously, we have shown that GPC3 is overexpressed in hepatocellular carcinoma (HCC). In this study, we demonstrated the mechanisms for GPC3-mediated oncogenesis. Firstly, GPC3 overexpression in NIH3T3 cells gave to cancer cell phenotypes including growing in serum-free medium and forming colonies in soft agar, or on the other way, GPC3 knockdown in HuH-7 cells decreased oncogenecity. We further demonstrated that GPC3 bound specifically through its N-terminal proline-rich region to both Insulin-like growth factor (IGF)-II and IGF-1R. GPC3 stimulated the phosphorylation of IGF-1R and the downstream signaling molecule extracellular signal-regulated kinase (ERK) in an IGF-II-dependent way. Also, GPC3 knockdown in HCC cells decreased the phosphorylation of both IGF-1R and ERK. Therefore, GPC3 confers oncogenecity through the interaction between IGF-II and its receptor, and the subsequent activation of the IGF-signaling pathway. This data are novel to the current understanding of the role of GPC3 in HCC and will be important in future developments of cancer therapy.

Inhibition of proliferation of Pseudomonas aeruginosa by KGF in an experimental burn model using human cultured keratinocytes

Experimental models showed the ability of Pseudomonas aeruginosa to interact with epidermal keratinocytes [Green H, Kehinde O, Thomas J. Growth of cultured human epidermal cells into multiple epithelia suitable for grafting. Proc Natl Acad Sci USA 1979;76(11):5665-8], stimulating these cells to produce specific peptides that start an immunological chain reaction in the epidermis [O'Connor NE, Mulliken JB, Banks-Schlegel S, Kehinde O, Green H. Grafting of burns with cultured epithelium prepared from autologous epidermal cells. Lancet 1981;(1):75-8]. The immune reaction causes the release of cytokines and growth factors. The objective of this study was to test whether the presence of keratinocyte growth factor (KGF) alters P. aeruginosa proliferation in an experimental burn model.

METHODS

Human keratinocytes derived from neonatal foreskins were isolated and cultured following standard methods [Gallico III, GG, O'Connor NE, Compton CC, Kehinde O, Green H. Permanent coverage of large burn wounds with autologous cultured human epithelium. N Engl J Med 1984;311(7):448-51]. Some of these cells were genetically modified to produce KGF, and the other cells were supplemented with different KGF concentrations in the culture media. Both groups of keratinocytes were seeded in collagen matrices and cultured to form stratified epithelia. A hot plate was used to produce burn defects. Each matrix was inoculated with luminescent P. aeruginosa strain. Experiments were made using keratinocytes without KGF, keratinocytes supplemented with different concentrations of KGF, and keratinocytes genetically modified to produce KGF. Statistical analyses were made using Wilcoxon paired test.

RESULTS

When KGF was added to P. aeruginosa in the presence of keratinocytes, bacterial growth was inhibited, and the same was observed when genetically modified keratinocytes were used.

CONCLUSION

Many studies have been done on KGF, where its known properties were defined as a mitogen for keratinocytes [Munster AM. Cultured skin for massive burns: a prospective, controlled trial. Ann Surg 1996;224(3):372-7]. This method allows for a qualitative and quantitative evaluation in real time of the bacterial growth in wound sites after bacterial inoculation. KGF was involved in the reduction of bacterial viability. However, as KGF alone did not produce any effect on P. aeruginosa, it seems to modulate the skin innate immune response.

Ectopic expression of syndecan-1 in basal epidermis affects keratinocyte proliferation and wound re-epithelialization

Epidermal proliferation and differentiation can be regulated by soluble morphogens and growth factors. Heparan sulfate proteoglycans (HSPGs) modulate the action of several of these effector molecules, such as members of the fibroblast growth factor (FGF) and Wnt families. Syndecan-1 is a cell-surface proteoglycan that is expressed in differentiating keratinocytes and transiently upregulated in all layers of the epidermis upon tissue injury. To address the role of syndecan-1 in the regulation of keratinocyte proliferation and differentiation, we generated transgenic mice that overexpress syndecan-1 under K14 keratin promoter in the basal layer of the epidermis. We observed epidermal hyperproliferation in newborn transgenic mice, as evidenced by increased number of suprabasal cell layers, elevated proliferating cell nuclear antigen (PCNA) expression in both basal and suprabasal cell layers and by expression of keratin 6 in the interfollicular epidermis. Compared to both wild-type and syndecan-1-null animals, the transgene expression interfered with skin wound healing in adult mice by decreasing cell proliferation in the re-epithelialized epidermis. Thus, syndecan-1 regulates keratinocyte proliferation differently during skin development and in healing wounds.

