Insulin-Like Growth Factor-I Is an Important Antiapoptotic Factor for Rat Leydig Cells during Postnatal Development

Abstract

The present investigation examines the influence of IGF-I and the role of IGF-I receptor (IGF-IR) in the apoptosis/survival of Leydig cells. Immunohistochemical analysis of the rat testis at different ages revealed that the level of the phosphorylated IGF-IR increases from birth to d 20 of postnatal life, remaining high in the adult testis. Western blotting revealed that this level is higher in Leydig cells isolated from 40-d-old than from 10- or 60-d-old rats. Application of the terminal deoxyribonucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling assay revealed that IGF-I decreases the level of apoptosis in Leydig cells at all stages of development, and the selective inhibitor of IGF-IR, picropodophyllin, blocks this antiapoptotic effect. The mechanism underlying the antiapoptotic action of IGF-I involves the phosphatidylinositol 3-kinase/Akt pathway, and in immature Leydig cells, this growth factor enhances the expression of Bcl-2 and cellular inhibitor of apoptosis proteins 2, while preventing activation of caspase-3 by cleavage. Furthermore, IGF-II and high concentrations of insulin also evoke phosphorylation of IGF-IR and, like IGF-I, enhance the expression of the steroidogenic acute regulatory protein by Leydig cells. Inhibition of IGF-IR by picropodophyllin decreases the survival of Leydig cells, both in the presence and absence of IGF-I, demonstrating that signaling via the IGF-IR plays an important role in Leydig cell survival.

Insulin-Like Growth Factor Type-I Receptor-Dependent Phosphorylation of Extracellular Signal-Regulated Kinase 1/2 but not Akt (Protein Kinase B) Can Be Induced by Picropodophyllin

“…Although PPP inhibits IGF-1-induced IGF-1R phosphorylation Vasilcanu et al, 2004;Colón et al, 2007;Conti et al, 2007), the present results suggest that PPP induces IGF-1R ubiquitination without receptor phosphorylation. Because IGF-1R phosphorylation is required for its ubiquitination , the existence of a causal relationship between the two actions of PPP on IGF-1R seems to be uncertain.…”

“…PPP inhibits activity of IGF-1R Vasilcanu et al, 2004Vasilcanu et al, , 2006Ulfarsson et al, 2005;Strömberg et al, 2006;Colón et al, 2007;Conti et al, 2007;Guha et al, 2007;Razuvaev et al, 2007;Shields et al, 2007) but also down-regulates the receptor (Vasilcanu et al, 2008). Even though PPP may have other effects, we conclude that it does not cross-target the highly homologous insulin receptor .…”

“…We and others demonstrated that PPP is an inhibitor of the IGF-1R tyrosine phosphorylation Colón et al, 2007;Conti et al, 2007;Shields et al, 2007). In contrast, PPP did not inhibit the tyrosine kinase activity of insulin receptor or other major cancer-relevant growth factor receptors .…”

Picropodophyllin induces downregulation of the insulin-like growth factor 1 receptor: potential mechanistic involvement of Mdm2 and β-arrestin1

“…Moreover, PPP reduced phosphorylation of Akt, induced caspase 3 activation and apoptosis and caused tumor regression in grafted mice Vasilcanu et al, 2004;Catrina et al, 2005;Ulfarsson et al, 2005;Conti et al, 2007;Menu et al, 2006;Stromberg et al, 2006;Tomizawa and Saisho, 2006;Vasilcanu et al, 2006;Colon et al, 2007;Razuvaev et al, 2007). PPP did, however, not affect growth of established R-vsrc (IGF-1RÀ/À) tumor grafts , suggesting selectivity for IGF-1R regarding tumor growth.…”

Insulin-Like Growth Factor-I, Regulating Aromatase Expression through Steroidogenic Factor 1, Supports Estrogen-Dependent Tumor Leydig Cell Proliferation

“…It has been suggested that IGF-I can influence Leydig cell survival and proliferation (40,41). Moreover, it has been shown that IGF-I up-regulates aromatase expression in primary cultures of rat Leydig cells through a cAMP-independent mechanism (42).…”