FGF 18 Human

What is FGF-18 and what are its primary functions in human biology?

FGF-18 is a bioactive substance that conducts biological signals, regulates cell growth, and participates in tissue repair. In human biology, FGF-18 functions as a signaling molecule that promotes cell proliferation and survival under various conditions, including hypoxia or nutrient deprivation. It plays essential roles in tissue development, particularly in cartilage formation and lung branching morphogenesis, while maintaining progenitor cell populations in developing tissues.

FGF-18 exerts its effects through binding to FGF receptors (primarily FGFR3) and activating downstream signaling pathways including AKT/ERK phosphorylation. The protein has been found to be expressed in various human tissues, with particular significance in developmental contexts and certain pathological conditions .

How does FGF-18 signaling differ from other FGF family members in human systems?

FGF-18 exhibits distinct signaling patterns compared to other FGF family members like FGF7, FGF9, and FGF10, particularly in human fetal lung development:

• While FGF7, FGF9, and FGF10 impair branching and promote cysting in human fetal lung explants, FGF-18 significantly increases branching by approximately 17.4% compared to controls

• FGF-18 uniquely promotes epithelial proliferation in human fetal lung tissue, with a significant increase in KI67+ cells specifically within the epithelium

• FGF-18 fails to support distal epithelial cell fate in isolated lung epithelial organoids, suggesting it may promote proximal cell fate, in contrast to other FGF family members

• Unlike mouse lung development where Fgf18 knockout affects alveolar formation but not branching, FGF-18 appears directly involved in the human lung branching process

These differences highlight FGF-18's unique properties and suggest distinct receptor preferences and downstream pathway activation compared to other FGF family members.

What evidence supports the efficacy of recombinant human FGF-18 (rhFGF-18) in cartilage healing?

Systematic review evidence indicates that rhFGF-18 significantly improves cartilage healing at 6 months postoperatively following microfracture or orthopedic procedures. Multiple studies have reported improved cartilage repair quality with FGF-18 treatment as measured by standardized scoring systems:

The tissue infill percentage, when reported, showed significant improvement with FGF-18 treatment (72.50% compared to 26.88% in controls), demonstrating enhanced cartilage regeneration capacity .

What experimental parameters should be considered when designing FGF-18 cartilage repair studies?

When designing studies to investigate FGF-18 for cartilage repair, researchers should consider multiple parameters that have been validated in previous research:

• Dosing: Studies have tested doses ranging from 10-100 μg administered via intra-articular (IA) injection

• Treatment timing: Typically following microfracture or similar orthopedic procedures

• Assessment timepoints: Follow-up periods of 5.5-6 months have shown significant effects

• Defect characteristics: Previous studies have used defined defect sizes (e.g., 15 mm in equine models)

• Delivery method: Intra-articular injection has been the predominant administration route

• Control groups: Surgically treated models without FGF-18 augmentation serve as appropriate controls

• Quality assessment: Implement risk-of-bias measures using tools like SYRCLE to ensure methodological rigor

Research designs should include appropriate randomization, blinding, and allocation concealment to minimize bias, as these factors have been positively evaluated in previous high-quality studies.

How is FGF-18 expression altered in human cancers and what are the clinical implications?

FGF-18 expression shows significant alterations across multiple cancer types, with consistent patterns of upregulation compared to corresponding normal tissues:

Colorectal Cancer (CRC):

Hepatocellular Carcinoma (HCC):

• Quantitative RT-PCR analyses have demonstrated significantly increased FGF-18 mRNA levels in HCC compared to normal liver tissue

• Both mRNA and protein levels of FGF-18 are significantly higher in HCC patient tissue samples and human liver cancer cell lines (HepG2 and HuH7) compared to normal liver tissue and normal liver epithelial cell line LO2

Ovarian Cancer (OC):

• Transcriptome analysis shows significantly increased FGF-18 expression in OC tissues

• ELISA verification confirms significantly increased serum FGF-18 expression in OC patients relative to matched normal controls

• Transcriptome sequencing identified FGF-18 as an up-regulated gene in ovarian serous cancer cell line SKOV-3 compared to benign ovarian epithelial tumor cell line MCV152

These expression patterns suggest FGF-18 may serve as a potential biomarker for cancer diagnosis, prognosis, or therapeutic targeting.

What signaling pathways does FGF-18 regulate in human cancer cells?

FGF-18 activates several critical signaling pathways in human cancer cells, contributing to cancer progression through multiple mechanisms:

In Hepatocellular Carcinoma (HCC):

• FGF-18 reverses serum starvation-induced apoptosis in HepG2 or Hep3B human liver cancer cells, improving cancer cell survival under hypoxia or nutrient deprivation

• FGF-18 promotes HCC angiogenesis by enhancing the FGFR3 pathway and increasing phosphorylation levels of AKT/ERK proteins

• FGF-18 is regulated upstream by the Wnt/β-catenin signaling pathway

• Ribosomal protein s15a (RPS15A) increases FGF-18 expression in HCC cells through the Wnt/β-catenin signaling pathway

• Co-culture experiments with HuH7 and human umbilical vein endothelial cells demonstrate FGF-18's role in promoting angiogenesis

The interconnected signaling network involving FGF-18 in cancer cells suggests multiple intervention points for potential therapeutic strategies. The activation of FGFR3 and subsequent phosphorylation of AKT/ERK represents a classic growth factor signaling cascade that promotes cell proliferation, survival, and angiogenesis in the tumor microenvironment.