A protective role for keratinocyte growth factor in a murine model of chemotherapy and radiotherapy-induced mucositis

PURPOSE

To evaluate the activity of palifermin (rHuKGF) in a murine model of mucosal damage induced by a radiotherapy/chemotherapy (RT/CT) regimen mimicking treatment protocols used in head-and-neck cancer patients.

METHODS AND MATERIALS

A model of mucosal damage induced by RT/CT was established by injecting female BDF1 mice with cisplatin (10 mg/kg) on Day 1; 5-fluorouracil (40 mg/kg/day) on Days 1-4, and irradiation (5 Gy/day) to the head and neck on Days 1-5. Palifermin was administered subcutaneously on Days -2 to 0 (5 mg/kg/day) and on Day 5 (5 mg/kg). Evaluations included body weight, organ weight, keratinocyte growth factor receptor expression, epithelial thickness, and cellular proliferation.

RESULTS

Initiation of the radiochemotherapeutic regimen resulted in a reduction in body weight in control animals. Palifermin administration suppressed weight loss and resulted in increased organ weight (salivary glands and small intestine), epithelial thickness (esophagus and tongue), and cellular proliferation (tongue and salivary glands).

CONCLUSIONS

Administration of palifermin before RT/CT promotes cell proliferation and increases in epithelial thickness in the oral mucosa, salivary glands, and digestive tract. Palifermin administration before and after RT/CT mitigates weight loss and a trophic effect on the intestinal mucosa and salivary glands, suggesting that palifermin use should be investigated further in the RT/CT settings, in which intestinal mucositis and salivary gland dysfunction are predominant side effects of cytotoxic therapy.

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

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

Heparin and fibroblast growth factors affect surfactant protein gene expression in type II cells

The stimulation and maintenance of the pulmonary alveolar type II cell's capacity to biosynthesize, store, and secrete surfactant proteins (SPs) are modulated to a great extent by growth factors, extracellular matrix (ECM) components, and hormones. It is possible that differences in ECM composition, as exist between type I and II cells normally or as might occur with excessive cell surface shedding during inflammation or injury states, may specifically alter SP expression. Here, isolated type II cells were exposed to the model sulfated ECM heparin; desulfated heparin; and/or fibroblast growth factor (FGF)-1, -2, or -7 for 24 h to examine by quantitative real-time polymerase chain reaction their effects on SP gene expression. Aquaporin 5 (AQP-5) gene expression was also examined as a phenotypic marker for the type I cell. SP-B mRNA abundance was increased 4- to 8-fold by all three FGFs. Heparin at low concentrations (5 microg/ml) or desulfated heparin at high concentrations (500 microg/ml) enhanced the effects of FGF-2 and -7, while high heparin concentrations (500 microg/ml) were inhibitory. In contrast, SP-B mRNA abundance was increased by heparin in a dose- and sulfation-dependent manner when used in combination with FGF-1. SP-C and AQP-5 mRNA levels were increased by heparin alone in a dose- and sulfation-dependent manner, while all FGFs lacked effect on SP-C or AQP-5 mRNA levels. These data indicate that heparin can be stimulatory to SP gene expression depending on concentration, degree of sulfation, and surrounding FGF environment, and that heparin plays a significant role in modulating alveolar epithelial cell phenotype in vitro.

Control of fibroblast growth factor (FGF) 7- and FGF1-induced mitogenesis and downstream signaling by distinct heparin octasaccharide motifs

Variation in length, disaccharide composition, and sulfation of heparan sulfate (HS) affects fibroblast growth factor (FGF) signaling. However, it is unclear whether the specific distribution of groups within oligosaccharides or random variations in charge density underlies the effects. Recently we showed that a mixture of undersulfated octasaccharides exhibiting 7 and 8 sulfates (7,8-S-OctaF7) generated from heparin had the highest affinity for FGF7 monitored by salt resistance (>0.60 M salt) of octasaccharide-FGF7 complexes. 7,8-S-OctaF7 also had the highest specific activity for formation of a complex with dimeric FGFR2IIIb competent to bind FGF7. Here we show that when endogenous HS was inhibited by chlorate treatment, 7,8-S-OctaF7 specifically supported FGF7-stimulated DNA synthesis and downstream signaling in FGFR2IIIb-expressing mouse keratinocytes. It failed to support FGF1 signaling in both HS-deficient mouse keratinocytes and 3T3 fibroblasts. In contrast, abundant, more highly sulfated and heterogenous mixtures of octasaccharides with lower affinity (0.30-0.60 M salt) for FGF7 supported FGF1-induced signaling in both cell types. In contrast to the two-component 7,8-S-OctaF7 mixture from FGF7, the high affinity octasaccharide fraction from FGF1 was a heterogeneous mixture with components ranging from 8 to 12 sulfates with 11-S-octasaccharides the most abundant. The high affinity fraction exhibited similar properties to the lower affinity fractions from both FGF1 and FGF7. Octasaccharide mixtures eluting from FGF1 between 0.30 and 0.60 M and above 0.60 M salt were nearly equal in support of FGF1 signaling in fibroblasts and keratinocytes. Both were deficient in support of FGF7-induced signaling in keratinocytes. The results show that both variations in overall charge density and specific distribution of charged groups within HS motifs exhibit FGF-specific control over formation of FGF-HS-FGFR complexes and downstream signaling.

