Insulin-like growth factor-I (IGF-I) gene expression is regulated by the nutritional and metabolic state, consistent with changes in the somatic or tissue-specific growth program. An important clinical example is the renal hypertrophy which occurs in the initial stages of diabetic nephropathy. Glucose and IGF-I stimulate renal matrix synthesis and hemodynamics, and IGF-I is transiently increased in diabetic kidney. Thus, hyperglycemia per se may stimulate kidney IGF-l gene expression in insulinopenic diabetes. The overall goal of the research program is to characterize the mechanisms by which altered glucose metabolism leads to transcriptional and translational control of IGF-I gene expression.
The specific aims proposed toward achieving this goal are: 1) To test the hypothesis that glucose or a consequence of glucose action directly stimulates IGF-I transcription. The effects of glucose and glucose metabolites and analogs on IGF-I mRNA levels, transcription rates and promoter activity in GH3 and in C6 cells will be characterized; 2) To test the hypothesis that tissue- specific changes in IGF-I gene expression in liver, kidney and possibly other tissues from insulinopenic diabetic animals are due to distinct effects of hypoinsulinemia and hyperglycemia. IGF-I mRNA levels and transcription rates will be determined in streptozotocin (STZ)-diabetic rats receiving either insulin replacement, or phlorizin to lower blood glucose without restoring insulin levels. Changes in IGF-I mRNA levels will be localized to distinct kidney regions and cell types using in situ hybridization; 3) To test the hypothesis that the tandem translation initiation and termination codons in the 5'-UTR of some IGF-I mRNAs necessitate re-initiation at the preproIGF-I open reading frame and thereby reduce the efficiency of preproIGF-I translation. The effect of mutation of the upstream initiation and termination codons in IGF-I mRNA 5'-UTRs on translational efficiency in vitro and in intact cells will be characterized; 4) To test the hypothesis that the efficiency of translation re-initiation in IGF-I mRNAs containing upstream initiation and termination codons in the 5'-UTR can be regulated by the level or activity of initiation factors eIF-2 and elF-2B. The effect of exogenous eIF-2 and eIF-2B and of phosphorylation of endogenous eIF-2 on the in vitro translation of IGF-I mRNAs with different 5'-UTRs will be characterized. This mechanism is potentially pertinent to glucose regulation of IGF-l gene expression because glucose regulates the activities of eIF-2 and eIF-2B. Positive outcomes of these specific aims will provide novel insights into transcriptional and translational control mechanisms which regulate IGF-I gene expression. The studies will provide a paradigm for the cellular mechanisms by which glucose regulates IGF-I gene expression as well as the expression of other genes. These studies will provide an essential foundation for determining how altered glucose levels and metabolism regulate IGF-I biosynthesis and growth in normal physiology and in disease states such as type II diabetes, malnutrition, and catabolic and tissue-wasting syndromes.
| Ney, D M; Huss, D J; Gillingham, M B et al. (1999) Investigation of insulin-like growth factor (IGF)-I and insulin receptor binding and expression in jejunum of parenterally fed rats treated with IGF-I or growth hormone. Endocrinology 140:4850-60 |
