Malgorzata Zakrzewska

FGF-1: From Biology Through Engineering to Potential Medical Applications

Human fibroblast growth factor 1 (FGF-1) is one of the best characterized members of the FGF superfamily. FGF-1 is a powerful mitogen exhibiting strong action on numerous different cell types. It plays a role in various stages of development and morphogenesis, as well as in angiogenesis and wound healing processes. Engineering of FGFs can bring many advantages. Design and construction of different mutants can contribute to a better understanding of the mechanism of action of protein growth factors. Moreover, application of FGFs as recombinant polypeptides in the treatment of wound and fracture healing, cardiovascular diseases and neurodegenerative diseases seems to be a rational medical approach. However, low thermal stability and high sensitivity to proteases limit the potential pharmaceutical use of wild-type FGFs. Thus, advanced protein design techniques and recombinant protein production can help to obtain new variants of FGFs with radically increased thermodynamic stability, prolonged half-life and improved proteolytic resistance. Such studies can provide a good starting point to convert short-lived and/or sensitive growth factors to effective therapeutic proteins.

Increased protein stability of FGF1 can compensate for its reduced affinity for heparin

Human FGF1 (fibroblast growth factor 1) is a powerful signaling molecule with a short half-life in vivo and a denaturation temperature close to physiological. Binding to heparin increases the stability of FGF1 and is believed to be important in the formation of FGF1·fibroblast growth factor receptor (FGFR) active complex. In order to reveal the function of heparin in FGF1·FGFR complex formation and signaling, we constructed several FGF1 variants with reduced affinity for heparin and with diverse stability. We determined their biophysical properties and biological activities as well as their ability to translocate across cellular membranes. Our study showed that increased thermodynamic stability of FGF1 nicely compensates for decreased binding of heparin in FGFR activation, induction of DNA synthesis, and cell proliferation. By stepwise introduction of stabilizing mutations into the K118E (K132E) FGF1 variant that shows reduced affinity for heparin and is inactive in stimulation of DNA synthesis, we were able to restore the full mitogenic activity of this mutant. Our results indicate that the main role of heparin in FGF-induced signaling is to protect this naturally unstable protein against heat and/or proteolytic degradation and that heparin is not essential for a direct FGF1-FGFR interaction and receptor activation.

Phosphorylation of Fibroblast Growth Factor (FGF) Receptor 1 at Ser777 by p38 Mitogen-Activated Protein Kinase Regulates Translocation of Exogenous FGF1 to the Cytosol and Nucleus▿

ABSTRACT Exogenous fibroblast growth factor 1 (FGF1) signals through activation of transmembrane FGF receptors (FGFRs) but may also regulate cellular processes after translocation to the cytosol and nucleus of target cells. Translocation of FGF1 occurs across the limiting membrane of intracellular vesicles and is a regulated process that depends on the C-terminal tail of the FGFR. Here, we report that translocation of FGF1 requires activity of the α isoform of p38 mitogen-activated protein kinase (MAPK). FGF1 translocation was inhibited after chemical inhibition of p38 MAPK or after small interfering RNA knockdown of p38α. Translocation was increased after stimulation of p38 MAPK with anisomycin, mannitol, or H2O2. The activity level of p38 MAPK was not found to affect endocytosis or intracellular sorting of FGF1/FGFR1. Instead, we found that p38 MAPK regulates FGF1 translocation by phosphorylation of FGFR1 at Ser777. The FGFR1 mutation S777A abolished FGF1 translocation, while phospho-mimetic mutations of Ser777 to Asp or Glu allowed translocation to take place and bypassed the requirement for active p38 MAPK. Ser777 in FGFR1 was directly phosphorylated by p38α in a cell-free system. These data demonstrate a crucial role for p38α MAPK in the regulated translocation of exogenous FGF1 into the cytosol/nucleus, and they reveal a specific role for p38α MAPK-mediated serine phosphorylation of FGFR1.

ERK-Mediated Phosphorylation of Fibroblast Growth Factor Receptor 1 on Ser777 Inhibits Signaling