Human papillomaviruses bind a basal extracellular matrix component secreted by keratinocytes which is distinct from a membrane-associated receptor

Human papillomaviruses (HPVs) have previously been shown to adsorb to cultured cells via membrane-associated heparan sulfate (HS) and alpha6 integrin. We demonstrate that cultured keratinocytes uniquely secrete a component into the basal extracellular matrix (ECM) which can function to adsorb HPV particles which can then be internalized by adherent cells. This uncharacterized basal ECM adsorption receptor was secreted by normal human epidermal keratinocytes (NHEK) and by each of the four keratinocyte-derived cell lines we examined, but not by non-keratinocyte cell lines. Multiple HPV types bound preferentially to this keratinocyte-specific receptor over the membrane-associated receptor, and binding to the basal ECM adsorption receptor was refractory to inhibition by heparin. Like the membrane-associated receptor, this basal ECM component was functional as an adsorption receptor in our in vitro infection model using HPV-11. Unlike particle adsorption, however, successful infection with HPV-11 virions remained sensitive to the pretreatment of virions with heparin. The secreted basal ECM receptor did not colocalize with antibodies against HS, perlecan, or alpha6 integrin, but colocalized with antibody against laminin-5, a marker of keratinocyte ECM and an abundant component of the basement membrane in mucosa and skin. These findings suggest a model for natural infections in which HPV virions, nonspecifically adsorbed to HS on suprabasal keratinocytes throughout an epithelial wound, might be transferred to mitotically active migrating keratinocytes via an intermediate association with the ECM secreted by these cells as they reestablish the basement membrane.

Keratinocyte growth factor/fibroblast growth factor 7, a homeostatic factor with therapeutic potential for epithelial protection and repair

Keratinocyte growth factor (KGF) is a paracrine-acting, epithelial mitogen produced by cells of mesenchymal origin. It is a member of the fibroblast growth factor (FGF) family, and acts exclusively through a subset of FGF receptor isoforms (FGFR2b) expressed predominantly by epithelial cells. The upregulation of KGF after epithelial injury suggested it had an important role in tissue repair. This hypothesis was reinforced by evidence that intestinal damage was worse and healing impaired in KGF null mice. Preclinical data from several animal models demonstrated that recombinant human KGF could enhance the regenerative capacity of epithelial tissues and protect them from a variety of toxic exposures. These beneficial effects are attributed to multiple mechanisms that collectively act to strengthen the integrity of the epithelial barrier, and include the stimulation of cell proliferation, migration, differentiation, survival, DNA repair, and induction of enzymes involved in the detoxification of reactive oxygen species. KGF is currently being evaluated in clinical trials to test its ability to ameliorate severe oral mucositis (OM) that results from cancer chemoradiotherapy. In a phase 3 trial involving patients who were treated with myeloablative chemoradiotherapy before autologous peripheral blood progenitor cell transplantation for hematologic malignancies, KGF significantly reduced both the incidence and duration of severe OM. Similar investigations are underway in patients being treated for solid tumors. On the basis of its success in ameliorating chemoradiotherapy-induced OM in humans and tissue damage in a variety of animal models, additional clinical applications of KGF are worthy of investigation.

Low doses and high doses of heparin have different effects on osteoblast-like Saos-2 cells in vitro

Long-term treatment with heparin has been associated with an increased risk of osteoporosis. Given the importance of heparan sulfate proteoglycans for bone metabolism, it can be anticipated that heparin due to its structural similarity with heparan sulfate chains somehow interferes with the biological activities of these cell surface- and extracellular matrix-associated molecules. Initially in order to study the effect(s) of heparin on osteoblasts that possibly contribute to the development of heparin-induced osteoporosis, we treated osteoblast-like Saos-2 cells in monolayer culture for different periods of time with different concentrations of heparin. None of the heparin concentrations tested led to an inhibition of osteoblast proliferation during the early proliferative phase. After longer incubation times, however, cultures treated with higher concentrations of heparin (>/=5 microg/ml) exhibited a reduction in cell number as well as an inhibition of matrix deposition and mineralization. These effects could not be observed with lower heparin concentrations. On the contrary, low concentrations of heparin (5-500 ng/ml) even promoted matrix deposition and its subsequent mineralization. Apparently, heparin has a biphasic effect on osteoblast-like Saos-2 cells, being inhibitory at high concentrations but stimulatory at low concentrations. These results imply that heparin at concentrations well below those used for antithrombotic therapy might eventually turn out to be beneficial for bone formation.