How does FGF-18 contribute to human lung branching morphogenesis?

FGF-18 plays a crucial role in human lung development, particularly during the pseudoglandular and canalicular stages:

• RNA sequencing data shows increased FGF-18 expression during the pseudoglandular stage of human lung development

• Treatment of human fetal lung explants with recombinant human FGF-18 (100 ng/mL for 48 hours) significantly increases branching by approximately 17.4% compared to controls

• FGF-18 treatment promotes epithelial proliferation, with a significant increase in KI67+ cells specifically within the epithelium of treated explants

• FGF-18 maintains double-positive SOX2/SOX9 distal bud progenitor cells in human lung development

• FGF-18 influences mesenchymal progenitor cells, regulating the formation of cartilaginous airways, which in humans extend deep into the lung up to the 12th generation of branching

These findings highlight important species differences: unlike in mice, where Fgf18 knockout impairs alveolar formation without affecting branching, FGF-18 appears to be directly involved in the branching process in human lung development.

How does FGF-18 influence cell fate decisions in developing human tissues?

FGF-18 exerts specific effects on cell fate decisions in developing human tissues, with distinct impacts on different cell populations:

In Mesenchymal Cells:

• FGF-18 treatment of isolated human fetal lung mesenchyme results in a more condensed and organized structure compared to untreated cultures

• FGF-18 significantly increases expression of SOX9 (P = 0.0452), COL2A1 (P = 0.049), and CDH1 (P = 0.0234) in treated mesenchymal cultures, promoting chondrogenic cell fate

• FGF-18 significantly decreases expression of WT1 (P = 0.0055) with trends toward decreased expression of SNAIL2 and ACTA2, suggesting reorganization of cell-cell and cell-matrix adhesion

• These effects explain the formation of cartilage at epithelial branch bifurcations within the developing human lung up to the twelfth branching generation

In Epithelial Cells:

• When tested on isolated epithelium (bud tip organoids derived from human fetal lung), FGF-18 alone fails to support growth and expansion

• FGF-18 treatment results in undetectable levels of SFTPC expression by RT-qPCR, suggesting it fails to support distal epithelial cell fate and may promote proximal cell fate instead

• Bud tip organoid culture experiments demonstrate that FGF-18 appears to inhibit distal commitment in epithelial progenitors

These findings demonstrate FGF-18's context-dependent effects on cell fate determination, with particular influence on chondrogenic differentiation in mesenchymal cells and possible promotion of proximal fate in epithelial cells.

What experimental models are most effective for studying human FGF-18 function?

Several experimental models have proven valuable for studying FGF-18 function in human contexts:

Human Fetal Lung Explant Cultures:

• Air-liquid interface culture of human fetal lung explants within the pseudoglandular stage (10-13 weeks gestational age)

• Allows assessment of branching patterns, proliferation, and cellular responses to exogenous FGF-18

• Quantification of terminal branches and immunostaining for markers like KI67 provide measurable outcomes

Isolated Mesenchymal Cultures:

• Manual dissection and isolation of mesenchyme from human fetal lungs cultured in growth factor reduced Matrigel

• Enables assessment of direct effects on mesenchymal organization and gene expression

• RT-qPCR analysis of key genes like SOX9, COL2A1, CDH1, WT1, SNAIL2, and ACTA2 provides quantitative measures of FGF-18 effects

Bud Tip Organoid Cultures:

• Derived from human fetal lung between 8 and 11.5 weeks postconception

• Enzymatically and mechanically separated and cultured in Matrigel

• Treated with FGF-18 alone or in combination with CHIR-99021 and all-trans retinoic acid (RA)

• Enables study of epithelial-specific responses to FGF-18

Cancer Cell Line Models:

• Human liver cancer cell lines (HepG2, HuH7, Hep3B)

• Allows investigation of FGF-18's role in cancer cell survival, proliferation, and signaling pathway activation

The choice of model depends on the specific research question, with ex vivo human tissue cultures offering advantages for developmental studies and cell lines providing accessibility for mechanistic investigations.

What techniques provide the most reliable assessment of FGF-18 activity in experimental settings?

Researchers employ several complementary approaches to accurately measure FGF-18 activity:

Morphological Assessment:

• Quantification of branching patterns in explant cultures (counting terminal branches before and after treatment)

• Assessment of structural organization in mesenchymal cultures (condensation and organization)

Proliferation Analysis:

• Immunostaining for proliferation markers like KI67

• Quantification of positive cells in specific tissue compartments (epithelium vs. mesenchyme)

• Statistical analysis of proliferation changes (e.g., significance level P = 0.0403 observed in FGF-18 studies)

Gene Expression Analysis:

• RT-qPCR for target genes regulated by FGF-18 signaling:

  • Chondrogenic markers: SOX9, COL2A1

  • Epithelial markers: SFTPC, CDH1

  • Mesenchymal markers: WT1, SNAIL2, ACTA2

Histological Scoring Systems for Cartilage Research:

• ICRS (International Cartilage Repair Society) Score

• Modified O'Driscoll Score

• Tissue infill percentage measurements

Quality Control Measures:

• Risk-of-bias assessment using tools like SYRCLE for preclinical studies

• Proper experimental design including randomization, allocation concealment, and blinding

• Assessment of baseline group similarities and appropriate statistical analysis

Combining these approaches provides a comprehensive evaluation of FGF-18 activity across different biological contexts and experimental systems.