Folding of Fibroblast Growth Factor 1 Is Critical for Its Nonclassical Release

Cellular Mechanisms of FGF-Stimulated Tissue Repair

Growth factors belonging to the FGF family play important roles in tissue and organ repair after trauma. In this review, I discuss the regulation by FGFs of the aspects of cellular behavior important for reparative processes. In particular, I focus on the FGF-dependent regulation of cell proliferation, cell stemness, de-differentiation, inflammation, angiogenesis, cell senescence, cell death, and the production of proteases. In addition, I review the available literature on the enhancement of FGF expression and secretion in damaged tissues resulting in the increased FGF supply required for tissue repair.

Through the back door: Unconventional protein secretion

Proteins are secreted from eukaryotic cells by several mechanisms besides the well-characterized classical secretory system. Proteins destined to enter the classical secretory system contain a signal peptide for translocation into the endoplasmic reticulum. However, many proteins lacking a signal peptide are secreted nonetheless. Contrary to conventional belief, these proteins are not just released as a result of membrane damage leading to cell leakage, but are actively packaged for secretion in alternative pathways. They are called unconventionally secreted proteins, and the best-characterized are from fungi and mammals. These proteins have extracellular functions including cell signaling, immune modulation, as well as moonlighting activities different from their well-described intracellular functions. Among the pathways for unconventional secretion are direct transfer across the plasma membrane, release within plasma membrane-derived microvesicles, use of elements of autophagy, or secretion from endosomal/multivesicular body-related components. We review the fungal and metazoan unconventional secretory pathways and their regulation, and propose experimental criteria to identify their mode of secretion.

Intracellular partners of fibroblast growth factors 1 and 2 - implications for functions

Fibroblast growth factors 1 and 2 (FGF1 and FGF2) are mainly considered as ligands of surface receptors through which they regulate a broad spectrum of biological processes. They are secreted in non-canonical way and, unlike other growth factors, they are able to translocate from the endosome to the cell interior. These unique features, as well as the role of the intracellular pool of FGF1 and FGF2, are far from being fully understood. An increasing number of reports address this problem, focusing on the intracellular interactions of FGF1 and 2. Here, we summarize the current state of knowledge of the FGF1 and FGF2 binding partners inside the cell and the possible role of these interactions. The partner proteins are grouped according to their function, including proteins involved in secretion, cell signaling, nucleocytoplasmic transport, binding and processing of nucleic acids, ATP binding, and cytoskeleton assembly. An in-depth analysis of the network of these binding partners could indicate novel, non-classical functions of FGF1 and FGF2 and uncover an additional level of a fine control of the well-known FGF-regulated cellular processes.

Adipocyte Piezo1 mediates obesogenic adipogenesis through the FGF1/FGFR1 signaling pathway in mice

White adipose tissue (WAT) expansion in obesity occurs through enlargement of preexisting adipocytes (hypertrophy) and through formation of new adipocytes (adipogenesis). Adipogenesis results in WAT hyperplasia, smaller adipocytes and a metabolically more favourable form of obesity. How obesogenic WAT hyperplasia is induced remains, however, poorly understood. Here, we show that the mechanosensitive cationic channel Piezo1 mediates diet-induced adipogenesis. Mice lacking Piezo1 in mature adipocytes demonstrated defective differentiation of preadipocyte into mature adipocytes when fed a high fat diet (HFD) resulting in larger adipocytes, increased WAT inflammation and reduced insulin sensitivity. Opening of Piezo1 in mature adipocytes causes the release of the adipogenic fibroblast growth factor 1 (FGF1), which induces adipocyte precursor differentiation through activation of the FGF-receptor-1. These data identify a central feed-back mechanism by which mature adipocytes control adipogenesis during the development of obesity and suggest Piezo1-mediated adipocyte mechano-signalling as a mechanism to modulate obesity and its metabolic consequences.