Molecular control of epithelial-mesenchymal interactions during hair follicle cycling

Epithelial-mesenchymal interactions play pivotal roles in the morphogenesis of many organs and various types of appendages. During hair follicle development, extensive interactions between two embryologically different hair follicle compartments (epidermal keratinocytes and dermal papilla fibroblasts) lead to the formation of the hair shaft-producing mini-organ that shows cyclic activity during postnatal life with periods of active growth, involution and resting. During the hair cycle, the epithelium and the mesenchyme are regulated by a distinct set of molecular signals that are unique for every distinct phase of the hair cycle. In telogen hair follicles, epithelial-mesenchymal interactions are characterized by a predominance of inhibitory signals that retain the hair follicle in a quiescent state. During anagen, a large variety of growth stimulatory pathways are activated in the epithelium and in the mesenchyme, the coordination of which are essential for proper hair fiber formation. During catagen, the termination of anagen-specific signaling interactions between the epithelium and the mesenchyme leads to apoptosis in the hair follicle epithelium, while activation of selected signaling pathways promotes the transition of the dermal papilla into a quiescent state. The signaling exchange between the follicular epithelium and the mesenchyme is modulated by proteoglycans, such as versican, which may significantly enhance or reduce the biological activities of secreted growth stimulators. However, additional research will be required to bridge the gap between our current understanding of mechanisms underlying epithelial-mesenchymal interactions in hair follicles and the potential clinical application of growth modulators involved in those interactions. Further progress in this area of research will hopefully lead to the development of new drugs for the treatment of hair growth disorders.

Establishment and characterisation of human papillomavirus type 16 DNA immortalised human tonsillar epithelial cell lines

Oncogenic human papillomavirus (HPV) plays a possible aetiological role in a subset of head and neck cancers, particularly in tonsillar carcinomas. For establishing a model to study mechanisms involved in HPV-associated tonsillar carcinogenesis, normal human tonsillar epithelial (HTE) cells were transfected with full-length HPV-16 DNA. The transfections produced four immortalised cell lines, designated HTE-114/K1, HTE-114/K2, HTE-114/K3 and HTE-114/B. All transfected HTE cell lines were cytogenetically abnormal. They exhibited altered morphology and impaired expression of cytokeratins in organotypic cultures. They failed to form colonies in soft agarose and formed no tumours in nude mice within 6 months. Each of them contained integrated viral DNA in a distinctive pattern as shown by Southern blot hybridisation. Early viral transcripts containing the E7 gene were detected by northern blot hybridisation. In conclusion, primary HTE cells can be immortalised following transfection with full-length HPV-16 DNA; the immortalised cell lines had partially retained epithelial characteristics in their morphology and function. They seem to represent early stages of premalignant epithelial cells and thus provide a useful model for studying further the multistep molecular events of HPV-16-associated tonsillar carcinogenesis.

Dermatan sulfate binds and potentiates activity of keratinocyte growth factor (FGF-7)

FGF-7 is induced after injury and induces the proliferation of keratinocytes. Like most members of the FGF family, the activity of FGF-7 is strongly influenced by binding to heparin, but this glycosaminoglycan is absent on keratinocyte cell surfaces and minimally present in the wound environment. In this investigation we compared the relative activity of heparan sulfate and chondroitin sulfate B (dermatan sulfate), glycosaminoglycans that are present in wounds. A lymphoid cell line (BaF/KGFR) containing the FGF-7 receptor (FGFR2 IIIb) was treated with FGF-7 and with various glycosaminoglycans. FGF-7 did not support cell proliferation in the absence of glycosaminoglycan or with addition of heparan sulfate or chondroitin sulfate A/C but did stimulate BaF/KGFR division in the presence of dermatan sulfate or highly sulfated low molecular weight fractions of dermatan. Dermatan sulfate also enabled FGF-7-dependent phosphorylation of mitogen-activated protein kinase and promoted binding of radiolabeled FGF-7 to FGFR2 IIIb. In addition, dermatan sulfate and FGF-7 stimulated growth of normal keratinocytes in culture. Thus, dermatan sulfate, the predominant glycosaminoglycan in skin, is the principle cofactor for FGF-7.

