Strategic advantages of insulin-like growth factor-I expression for cardioprotection.

Background: Insulin-like growth factor-I (IGF-I) peptide has beneficial effects on cardiomyocyte function and survival, many of which are mediated through the serine-threonine kinase, Akt. However, concerns about systemic effects of IGF-I peptide limit its clinical application. The present study tested whether local IGF-I expression could mediate cardioprotection without elevating serum [IGF-I].

Methods: The ability of a recombinant adenovirus encoding IGF-IB (Ad.IGF-I) to activate Akt and protect cardiomyocytes from hypoxia-induced apoptosis in vitro was compared with the effects of IGF-I peptide or expression of constitutively active Akt (myr-Akt). In vivo, cardiac IGF-I gene transfer was performed prior to ischemia-reperfusion injury (IRI). Effects on the ischemic and infarcted areas were assessed while serum [IGF-I] was measured by radioimmunoassay.

Results: Compared with IGF-I peptide, Ad.IGF-I achieved more sustained activation of Akt and reduced hypoxia-induced apoptosis at lower media IGF-I concentrations. In a co-culture system, Ad.IGF-I protected both infected and uninfected cells from hypoxic injury, while myr-Akt protected only infected cells. In vivo cardiac injection of Ad.IGF-I mediated significant local IGF-I expression, without affecting serum [IGF-I] levels. After IRI, Ad.IGF-I did not affect the ischemic area but reduced infarct size approximately 50% (32 +/- 13 vs. 64 +/- 14% AAR in Ad.GFP rats, p < 0.003), although the transgene was expressed in only approximately 15% of the ischemic region, consistent with possible paracrine benefit.

Conclusions: Somatic gene transfer of IGF-I may offer strategic advantages over both systemic delivery of IGF-I peptide and expression of cell autonomous cardioprotective transgenes such as Akt by mediating autocrine and paracrine cardiomyocyte protection without elevating serum [IGF-I] levels.