Pathophysiological Role of Growth Factors in Diabetic Kidney Disease: Focus on Innovative Therapy

The physiological and pathophysiological roles of the GH/IGF-axis in the kidney: lessons from experimental rodent models

The growth hormone (GH)/insulin-like growth factor (IGF) system plays an important role in renal development, growth, function and pathophysiology. IGF-I has been associated with renal/glomerular hypertrophy and compensatory renal growth. Potential effects on glomerular size are of interest, since an increase in glomerular size may be permissive for the development of glomerulosclerosis. In an effort to abolish the decline of renal function and possibly to restore the renal structure, different approaches have been tested in experimental models of nephropathy, focusing mainly on early renal changes. The involvement of the GH/IGF system in renal pathophysiology has been studied in much detail in the rat. In view of the growing interest in murine physiology, occurring in large part by genetically modified animals, this review examines those aspects of GH, IGFs, their receptors and binding proteins that relate both to mouse kidney physiology and to a number of conditions characterized by pathophysiological renal changes. A deeper understanding of the role of the GH/IGF system in renal dysfunction may stimulate the development of novel therapeutic approaches aiming at preventing or retarding various kidney diseases.

Role of insulin and the IGF system in renal hypertrophy in diabetic Psammomys obesus (sand rat)

BACKGROUND

Diabetic nephropathy is caused by multiple factors related to the altered metabolic environment in diabetes mellitus (DM). Experimental diabetic kidney disease is characterized by renal hypertrophy associated with increased tissue concentrations of insulin-like growth factor I (IGF-I). To assess the specific roles of serum insulin and glucose in mediating the development of diabetic nephropathy, the effects of both hyperinsulinaemic and hypoinsulinaemic DM were studied in Psammomys obesus (sand rat), a model of type 2 DM.

METHODS

The IGF-I system was studied in normal Psammomys obesus gerbils and at 5, 15 and 70 days after the induction of either hyper- or hypoinsulinaemic DM. To induce hyperinsulinaemic DM, Psammomys were raised on a high-energy diet. Hypoinsulinaemic DM was induced by either administration of streptozotocin or a specially designed diet.

RESULTS

Hyperinsulinaemic hyperglycaemic Psammomys did not exhibit renal hypertrophy (unchanged kidney/body-weight ratio) and renal IGF-I levels were in the normal range on days 5, 15 and 70. In contrast, Psammomys with hypoinsulinaemic hyperglycaemia induced either by streptozotocin injection or by pancreas exhaustion brought on by a long-term caloric excess diet, had significant increases in kidney/body-weight ratio which were associated with elevated renal IGF-I and mRNA and protein levels of kidney IGF binding protein I.

CONCLUSIONS

This study shows that serum insulin levels in the presence of hyperglycaemia have an important role in the development of experimental diabetic nephropathy in the Psammomys model. The implication of this finding is that the pathophysiological mechanisms for diabetic kidney disease in experimental models may be different for type 1 and type 2 DM.

Specificity is complex and time consuming: mutual exclusivity in tyrosine kinase-mediated signaling

Most fundamental cellular processes are transduced through tyrosine kinase (TK)-mediated pathways. For transduction without corruption, the protein-protein interactions involved have to be mutually exclusive. Many of these proteins bind via homologous domains whose binding characteristics suggest that their innate specificity is not sufficiently high to account for the integrity of signal transduction. Stimulation of TK-mediated signals is often accompanied by recruitment of a precise, multimolecular protein complex that is itself capable of imposing specificity. Furthermore, this complex provides protection against phosphatase activity, controlling the longevity of the active signaling complex, and thus influencing outcomes in subsequent downstream events.

C-peptide: a new potential in the treatment of diabetic nephropathy

C-peptide is formed in the biosynthesis of insulin and the two peptides are subsequently released in equimolar amounts to the circulation. C-peptide has long been considered to be without physiologic effects. Recent data now demonstrate that C-peptide in the nanomolar concentration range binds specifically to cell surfaces, probably to G protein-coupled receptors, with subsequent activation of Ca(2+)-dependent intracellular signaling pathways and stimulation of Na+, K(+)-ATPase activities. C-peptide replacement in animal models of type 1 diabetes results in diminished hyperfiltration, improved functional reserve, reduction of urinary albumin excretion, and prevention of glomerular and renal hypertrophy. Administration of C-peptide to physiologic concentrations in patients with type 1 diabetes and incipient nephropathy for periods of 3 hours to 3 months is accompanied by reduced glomerular hyperfiltration and filtration fraction, and diminished urinary albumin excretion. C-peptide replacement together with insulin therapy may be beneficial in type 1 diabetes patients with nephropathy.