Fibroblast growth factor receptors 1 and 2 interact differently with heparin/heparan sulfate. Implications for dynamic assembly of a ternary signaling complex

Heparan sulfate (HS) regulates the kinetics of fibroblast growth factor 2 (FGF2)-stimulated intracellular signaling and differentially activates cell proliferation of cells expressing different FGF receptors (FGFRs). Evidence suggests that HS interacts with both FGFs and FGFRs to form active ternary signaling complexes. Here we compare the interactions of two FGFRs with HS. We show that the ectodomains of FGFR1 IIIc and FGFR2 IIIc exhibit specific interactions with different characteristics for both heparin and porcine mucosal HS. These glycans are both known to activate FGF signaling via these receptors. FGFR2 interacts with a higher apparent affinity than FGFR1 despite both involving 6-O-, 2-O-, and N-sulfates. FGFR1 and FGFR2 bind heparin with mean association rate constants of 1.9 x 10(5) and 2.1 x 10(6) m(-1)s(-1), respectively, and dissociation rate constants of 1.2 x 10(-2) and 2.7 x 10(-2) s(-1), respectively. These produced calculated affinities of 63 and 13 nm, respectively. Hence, FGFR1 and FGFR2 bind to heparin chains with markedly different kinetics and affinities. We propose a mechanistic model where the kinetic parameters of the HS/FGFR interaction are a key element regulating the formation of ternary complexes and the resulting FGF signaling outcomes.

Molecular signaling in bioengineered tissue microenvironments

Biological tissues and organs consist of specialized living cells arrayed within a complex structural and functional framework known generally as the extracellular matrix (ECM). The great diversity observed in the morphology and composition of the ECM contributes enormously to the properties and function of each organ and tissue. For example, the ECM contributes to the rigidity and tensile strength of bone, the resilience of cartilage, the flexibility and hydrostatic strength of blood vessels, and the elasticity of skin. The ECM is also important during growth, development, and wound repair: its own dynamic composition acts as a reservoir for soluble signaling molecules and mediates signals from other sources to migrating, proliferating, and differentiating cells. Artificial three-dimensional substitutes for ECM, called tissue scaffolds, may consist of natural or synthetic polymers or a combination of both. Scaffolds have been used successfully alone and in combination with cells and soluble factors to induce tissue formation or promote tissue repair. Appropriate numbers of properly functioning living cells are central to many tissue-engineering strategies, and significant efforts have been made to identify and propagate pluripotent stem cells and lineage-restricted progenitor cells. The study of these and other living cells in artificial microenvironments, in turn, has led to the identification of signaling events important for their controlled proliferation, proper differentiation, and optimal function.

Keratinocyte and hepatocyte growth factors in the lung: roles in lung development, inflammation, and repair

A growing body of evidence indicates that the epithelial-specific growth factors keratinocyte growth factor (KGF), fibroblast growth factor (FGF)-10, and hepatocyte growth factor (HGF) play important roles in lung development, lung inflammation, and repair. The therapeutic potential of these growth factors in lung disease has yet to be fully explored. KGF has been best studied and has impressive protective effects against a wide variety of injurious stimuli when given as a pretreatment in animal models. Whether this protective effect could translate to a treatment effect in humans with acute lung injury needs to be investigated. FGF-10 and HGF may also have therapeutic potential, but more extensive studies in animal models are needed. Because HGF lacks true epithelial specificity, it may have less potential than KGF and FGF-10 as a targeted therapy to facilitate lung epithelial repair. Regardless of their therapeutic potential, studies of the unique roles played by these growth factors in the pathogenesis and the resolution of acute lung injury and other lung diseases will continue to enhance our understanding of the complex pathophysiology of inflammation and repair in the lung.

Not all perlecans are created equal: interactions with fibroblast growth factor (FGF) 2 and FGF receptors

Human basement membrane heparan sulfate proteoglycan (HSPG) perlecan binds and activates fibroblast growth factor (FGF)-2 through its heparan sulfate (HS) chains. Here we show that perlecans immunopurified from three cellular sources possess different HS structures and subsequently different FGF-2 binding and activating capabilities. Perlecan isolated from human umbilical arterial endothelial cells (HUAEC) and a continuous endothelial cell line (C11 STH) bound similar amounts of FGF-2 either alone or complexed with FGFRalpha1-IIIc or FGFR3alpha-IIIc. Both perlecans stimulated the growth of BaF3 cell lines expressing FGFR1b/c; however, only HUAEC perlecan stimulated those cells expressing FGFR3c, suggesting that the source of perlecan confers FGF and FGFR binding specificity. Despite these differences in FGF-2 activation, the level of 2-O- and 6-O-sulfation was similar for both perlecans. Interestingly, perlecan isolated from a colon carcinoma cell line that was capable of binding FGF-2 was incapable of activating any BaF3 cell line unless the HS was removed from the protein core. The HS chains also exhibited greater bioactivity after digestion with heparinase III. Collectively, these data clearly demonstrate that the bioactivity of HS decorating a single PG is dependent on its cell source and that subtle changes in structure including secondary interactions have a profound effect on biological activity.

