In modern biomedical sciences, there is a continuous search for informative biomarkers that are present in biological pathways or pathological processes which can be identified for an accurate diagnosis or treatment. Follistatin-like proteins are one of these groups of potential biomarkers belonging to the family of acidic cysteine-rich secreted glycoproteins (SPARC) that are seen in the sequence and domain structure of the activin-binding protein follistatin (FST) (Parfenova et al., 2021). There are a total of five follistatin-like proteins present in virtually every organ system and tissue in the body and change their expression with shifts in pathological processes depending on severity (Parfenova et al., 2021). Since follistatin-like proteins are involved in intracellular signaling pathways, they have the potential to become key biomarkers for diagnostics.
The most studied gene is follistatin-like protein 1 (FSTL1), which is located on chromosome 3q13.33 and encompasses 11 exons (Parfenova et al., 2021). According to transgenic models, FSTL1 is directly a part of a range of physiological signaling pathways and pathological states. It interacts with multiple TGF-β receptors that serve to differentiate between the type I transforming growth factor beta receptor (TGFBR1) and bone morphogenetic protein receptors (BMP-receptors) (Parfenova et al., 2021). The gene interacts with “disco-interacting protein 2 (DIP2A), toll-like receptor 4 (TLR4), and glycosylphosphatidylinositol (GPI)-anchored protein (CD14)” (Parfenova et al., 2021). Using these interactions and signal pathways, FSTL1 can demonstrate its activity through gene expression using cytokines and chemokines, as well as instigating a pro-inflammatory response. Signaling molecules are a critical part of pathophysiological processes, the anti-inflammatory response, and FSTL1 can regulate the proliferation of cell types using phosphorylation of SMAD transcription modulators (Parfenova et al., 2021).
Metabolic syndrome is the medical term for a group of conditions associated with an increased risk of heart disease, stroke, and type-II diabetes. The conditions include high blood pressure, high blood sugar, abnormal cholesterol levels (low HDL or high triglycerides), and increased fat deposits in the abdominal area (typically measured by waist circumference as well as BMI) (Wang et al., 2020). The greater number of these conditions affect an individual, the higher the risk for one of the critical diseases listed above. Metabolic syndrome affects a range of organ systems, including the metabolic, cardiovascular, endocrine, and central nervous systems. It is associated with obesity and insulin resistance, which along with the conditions, lead to fatty buildups in artery walls (atherosclerosis) (Wang et al., 2020). Since metabolic syndrome is directly responsible for type-II diabetes and heart disease, any single one of the components being abnormal is a serious health indicator, and it is best to identify metabolic syndrome in patients as early as possible.
Obesity is one of the overarching characteristics that can be both a cause and a consequence of metabolic disorders. Mattiotti et al. (2018) indicate that serum levels of FSTL1 are increasingly higher, correlating to BMI. However, it is not consistent, peaking specifically during differentiation of pre-adipocytes (3T3–L1) into adipocytes and then falling to background levels at the mRNA level. This occurs via two distinct mechanisms associated with the regulation of FSTL1 expression, but it is observed that there is an upregulation of pro-inflammatory cytokines. The shifting of the ratio of pro- and anti-inflammatory cytokines is the potential causal factor in the chronic inflammation seen during obesity which then results in insulin resistance and other associated conditions (Mattiotti et al., 2018).
As mentioned, due to the critical nature of FSTL1 in gene expression and pathway signaling, there is increased research on it being used as a diagnostic and prognostic biomarker due to its unique properties. Research exists on its role as a biomarker for arthritis and tumors, but there is yet limited information on the potential of FSTL1 as a biomarker for metabolic syndrome described above (Parfenova et al., 2021). However, there are promising results from the few studies aimed at researching this objective.
One of the key pieces of information necessary to use FSTL1 as a biomarker for metabolic disorders is determining the FSTL1 levels in both healthy and unhealthy metabolic states. A study by Lee et al. (2019) sought to evaluate the connection between FSTL1 levels, the metabolic state, and the presence of subclinical coronary atherosclerosis (plaques in the coronary artery), a known consequence of metabolic syndrome. Their findings identified that FSTL1 levels were increased in subjects with an unhealthy metabolic state but not in those with obesity. Furthermore, higher levels were observed in patients with plaques. The authors suggest that based on their findings and supporting evidence, FSTL1 may be a viable biomarker to characterize a metabolically unhealthy status and risk of CVD. However, it is unclear if FSTL1 indicates the underlying metabolic state or remains a mediator of metabolic risk (Lee et al., 2019).
