Insulin-like growth factor-I receptor increases in aortic endothelial cells from diabetic rats

The multifunctional role of IGF-1 in peripheral nerve regeneration

The elements of peripheral nerve regeneration comprise a complex combination of nerve growth, muscle satellite cells proliferation and differentiation and vessel growth. There is also increasing evidence that growth factors may act at multiple levels in the regenerative response. One such factor affecting multiple cell processes is insulin-like growth factor (IGF-1). As a neurotrophic factor IGF-1 is known to promote nerve elongation and branching. As a myogenic factor, IGF-1 promotes satellite cell proliferation, differentiation and muscle hypertrophy. As an angiogenic factor, IGF-1 is known to promote angiogenesis in regenerating skeletal muscle by activating VEGF and VEGF receptors. Additionally, recent studies show that IGF-1 may also promote the activation of muscle stem cells during the regenerative process. This review will outline the pathways by which IGF-1 affects multiple layers of the regenerative response and how these pathways converge to promote the regeneration of nerves.

Insulin-like growth factor-1 increases endothelin receptor A levels and action in cultured rat aortic smooth muscle cells

Insulin is known to cause an increase in endothelin-1 (ET-1) receptors in vascular smooth muscle cells (SMCs), but the effect of insulin-like growth factor 1 (IGF-1) on ET-1 receptor expression is not known. We therefore carried out the present study to determine the effect of IGF-1 on the binding of ET-1 to, and ET type A receptor (ETAR) expression and ET-1-induced 3H-thymidine incorporation in, vascular SMCs. In serum-free medium, IGF-1 treatment increased the binding of 125I-ET-1 to SMC cell surface ET receptors from a specific binding of 20.1%+/-3.1% per mg of protein in control cells to 45.1%+/-8.6% per mg of protein in cells treated with IGF-1 (10 nM). The effect of IGF-1 was dose-related, with a significant effect (1.4-fold) being seen at 1 nM. The minimal time for IGF-1 treatment to be effective was 30 min and the maximal effect was reached at 6 h. Immunoblotting analysis showed that ETAR expression in IGF-1-treated cells was increased by 1.7-fold compared to controls. Levels of ETAR mRNA measured by the RT-PCR method and Northern blotting were also increased by 2-fold in IGF-1-treated SMCs. These effects of IGF-1 were abolished by cycloheximide or genistein. Finally, ET-1-stimulated thymidine uptake and cell proliferation were enhanced by IGF-1 treatment, with a maximal increase of 3.2-fold compared to controls. In conclusion, in vascular SMCs, IGF-1 increases the expression of the ET-1 receptor in a dose- and time-related manner. This effect is associated with increased thymidine uptake and involves tyrosine kinase activation and new protein synthesis. These findings support the role of IGF-1 in the development of atherosclerotic, hypertensive, and diabetic vascular complications.

Insulin-like growth factor: receptor and binding proteins in human retinal endothelial cell cultures of diabetic and non-diabetic origin

Human retinal endothelial cell (HREC) cultures of diabetic and non-diabetic origin were examined for the production of insulin-like growth factor I (IGF-I), IGF-I receptor and IGF-binding proteins (IGFBPs) using colloidal gold quantitative immunocytochemistry and quantitative reverse transcriptase-polymerase chain reaction (QRT-PCR). The levels of immunoreactivity for IGF-I receptor and for four IGFBPs (IGFBP-1, -2, -3 and -5) were significantly increased in diabetic HREC cultures. Moreover, diabetic HREC cultures showed significantly less immunoreactivity for IGF-I and for IGFBP-4 as compared to non-diabetic HREC cultures. Message levels for IGF-I decreased two-fold in diabetic HREC and correlated with protein levels. Message levels for IGFBP-1, -2 and -5 increased 1.5-, 1.7- and 1.6-fold, respectively, in diabetic HREC and correlated with protein levels. However, the protein levels for IGF-I receptor and IGFBP-3 and -4 did not correlate with mRNA levels. There were no differences in mRNA levels for IGF-I receptor and IGFBP-3 and -4 between diabetic and non-diabetic HREC cultures, suggesting a post-transcriptional regulation of IGF-I receptor and the two IGFBPs. The net effect, however, supports enhanced IGF-I action in HREC cultures of diabetic origin which is an important cellular event in diabetic retinopathy.

Insulin and insulin-like growth factor-I: their role as risk factors in the development of diabetic cardiovascular disease

Experimental data support a role for insulin and insulin-like growth factor-I (IGF-I) in the pathophysiology of vascular complications in diabetes. Clinical data for both hormones are less convincing, mainly because the various studies vary in methodologies, sample sizes, and populations. So far, by epidemiological means, insulin's vascular toxicity has been shown only in middle-aged non-diabetic men. Furthermore, serious methodological problems hamper the clear understanding of IGF-I's significance in this context. Definitive determination of the role of insulin, IGFs and other growth factors in the development of diabetic vascular complications needs considerably more work. In any case, hyperinsulinemia is associated with a cluster of other accepted risk factors for cardiovascular disease which altogether resemble the entire insulin-resistance syndrome.