Differential effects of heparin saccharides on the formation of specific fibroblast growth factor (FGF) and FGF receptor complexes

Heparan sulfates (HS) play an important role in the control of cell growth and differentiation by virtue of their ability to modulate the activities of heparin-binding growth factors, an issue that is particularly well studied for fibroblast growth factors (FGFs). HS/heparin co-ordinate the interaction of FGFs with their receptors (FGFRs) and are thought to play a critical role in receptor dimerization. Biochemical and crystallographic studies, conducted mainly with FGF-2 or FGF-1 and FGF receptors 1 and 2, suggests that an octasaccharide is the minimal length required for FGF- and FGFR-induced dimerization and subsequent activation. In addition, 6-O-sulfate groups are thought to be essential for binding of HS to FGFR and for receptor dimerization. We show here that oligosaccharides shorter than 8 sugar units support activation of FGFR2 IIIb by FGF-1 and interaction of FGFR4 with FGF-1. In contrast, only relatively long oligosaccharides supported receptor binding and activation in the FGF-1.FGFR1 or FGF-7.FGFR2 IIIb setting. In addition, both 6-O- and 2-O-desulfated heparin activated FGF-1 signaling via FGFR2 IIIb, whereas neither one stimulated FGF-1 signaling via FGFR1 or FGF-7 via FGFR2 IIIb. These findings indicate that the structure of HS required for activating FGFs is dictated by the specific FGF and FGFR combination. These different requirements may reflect the differences in the mode by which a given FGFR interacts with the various FGFs.

Purification and characterization of the endoglycosidase heparanase 1 from human plantar stratum corneum: a key enzyme in epidermal physiology?

A protein exhibiting endoglycosidase activity was purified from plantar stratum corneum to apparent homogeneity in two sequential column chromatographic steps. Protein sequencing revealed its identity with the recently cloned human heparanase 1, an enzyme, the expression of which is reported to be related to the metastasic potential of tumor cells. By using a heparanase 1 specific antibody we were able to demonstrate that, in the plantar stratum corneum, heparanase 1 exists in two forms, the active 50 kDa protein and the inactive 63 kDa form, probably a proform of the enzyme. The antibody also decorated numerous degradation fragments. Reverse transcription polymerase chain reaction studies as well as immunohistochemical analysis using reconstructed and normal human epidermis demonstrated clearly a keratinocyte differentiation related expression of heparanase 1. Interestingly, the antibody also strongly decorated dendritic cells, which after double labeling could be identified to be a subpopulation of the epidermal Langerhans cells. Based on our findings and the known history of this enzyme, we advanced the hypothesis that heparanase 1 has multiple physiologic functions in the epidermis: (i) it plays an important role in epidermal differentiation, possibly by modulating the liberation of heparan sulfate bound (growth) factors; (ii) in the stratum corneum, the endoglycosidase activity of heparanase 1 might be indispensable and represent the first step in the desquamation process; and (iii) in Langerhans cells, its catalytic activity is required for the trans-tissue migration of these cells.

A role for the perlecan protein core in the activation of the keratinocyte growth factor receptor

Perlecan, a widespread heparan sulphate (HS) proteoglycan, is directly involved in the storing of angiogenic growth factors, mostly members of the fibroblast growth factor (FGF) gene family. We have previously shown that antisense targeting of the perlecan gene causes a reduced growth and responsiveness to FGF7 [also known as keratinocyte growth factor (KGF)] in human cancer cells, and that the perlecan protein core interacts specifically with FGF7. In the present paper, we have investigated human colon carcinoma cells in which the perlecan gene was disrupted by targeted homologous recombination. After screening over 1000 clones, we obtained two clones heterozygous for the null mutation with no detectable perlecan, indicating that the other allele was non-functioning. The perlecan-deficient cells grew more slowly, did not respond to FGF7 with or without the addition of heparin, and were less tumorigenic than control cells. Paradoxically, the perlecan-deficient cells displayed increased FGF7 surface binding. However, the perlecan protein core was required for functional activation of the KGF receptor and downstream signalling. Because heparin could not substitute for perlecan, the HS chains are not critical for FGF7-mediated signalling in this cell system. These results provide the first genetic evidence that the perlecan protein core is a molecular entity implicated in FGF7 binding and activation of its receptor.

Dissociation of heparan sulfate and receptor binding domains of hepatocyte growth factor reveals that heparan sulfate-c-met interaction facilitates signaling

