Aging men develop a significant loss of muscle strength that occurs in conjunction with a decline in serum testosterone concentrations. We investigated the effects of testosterone administration to six healthy men [67 +/- 2 (SE) yr] on skeletal muscle protein synthesis, strength, and the intramuscular insulin-like growth factor I (IGF-I) system. Elderly men with serum testosterone concentrations of 480 ng/dl or less were given testosterone injections for 4 wk to produce serum concentrations equal to those of younger men. During testosterone administration muscle strength (isokinetic dynamometer) increased in both right and left hamstring and quadricep muscles as did the fractional synthetic rate of muscle protein (stable-isotope infusion). Ribonuclease protection assays done on total RNA from muscle showed that testosterone administration increased mRNA concentrations of IGF-I and decreased mRNA concentrations of insulin-like growth factor binding protein-4. We conclude that increasing testosterone concentrations in elderly men increases skeletal muscle protein synthesis and strength. This increase may be mediated by stimulation of the intramuscular IGF-I system.

Since C-peptide can only be derived from de novo synthesis and processing of pre-proinsulin mRNA and protein, we conclude that these results are the first demonstration that human umbilical cord blood-derived stem cells can be engineered to engage in de novo synthesis of insulin.

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An insulin-like growth factor (IGF) I response element (IGFRE) in the porcine P-450 cholesterol side-chain cleavage gene (P450scc) regulates transcription through the binding of two proteins, Sp1 and polypyrimidine tract-binding protein-associated splicing factor (PSF). PSF is a component of spliceosomes and contains RNA-binding domains. In this study, we localized the NH2-terminal amino acid residues necessary for binding of PSF to the IGFRE. Three COOH-terminal truncated proteins (aa 304, 214, and 134) of PSF were designed to empirically partition the NH2-terminal region while excluding the RNA-binding domains. Southwestern analysis showed that only the largest expressed truncated protein, P3, strongly bound the porcine P450scc IGFRE. Truncated PSF protein expression in Y1 adrenal cells showed that P3 repressed transcriptional activity of the IGFRE similar to full-length PSF, whereas P2 (minimal binding to the IGFRE) had no effect. In conclusion, the NH2-terminal region of PSF contains the amino acid residues necessary for binding to the porcine P450scc IGFRE and repressing the transcriptional activity of the element.

An insulin-like growth factor I (IGF-I) response element (IGFRE) in the porcine P-450 cholesterol side-chain cleavage gene (P450scc) binds two transcription factors, Sp1 and polypyrimidine tract-binding protein-associated splicing factor (PSF). In this study, we investigated expression of these transcription factors in mouse Y1 adrenal cells, a cell line that does not increase P450scc expression in response to IGF-I. Western blot analysis showed a greater expression of PSF in Y1 cells when compared with a mouse fibroblast cell line (NWTb3) in which IGF-I stimulates the P450scc IGFRE. The two cell lines expressed Sp1 equally, and IGF-I did not increase expression of either transcription factor. Chromatin immunoprecipitation analysis with Y1 chromatin confirmed that PSF and Sp1 bound to the IGFRE. When increasing amounts of Sp1 were expressed in Y1 cells, the IGFRE became responsive to IGF-I. Moreover, a mutant oligonucleotide IGFRE reporter construct that lacks PSF binding was responsive to IGF-I. In conclusion, Y1 adrenal cells are a physiological example of PSF repression of growth factor-stimulated (IGF-I) gene expression (P450scc). The dynamic nature of this repression is consistent with PSF functioning as a regulator of growth factor-stimulated gene expression in mammalian cells.