Adipose tissue expansion occurs via enlargement of adipocytes as well as the generation of new fat cells, the latter being associated with more favorable metabolic outcomes. Here, the authors show that activation of adipocyte Piezo1 results in release of FGF1 and stimulates the differentiation of adipocyte precursor cells.

miRNA-210-3p regulates trophoblast proliferation and invasiveness through fibroblast growth factor 1 in selective intrauterine growth restriction

Selective intrauterine growth restriction (sIUGR), which affects approximately 10%-15% of monochorionic (MC) twin pregnancies, is highly associated with intrauterine foetal death and neurological impairment in both twins. Data suggest that unequal sharing of the single placenta is the main contributor to birth weight discordance. While MC twins and their placenta derive from a single zygote and harbour almost identical genetic material, the underlying mechanisms of phenotypic discrepancies in MC twins remain unclear. MicroRNAs are small non-coding RNA molecules that regulate gene expression but do not change the DNA sequence. Our preliminary study showed that microRNA-210-3p (miR-210-3p) was significantly upregulated in the placental share of the smaller sIUGR twin. Here, we investigate the potential role of miR-210-3p in placental dysplasia, which generally results from dysfunction of trophoblast cells. Functional analysis revealed that miR-210-3p, induced by hypoxia-inducible factor 1α (HIF1α) under hypoxic conditions, suppressed the proliferation and invasiveness of trophoblast cell lines. Further RNA sequencing analysis and luciferase reporter assays were performed, revealing that fibroblast growth factor 1 (FGF1) is an influential target gene of miR-210-3p. Moreover, correlations among miR-210-3p levels, HIF1α and FGF1 expression and the smaller placental share were validated in sIUGR specimens. These findings suggest that upregulation of miR-210-3p may contribute to impaired placentation of the smaller twin by decreasing FGF1 expression in sIUGR.

Resistance to visceral obesity is associated with increased locomotion in mice expressing an endothelial cell-specific fibroblast growth factor 1 transgene

Overdevelopment of visceral adipose is positively correlated with the etiology of obesity-associated pathologies including cardiovascular disease and insulin resistance. However, identification of genetic, molecular, and physiological factors regulating adipose development and function in response to nutritional stress is incomplete. Fibroblast Growth Factor 1 (FGF1) is a cytokine expressed and released by both adipocytes and endothelial cells under hypoxia, thermal, and oxidative stress. Expression of Fibroblast Growth Factor 1 (FGF1) in adipose is required for normal depot development and remodeling. Loss of FGF1 leads to deleterious changes in adipose morphology, metabolism, and insulin resistance. Conversely, diabetic and obese mice injected with recombinant FGF1 display improvements in insulin sensitivity and a reduction in adiposity. We report in this novel, in vivo study that transgenic mice expressing an endothelial-specific FGF1 transgene (FGF1-Tek) are resistant to high-fat diet-induced abdominal adipose accretion and are more glucose-tolerant than wild-type control animals. Metabolic chamber analyses indicate that suppression of the development of visceral adiposity and insulin resistance was not associated with alterations in appetite or resting metabolic rate in the FGF1-Tek strain. Instead, FGF1-Tek mice display increased locomotor activity that likely promotes the utilization of dietary fatty acids before they can accumulate in adipose and liver. This study provides insight into the impact that genetic differences dictating the production of FGF1 has on the risk for developing obesity-related metabolic disease in response to nutritional stress.

Probing the role of proline -135 on the structure, stability, and cell proliferation activity of human acidic fibroblast growth factor

Human acidic fibroblast growth factor 1 (hFGF1) is a protein intricately involved in cell growth and tissue repair. In this study, we investigate the effect(s) of understanding the role of a conserved proline (P135), located in the heparin binding pocket, on the structure, stability, heparin binding affinity, and cell proliferation activity of hFGF1. Substitution of proline-135 with a positively charged lysine (P135K) resulted in partial destabilization of the protein; however, the overall structural integrity of the protein was maintained upon substitution of proline-135 with either a negative charge (P135E) or a polar amino acid (P135Q). Interestingly, upon heparin binding, an increase in thermal stability equivalent to that of wt-hFGF1 was observed when P135 was replaced with a positive (P135K) or a negative charge (P135E), or with a polar amino acid (P135Q). Surprisingly, introduction of negative charge in the heparin-binding pocket at position 135 (P135E) increased hFGF1's affinity for heparin by 3-fold, while the P135K mutation, did not alter the heparin-binding affinity. However, the enhanced heparin-binding affinity of mutant P135E did not translate to an increase in cell proliferation activity. Interestingly, the P135K and P135E double mutations, P135K/R136E and P135/R136E, reduced the heparin binding affinity by ∼3-fold. Furthermore, the cell proliferation activity was increased when the charge reversal mutation R136E was paired with both P135E (P135E/R136E) and P135K (P135K/R136E). Overall, the results of this study suggest that while heparin is useful for stabilizing hFGF1 on the cell surface, this interaction is not mandatory for activation of the FGF receptor.