Andrew Beenken, Moosa Mohammadi
9 min
The fibroblast growth factor (FGF) family consists of 18 mammalian ligands (FGF1–10 and FGF16–23) classified into subfamilies based on their phylogeny and signaling mechanisms. While historically known as paracrine regulators of developmental processes—such as branching morphogenesis and tissue patterning—recent research has shifted focus toward the endocrine functions of the FGF19 subfamily. These ligands regulate systemic homeostasis of bile acids, glucose, and phosphate, making them attractive targets for metabolic and renal disease therapy.
FGFs signal by activating the four FGF receptors (FGFR1–FGFR4), which are tyrosine kinase receptors. Signaling requires the presence of heparan sulfate glycosaminoglycans (HSGAGs) to facilitate the formation of a ternary FGF–FGFR–HSGAG complex. Specificity is tightly regulated by alternative splicing of FGFRs, which generates isoforms with distinct ligand-binding preferences, and by the spatial and temporal expression patterns of the ligands and receptors. Endocrine FGFs have evolved to bypass the reliance on HSGAGs for local signaling, instead requiring membrane-bound klotho proteins to increase their affinity for specific FGFRs.
Deregulated FGF signaling is implicated in various cancers and congenital syndromes. Gain-of-function mutations in FGFRs are commonly associated with skeletal syndromes (e.g., achondroplasia) and various malignancies, while loss-of-function mutations often lead to developmental disorders such as Kallmann syndrome. Therapeutic strategies currently explore recombinant FGFs for tissue repair and cytoprotection—most notably palifermin (FGF7) for oral mucositis—as well as small-molecule FGFR inhibitors to treat cancers characterized by overactive signaling.
Endocrine FGFs represent the next frontier in pharmacological development. FGF21 has shown promise in improving insulin sensitivity and beta-cell function without the mitogenic risks associated with other members. Conversely, while FGF19 is effective in metabolic regulation, its potential to induce hepatocellular carcinoma remains a significant clinical hurdle. Similarly, anti-FGF23 antibodies are being explored to treat hypophosphataemic disorders. Future success relies on balancing the potent metabolic benefits of these factors with strategies to minimize their potential for tumorigenesis.
Sam: The clearest example here is palifermin, a modified, more stable version of FGF7. It treats oral mucositis, injury to the mouth lining caused by chemotherapy and radiation. The review describes its approved use in patients undergoing bone marrow transplantation.
Alex: Mouth injury can sound like a secondary outcome beside cancer treatment. How severe was the condition being prevented?
Sam: The most severe grade makes oral feeding impossible. In the patient results summarized here, palifermin reduced its incidence from sixty-two percent to twenty percent. Patients also used fewer opioid painkillers.
Alex: That’s a substantial difference in severe injury. Did it shorten the illness as well?
Sam: Median duration fell from nine days to six days. The review describes adverse events as mild and transient. But this excerpt gives neither the trial size nor enough comparator details to independently assess those estimates.
Alex: That makes this a strong clinical example within the review, not a complete trial appraisal. What does the protein actually do?
Sam: Mainly, it increases cell proliferation and thickens the protective lining. Other proposed mechanisms include antioxidant and inflammatory responses. But benefits in one setting don’t establish benefits elsewhere: promising immune effects in animals had not been corroborated in clinical trials described here.
Alex: The obvious extension is to grow new blood vessels in poorly supplied tissue. Did that approach work as cleanly?
Sam: No. The cardiovascular evidence is much less consistent. FGF2 produced early improvements in some studies, but the FIRST trial found those improvements weren’t sustained. The placebo group continued improving.
Alex: Is that evidence against the biological idea, or a problem getting enough protein to the right place?
Sam: The review leaves room for delivery problems. Growth factors need prolonged local presence to maintain new vessels, yet injected protein can disappear quickly. Direct injection into heart muscle improves retention compared with delivery through coronary arteries, but choosing a route also depends on which vessels need to grow.
Alex: Keeping the signal around longer could help efficacy. But the review’s opening tension suggests that persistence could also create safety problems.
Sam: The authors raise concerns about abnormal vascular growth and new vessels within atherosclerotic plaques. Their summary is restrained: exogenous FGFs had not yet altered the course of human cardiovascular disease. That distinction between biological activity and clinical benefit is central.
Alex: Now take us to the circulating branch. Why did it renew therapeutic interest?
Sam: Its members regulate bile acids, glucose, vitamin D, and phosphate. That expands the family’s relevance beyond development and repair. The review gives particular attention to FGF21 because of its metabolic effects in animals and its comparatively reassuring safety findings.
Alex: Metabolic improvements in mice don’t necessarily travel well. Does the review include evidence beyond rodents?
Sam: It describes rhesus monkey experiments showing lower fasting glucose and improved lipid profiles after treatment. After six weeks, levels of high-density lipoprotein cholesterol were eighty percent higher than controls. The excerpt doesn’t provide the animal count.
Alex: Encouraging, but still not a human treatment result. Does human biology support the same account of what this protein does?
Sam: Not completely. A ketogenic diet induces FGF21 in mice, but the human studies described here did not show that response. Human levels increased only after prolonged fasting. The review also reports higher levels in people with diabetes, without establishing whether that reflects resistance or compensation.
Alex: So an association with disease doesn’t tell us whether to add more protein. What makes FGF21 look safer than its relatives?
Sam: The reviewed experiments found little evidence of growth-promoting activity, and treatment did not cause low blood glucose or fluid accumulation. By contrast, mice engineered to overexpress FGF19 developed liver cancers with age. Both proteins improved aspects of metabolism in animals, but their safety arguments differ sharply.
Alex: We’ve mostly discussed adding signals. Where does blocking a circulating signal make sense?
Sam: FGF23 regulates phosphate balance, including how much phosphate the kidneys retain. Excessive activity contributes to phosphate-wasting disorders. Neutralizing antibodies normalized phosphate and vitamin D concentrations in affected mice, supporting a therapeutic strategy, not yet demonstrating human benefit in this account.
Alex: Does that logic extend to kidney disease, where the review also reports high FGF23 levels?
Sam: Not automatically. The authors say whether those increases are beneficial or harmful remains debated. High levels predict worse outcomes, but prediction alone doesn’t show that blocking the protein would help.
Alex: With those distinctions in mind, who should read the full review, and where should they begin?
Sam: Researchers developing growth-factor or receptor-directed therapies should read it. Start with “The FGF–FGFR signalling system” for the specificity argument. Then compare the endocrine ligand sections, or read the cardiovascular treatment challenges if delivery is your concern.
Alex: And for someone who only needs the central lesson?
Sam: This is a map of therapeutic possibilities, not one uniformly validated treatment strategy. Its clinical examples, animal results, and unresolved mechanisms deserve different levels of confidence. Remember this: the right growth signal needs the right tissue, the right duration, and evidence that benefit outweighs unwanted growth.
Alex: Thanks for listening.