Similarly, a study by Yang et al. (2020) sought to examine the new cytokine FSTL-1 association with metabolic syndrome and insulin resistance. In a cross-sectional study of 487 subjects using complex bioinformatics to determine protein and protein interaction networks, the connection between the variables was assessed. The researchers found that FSTL-1 levels were significantly higher in patients with a recent diagnosis of metabolic syndrome (7.5 vs. 5.8) (Yang et al., 2020). A range of genes was identified, including FSTL-1 homologous FSTL-3, which enriched the transforming growth factor-β and signaling pathway for diabetic complications. It was also found that FSTL-1 levels positively correlate with a range of metabolic factors which contribute to metabolic syndrome, including fasting plasma glucose, waist circumferences, blood pressure, and triglyceride levels (Yang et al., 2020). Therefore, using binary logistic regression analysis, the authors determined that FSTL-1 levels are related to metabolic syndrome and insulin resistance at a level where it can be used as a biomarker.
These findings are yet again supported by Xu et al. (2020) that found FSTL-1 protein expression levels in the adipose tissue of participants with type-II diabetes and obesity were significantly higher than in healthy and lean members. These indicators were related to insulin resistance, adiponectin, and obesity, all metabolism-related factors. However, the authors note that as a cytokine response to shifts in metabolic status, the release of FSTL-1 in muscles and adipose tissue may be inhibited by high insulin and FFA levels (Xu et al., 2020). Overall, evidence demonstrates that FSTL-1 is an emerging cytokine that has tremendous potential and serves as a relatively accurate biomarker for metabolic syndrome. There are potential challenges that may require further research, such as the ability to differentiate between conditions to which increased FSTL-1 levels may point, as the gene expression has been associated with tumors, arthritis, and CVD. Another element to consider is the yet unknown role of FSTL-1 in these metabolic conditions, being just a messenger on signaling pathways or being part of the underlying metabolic state. The novel glycoprotein is associated with metabolic syndrome and insulin resistance and potentially serves as a critical biomarker for early diagnosis of metabolic disorders.
References
Lee, S.-Y., Kim, D.-Y., Kyung Kwak, M., Hee Ahn, S., Kim, H., Kim, B.-J., Koh, J.-M., Rhee, Y., Hwa Kim, C., Hyun Baek, K., Min, Y.-K., Hun Lee, S., & Kang, M.-I. (2019). High circulating follistatin-like protein 1 as a biomarker of a metabolically unhealthy state. Endocrine Journal, 66(3), 241–251.
Mattiotti, A., Prakash, S., Barnett, P., & van den Hoff, M. J. B. (2018). Follistatin-like 1 in development and human diseases. Cellular and Molecular Life Sciences, 75(13), 2339–2354.
Parfenova, O. K., Kukes, V. G., & Grishin, D. V. (2021). Follistatin-like proteins: Structure, functions and biomedical importance. Biomedicines, 9(8), 999.
Wang, H. H., Lee, D. K., Liu, M., Portincasa, P., & Wang, D. Q.-H. (2020). Novel insights into the pathogenesis and management of the metabolic syndrome. Pediatric Gastroenterology, Hepatology & Nutrition, 23(3), 189.
Xu, X., Zhang, T., Mokou, M., Li, L., Li, P., Song, J., Liu, H., Zhu, Z., Liu, D., Yang, M., & Yang, G. (2020). Follistatin-like 1 as a Novel adipomyokine related to insulin resistance and physical activity. The Journal of Clinical Endocrinology & Metabolism, 105(12), e4499–e4509.
Yang, S., Dai, H., Hu, W., Geng, S., Li, L., Li, X., Liu, H., Liu, D., Li, K., Yang, G., & Yang, M. (2020). Association between circulating follistatin‐like‐1 and metabolic syndrome in middle‐aged and old population: A cross‐sectional study. Diabetes/Metabolism Research and Reviews, 37(2).