Hepatocyte growth factor (HGF) is a secreted, heparan sulfate (HS) glycosaminoglycan-binding protein that stimulates mitogenesis, motogenesis, and morphogenesis in a wide array of cellular targets, including hepatocytes and other epithelial cells, melanocytes, endothelial cells, and hematopoietic cells. NK1 is an alternative HGF isoform that consists of the N-terminal (N) and first kringle (K1) domains of full-length HGF and stimulates all major HGF biological activities. Within NK1, the N domain retains the HS binding properties of full-length HGF and mediates HS-stimulated ligand oligomerization but lacks significant mitogenic or motogenic activity. In contrast, K1 does not bind HS, but it stimulates receptor and mitogen-activated protein kinase activation, mitogenesis, and motogenesis, demonstrating that structurally distinct and dissociable domains of HGF are the primary mediators of HS binding and receptor activation. Despite the absence of HS-K1 binding, K1 mitogenic activity in HS-negative cells is strictly dependent on added soluble heparin, whereas K1-stimulated motility is not. We also found that, like the receptors for fibroblast growth factors, the HGF receptor c-Met binds tightly to HS. These data suggest that HS can facilitate HGF signaling through interaction with c-Met that is independent of HGF-HS interaction and that the recruitment of specific intracellular effectors that mediate distinct HGF responses such as mitogenesis and motility is regulated by HS-c-Met interaction at the cell surface.

HGF- and KGF-induced activation of PI-3K/p70 s6 kinase pathway in corneal epithelial cells: its relevance in wound healing

In this study we have investigated the involvement of PI-3K and its downstream target p70 S6K in the signaling response of corneal epithelial cells after HGF and KGF stimulation. HGF induced three- to five-fold increase in PI-3K activity in 5-10 min, whereas KGF stimulation resulted in two- to three-fold increase in activity in 2-10 min. Both growth factors also caused the phosphorylation of p70 S6K and stimulation of its activity. HGF increased p70 S6K activity by 300% and KGF by about 200%. Protein kinase C (PKC) activator TPA also induced the phosphorylation of p70 S6K. Both the PI-3K inhibitor wortmannin and PKC inhibitor calphostin C blocked the phosphorylation of p70 S6K mediated by the growth factors. However, the mitogen-activated protein kinase (p42/44 MAPK) cascade inhibitor PD98059 had no effect on p70 S6K activation. Furthermore, HGF and KGF increased the rate of corneal epithelial wound healing in an organ culture model, and wortmannin and rapamycin (the p70 S6K inhibitor) blocked corneal epithelial wound healing promoted by the growth factors. These studies suggest that PI-3K and p70 S6K are important signal transducers in the stimulation of corneal epithelial cells by HGF and KGF. PKC is involved in the PI-3K-dependent activation of p70 S6K but not MAPK. Inhibition of wound closure by PI-3K and p70 S6K inhibitors suggests these enzymes play a significant role in corneal wound repair stimulated by HGF and KGF.

Binding of heparin/heparan sulfate to fibroblast growth factor receptor 4

Fibroblast growth factors (FGFs) are heparin-binding polypeptides that affect the growth, differentiation, and migration of many cell types. FGFs signal by binding and activating cell surface FGF receptors (FGFRs) with intracellular tyrosine kinase domains. The signaling involves ligand-induced receptor dimerization and autophosphorylation, followed by downstream transfer of the signal. The sulfated glycosaminoglycans heparin and heparan sulfate bind both FGFs and FGFRs and enhance FGF signaling by mediating complex formation between the growth factor and receptor components. Whereas the heparin/heparan sulfate structures involved in FGF binding have been studied in some detail, little information has been available on saccharide structures mediating binding to FGFRs. We have performed structural characterization of heparin/heparan sulfate oligosaccharides with affinity toward FGFR4. The binding of heparin oligosaccharides to FGFR4 increased with increasing fragment length, the minimal binding domains being contained within eight monosaccharide units. The FGFR4-binding saccharide domains contained both 2-O-sulfated iduronic acid and 6-O-sulfated N-sulfoglucosamine residues, as shown by experiments with selectively desulfated heparin, compositional disaccharide analysis, and a novel exoenzyme-based sequence analysis of heparan sulfate oligosaccharides. Structurally distinct heparan sulfate octasaccharides differed in binding to FGFR4. Sequence analysis suggested that the affinity of the interaction depended on the number of 6-O-sulfate groups but not on their precise location.

Ligand discrimination by ErbB receptors: differential signaling through differential phosphorylation site usage

The four members of the ErbB family of receptor tyrosine kinases (RTKs) mediate a variety of cellular responses to epidermal growth factor (EGF)-like growth factors, and serve as a model for the generation of both diversity and specificity in RTK signaling. Previous studies indicate that receptor-receptor interactions figure prominently in signaling through ErbB receptors. In addition to a role in receptor kinase activation, ligand-induced ErbB receptor homo- and heterodimerization is thought to account for the diversity of biological responses stimulated by EGF-like growth factors. Since each receptor has the potential to couple to different complements of signaling pathways, EGF-like ligands specify cellular response by dictating which pairs of receptors become activated. More recently evidence has been uncovered for ligand discrimination by individual ErbB receptor dimers; receptors appear to realize which ligand is binding and differentially respond through autophosphorylation site usage. These observations indicate that ligand stimulation of RTKs is not generic, and point to another layer in the ErbB signal diversification mechanism. The mechanistic implications of ligand discrimination are discussed.

