Changes in the circulating IGF system during short-term fasting and refeeding in rats

Little is known about free insulin-like growth factor I (IGF-I) and its regulatory proteins during fasting and refeeding. Prior research has shown that IGF-I and its binding proteins play roles in metabolic adaptation. However, the specific dynamics of these proteins during short-term fasting remain unclear. Established knowledge includes the role of IGF-I in growth and metabolism, but the interaction between IGF-I, binding proteins, and insulin during fasting is not fully understood. This gap motivated a closer look at the IGF system during fasting and refeeding. No prior work had resolved how IGF-binding protein (IGFBP)-1 and -3 change in response to metabolic shifts. That uncertainty drove the need to examine these proteins in a controlled rodent model. The study aimed to address this gap by tracking serum and hepatic changes over time.

The study aimed to investigate changes in the circulating IGF system during fasting and refeeding in rats. A specific problem is the lack of data on free IGF-I and its regulatory proteins under these conditions. The motivation comes from the need to understand how metabolic shifts affect IGF regulation. Researchers sought to clarify the relationship between IGF-I, IGFBP-1, and insulin during fasting and refeeding. The study also aimed to determine how hepatic gene expression responds to these metabolic changes. By tracking serum and liver markers, the authors hoped to identify patterns in IGF regulation. The focus was on short-term fasting and refeeding to capture acute physiological responses. This approach allows for a detailed analysis of IGF system dynamics.

The study used a rat model to examine fasting and refeeding effects on the IGF system. Rats were fasted for 0, 1, 2, and 3 days, followed by refeeding at various intervals. Serum was collected for analysis of insulin, C-peptide, growth hormone (GH), free and total IGF-I, IGFBP-1, IGFBP-3, and the acid-labile subunit (ALS). Liver mRNA levels for IGF-I, IGFBP-1, and ALS were also measured. The design allowed for tracking changes in both serum and hepatic markers. The approach combined biochemical assays with gene expression analysis. This method enabled the researchers to correlate protein levels with mRNA expression. The study used a controlled experimental setup to ensure accurate comparisons.

Fasting significantly reduced serum levels of GH, free and total IGF-I, IGFBP-3, and ALS. IGFBP-1 levels increased during fasting (P < 0.0001). Refeeding normalized IGFBP-1 at 3 hours and GH at 12 hours. Free IGF-I levels changed in parallel with total IGF-I, ALS, and IGFBP-3. Normalization of free IGF-I occurred at 48 hours of refeeding. IGFBP-1 (peptide and mRNA) showed an inverse correlation with insulin and C-peptide (P < 0.001). The correlation between IGFBP-1 peptide and mRNA was strong (r² = 0.36; P < 0.0001). The correlation was moderate for IGF-I (r² = 0.18; P < 0.0005) and insignificant for ALS.

The authors propose that insulin regulates IGFBP-1 in fasted and refed rats. However, the normal inverse relationship between free IGF-I and IGFBP-1 was absent in this study. Free IGF-I changed in parallel with total IGF-I and thus with ALS and IGFBP-3. These findings suggest a distinct regulatory pattern for free IGF-I during refeeding. The study highlights differences in the regulation of hepatic IGF-I, IGFBP-1, and ALS. The researchers suggest that the synthesis of these proteins is not tightly coordinated. The absence of the expected inverse relationship indicates a novel mechanism. These results contribute to understanding IGF regulation during metabolic transitions.