Serine phosphorylation limits signaling by a fibroblast growth factor receptor. Limiting Growth Factor Signaling Binding of fibroblast growth factors (FGFs) to receptor tyrosine kinases in the FGFR family triggers activation of these receptors through phosphorylation of tyrosine residues, thereby initiating signaling that stimulates cell proliferation and differentiation in various developmental processes, such as axonal growth. Zakrzewska et al. investigated the effect of phosphorylation of serine residues in FGFR1 on receptor activity. FGF-dependent activation of the mitogen-activated protein kinases ERK1 and ERK2 (ERK1/2) resulted in phosphorylation of Ser777 in the C-terminal region of the receptor, and this phosphorylation event was associated with decreased activation of FGFR1 and attenuated signaling. Dorsal root ganglion neurons expressing a S777A mutant FGFR1 exhibited enhanced and sustained receptor activation and extended longer axons than those expressing the wild-type receptor. In addition, previous stimulation of cells with epidermal growth factor, a ligand for a distinct receptor tyrosine kinase, resulted in the ERK1/2-mediated serine phosphorylation of FGFR1 and a reduction in subsequent FGF-dependent signaling. Together, these data suggest that an ERK-dependent mechanism initiated by activation of FGFR1 or other growth factor receptors prevents excessive FGFR1 signaling. Fibroblast growth factor 1 (FGF1) controls cellular activities through the activation of specific cell-surface FGF receptors (FGFRs). Transphosphorylation of tyrosine residues in the kinase domain of FGFRs leads to activation of intracellular signaling cascades, including those mediated by mitogen-activated protein kinases (MAPKs). FGFRs also contain a serine-rich C-terminal tail. We identified a regulatory mechanism of FGFR signaling involving phosphorylation of Ser777 in the C-terminal region of FGFR1 by the MAPKs extracellular signal–regulated kinase 1 (ERK1) and ERK2. Prevention of the phosphorylation of Ser777 in FGFR1 or mutation of Ser777 to alanine enhanced FGF-stimulated receptor tyrosine phosphorylation and increased cell proliferation, cell migration, and axonal growth. A form of FGFR1 with a phosphomimetic mutation at Ser777 exhibited reduced signaling. Activation of MAPKs by other receptor tyrosine kinases also resulted in phosphorylation of Ser777 in FGFR1, thereby enabling crosstalk regulation of FGFR activity by other signaling pathways. Our data reveal a negative feedback mechanism that controls FGF signaling and thereby protects the cell from excessive activation of FGFR.

Seprase, dipeptidyl peptidase IV and urokinase-type plasminogen activator expression in dysplasia and invasive squamous cell carcinoma of the esophagus. A study of 229 cases from Anyang Tumor Hospital, Henan Province, China.

Objective: Seprase, dipeptidyl peptidase IV (DPPIV) and urokinase-type plasminogen activator (uPA) play a crucial role in the degradation of the extracellular matrix and in the progression of various human tumors. However, their pathophysiologic significance in esophageal carcinoma has not yet been fully elucidated. Methods: The expression of seprase, DPPIV and uPA in esophageal dysplasia, squamous cell carcinoma (SCC) and normal epithelium was examined by immunohistochemistry. Results: Seprase, DPPIV and uPA immunoreactivity was found in dysplastic and cancer cells as well as in stromal cells adjacent to dysplasia and cancer sites, but not in normal epithelium. We found a significant association between uPA expression and sex, tumor size and histological classification in carcinomas. High expression of DPPIV in cancer cells correlated with longer survival of the patients. No significant associations between seprase and clinicopathological features either in dysplasia or in carcinomas were found. Finally, we demonstrated higher levels of seprase, DPPIV and uPA in SCC cell lines than in normal esophageal epithelial cell lines. Conclusions: Our results showed that seprase, DPPIV and uPA are expressed in both premalignant and malignant forms of SCC, but are lacking in normal esophageal epithelia, suggesting that they are involved in SCC neoplastic progression.

Clathrin- and Dynamin-Independent Endocytosis of FGFR3 – Implications for Signalling

Endocytosis of tyrosine kinase receptors can influence both the duration and the specificity of the signal emitted. We have investigated the mechanisms of internalization of fibroblast growth factor receptor 3 (FGFR3) and compared it to that of FGFR1 which is internalized predominantly through clathrin-mediated endocytosis. Interestingly, we observed that FGFR3 was internalized at a slower rate than FGFR1 indicating that it may use a different endocytic mechanism than FGFR1. Indeed, after depletion of cells for clathrin, internalization of FGFR3 was only partly inhibited while endocytosis of FGFR1 was almost completely abolished. Similarly, expression of dominant negative mutants of dynamin resulted in partial inhibition of the endocytosis of FGFR3 whereas internalization of FGFR1 was blocked. Interfering with proposed regulators of clathrin-independent endocytosis such as Arf6, flotillin 1 and 2 and Cdc42 did not affect the endocytosis of FGFR1 or FGFR3. Furthermore, depletion of clathrin decreased the degradation of FGFR1 resulting in sustained signalling. In the case of FGFR3, both the degradation and the signalling were only slightly affected by clathrin depletion. The data indicate that clathrin-mediated endocytosis is required for efficient internalization and downregulation of FGFR1 while FGFR3, however, is internalized by both clathrin-dependent and clathrin-independent mechanisms.