A ligand-receptor signaling threshold model of stem cell differentiation control: a biologically conserved mechanism applicable to hematopoiesis

A major limitation to the widespread use of hematopoietic stem cells (HSC) is the relatively crude level of our knowledge of how to maintain these cells in vitro without loss of the long-term multilineage growth and differentiation properties required for their clinical utility. An experimental and theoretical framework for predicting and controlling the outcome of HSC stimulation by exogenous cytokines would thus be useful. An emerging theme from recent HSC expansion studies is that a net gain in HSC numbers requires the maintenance of critical signaling ligand(s) above a threshold level. These ligand-receptor complex thresholds can be maintained, for example, by high concentrations of soluble cytokines or by extracellular matrix- or cell-bound cytokine presentation. According to such a model, when the relevant ligand-receptor interaction falls below a critical level, the probability of a differentiation response is increased; otherwise, self-renewal is favored. Thus, in addition to the identity of a particular receptor-ligand interaction being important to the regulation of stem cell responses, the quantitative nature of this interaction, as well as the dynamics of receptor expression, internalization, and signaling, may have a significant influence on stem cell fate decisions. This review uses examples from hematopoiesis and other tissue systems to examine existing evidence for a role of receptor activation thresholds in regulating hematopoietic stem cell self-renewal versus differentiation events.

A ligand-receptor signaling threshold model of stem cell differentiation control: a biologically conserved mechanism applicable to hematopoiesis

A major limitation to the widespread use of hematopoietic stem cells (HSC) is the relatively crude level of our knowledge of how to maintain these cells in vitro without loss of the long-term multilineage growth and differentiation properties required for their clinical utility. An experimental and theoretical framework for predicting and controlling the outcome of HSC stimulation by exogenous cytokines would thus be useful. An emerging theme from recent HSC expansion studies is that a net gain in HSC numbers requires the maintenance of critical signaling ligand(s) above a threshold level. These ligand-receptor complex thresholds can be maintained, for example, by high concentrations of soluble cytokines or by extracellular matrix- or cell-bound cytokine presentation. According to such a model, when the relevant ligand-receptor interaction falls below a critical level, the probability of a differentiation response is increased; otherwise, self-renewal is favored. Thus, in addition to the identity of a particular receptor-ligand interaction being important to the regulation of stem cell responses, the quantitative nature of this interaction, as well as the dynamics of receptor expression, internalization, and signaling, may have a significant influence on stem cell fate decisions. This review uses examples from hematopoiesis and other tissue systems to examine existing evidence for a role of receptor activation thresholds in regulating hematopoietic stem cell self-renewal versus differentiation events.

Similarities and differences between the effects of heparin and glypican-1 on the bioactivity of acidic fibroblast growth factor and the keratinocyte growth factor

The keratinocyte growth factor (KGF or FGF-7) is unique among its family members both in its target cell specificity and its inhibition by the addition of heparin and the native heparan-sulfate proteoglycan (HSPG), glypican-1 in cells expressing endogenous HSPGs. FGF-1, which binds the FGF-7 receptor with a similar affinity as FGF-7, is stimulated by both molecules. In the present study, we investigated the modulation of FGF-7 activities by heparin and glypican-1 in HS-free background utilizing either HS-deficient cells expressing the FGF-7 receptor (designated BaF/KGFR cells) or soluble extracellular domain of the receptor. At physiological concentrations of FGF-7, heparin was required for high affinity receptor binding and for signaling in BaF/KGFR cells. In contrast, binding of FGF-7 to the soluble form of the receptor did not require heparin. However, high concentrations of heparin inhibited the binding of FGF-7 to both the cell surface and the soluble receptor, similar to the reported effect of heparin in cells expressing endogenous HSPGs. The difference in heparin dependence for high affinity interaction between the cell surface and soluble receptor may be due to other molecule(s) present on cell surfaces. Glypican-1 differed from heparin in that it stimulated FGF-1 but not FGF-7 activities in BaF/KGFR cells. Glypican-1 abrogated the stimulatory effect of heparin, and heparin reversed the inhibitory effect of glypican-1, indicating that this HSPG inhibits FGF-7 activities by acting, most likely, as a competitive inhibitor of stimulatory HSPG species for FGF-7. The regulatory effect of glypican-1 is mediated at the level of interaction with the growth factor as glypican-1 did not bind the KGFR. The effect of heparin and glypican-1 on FGF-1 and FGF-7 oligomerization was studied employing high and physiological concentrations of growth factors. We did not find a correlation between the effects of these glycosaminoglycans on FGFs biological activity and oligomerization. Altogether, our findings argue against the heparin-linked dimer presentation model as key in FGFR activation, and support the notion that HSPGs primarily affect high affinity interaction of FGFs with their receptors.