Mrna levels of insulin-like growth factor 1 and II and binding proteins 4 and 5 in human skeletal muscle

IGFI, IGFII and their binding proteins are important components in growth promotion and tissue maintenance. Previous studies in cultured mouse muscle cells indicate that IGFBP5 plays a role in muscle development and IGFBP4 in proliferation. In the same study, IGFBP5 increased dramatically during differentiation in the presence of IGFs or insulin. The gene expression of these binding proteins in human skeletal muscle has not yet been demonstrated. In order to determine the presence and to assess whether the gene expression of these binding proteins, IGFI and II are altered acutely by nutrients and insulin we measured the abundance of their mRNA in human skeletal muscle. Thirteen healthy subjects were studied on two occasions. In the first study (control) they drank equal volumes of water every 20 minutes for 10 hours. Seven of these subjects drank polycose (CHO) in their second study. The six other subjects drank equal calories as a mixed meal with 13.9 mg/kg protein/every 20 minutes. Quadriceps muscle biopsies were taken at 10 hours. A quantitative polymerase chain reaction (QPCR) was designed to measure IGFI, IGFII, 1GFBP4 and IGFBP5 using the constitutively expressed glyceraldehyde phosphate dehydrogenase (GAPDH) to normalize samples. The mRNA were extracted from tissue samples, reverse transcribed, and PCR was performed by co-amplifying the target gene and GAPDH with biotinylated primers. The amplified products were hybridized with tris (2,2'-bipyridine) ruthenium (II) chelate (TBR) labeled probes and measured by electrochemiluminescence. The values were expressed as ratios using GAPDH as the normalizing factor. Reproducibility studies proved this procedure to be precise with CV's ranging from 4-16%. IGFI, IGFII, BP4 and BP5 mRNAs were found to be present in human skeletal muscle. mRNA expression of IGFI and II were correlated (r=0.60) in the control study but the correlation became highly significant during mixed meal (r=0.88) and CHO meal (r-0.99). BP5 mRNA levels were also correlated with IGFI (r=0.53) and IGFII (r=0.64) in the control study and after mixed meal with IGFI (r=0.81) and IGFII (r=0.99). Correlation with BP5 was lower in the CHO study with IGFI (r=0.32) and IGFII (r-0.43). There was no correlation between the BP4 mRNA levels with IGFI or IGFII in any conditions (r<0.2).