Design and characteristics of cytotoxic fibroblast growth factor 1 conjugate for fibroblast growth factor receptor-targeted cancer therapy

Fibroblast growth factor receptors (FGFRs) are attractive candidate cancer therapy targets as they are overexpressed in multiple types of tumors, such as breast, prostate, bladder, and lung cancer. In this study, a natural ligand of FGFR, an engineered variant of fibroblast growth factor 1 (FGF1V), was conjugated to a potent cytotoxic drug, monomethyl auristatin E (MMAE), and used as a targeting agent for cancer cells overexpressing FGFRs, similar to antibodies in antibody–drug conjugates. The FGF1V–valine–citrulline–MMAE conjugate showed a favorable stability profile, bound FGFRs on the cell surface specifically, and efficiently released the drug (MMAE) upon cleavage by the lysosomal protease cathepsin B. Importantly, the conjugate showed a prominent cytotoxic effect toward cell lines expressing FGFR. FGF1V–vcMMAE was highly cytotoxic at concentrations even an order of magnitude lower than those found for free MMAE. This effect was FGFR-specific as cells lacking FGFR did not show any increased mortality.

Translocation of exogenous FGF1 into cytosol and nucleus is a periodic event independent of receptor kinase activity

Fibroblast growth factor 1 (FGF1) has the property to become translocated from the extracellular space into the cell cytosol and nucleus. Membrane translocation of FGF1 occurs subsequent to endocytic uptake and is strictly FGF-receptor (FGFR) dependent. Here we have investigated the timing of FGF1 translocation in relation to FGFR1 signalling. We found that the translocation of FGF1 is a periodic event that occurs with 24h intervals. Serum-starved cells translocated the growth factor with peak occurrences ~6 h, ~30 h, and ~54 h after the addition of FGF1. The periodic FGF1 translocation was totally independent of the FGFR1 tyrosine kinase activity as it proceeded unchanged when the kinase activity was chemically inhibited or the kinase domain was deleted. Furthermore, FGF1 translocation was not restricted to a particular phase of the cell cycle or dependent on cell cycle progression. The results demonstrate that the FGF1/FGFR1 complex constitutes a signalling module that independently of the receptor tyrosine kinase can convey a signal that initiates a strictly timed and periodic release of endocytosed FGF1 into the cytosol/nucleus.

Identification of new FGF1 binding partners—Implications for its intracellular function

Besides its classical mode of action through activation of specific receptors at the cell surface, fibroblast growth factor 1 (FGF1) can also cross the cellular membrane and translocate into the cytosol and further to the nucleus. The mechanism of this translocation is described partially, but the role of FGF1 inside the cell remains unknown. The aim of our work was to identify novel binding partners of FGF1 to predict its intracellular functions. We combined three methods of identification of such partners based on different principles: yeast two‐hybrid screen and mass spectrometry (MS) analysis of complexes obtained by Tandem Affinity Purification (TAP) or by co‐precipitation from cell lysate using recombinant FGF1. Altogether, we identified twenty novel intracellular proteins interacting with FGF1. For selected proteins, their direct interaction with FGF1 was confirmed by pull‐down assays and SPR measurements. Interestingly, half of the proteins found are involved in processes related to cell viability, such as apoptosis, cell proliferation, and cell cycle regulation. Thus, our study indicates that the role of intracellular FGF1 is to protect the cell against stress conditions by providing an additional signal for cell survival, independently of receptor‐activated signaling cascades.

Protease Resistant Variants of FGF1 with Prolonged Biological Activity

Therapeutic potential of human acidic fibroblast growth factor (FGF1) resulting from its undeniable role in angiogenesis and wound healing processes is questioned due to its low stability and short half-life in vivo. Our previous studies showed that prolonged biological activity of FGF1 can be achieved by increasing its proteolytic resistance directly linked to improved global thermostability. In this study, we applied an alternative method of generation of long-lasting FGF1 variants by rigidification of the growth factors segment highly sensitive to proteases action. In order to determine regions the most prone to enzymatic degradation, we used limited proteolysis by trypsin combined with mass spectrometry analysis. We found that the initial proteolytic cleavages occurred mainly within the C-terminal region of the wild-type protein, pointing on its significant role in growth factor degradation. Based on bioinformatic analysis, we introduced two single mutations (C117P, K118V) within β-strand XI and combined them in a double mutant. We determined resistance to proteolysis, biophysical properties and biological activities of obtained variants. All of them occurred to be significantly less susceptible to trypsin (up to 100-fold) and also to chymotrypsin degradation comparing to the wild-type protein. Interestingly, all variants were not more thermostable than the wild-type FGF1. We attributed this dramatic increase in resistance to proteolysis to entropic stabilization of C-terminal region.