Cyclolignans as Inhibitors of the Insulin-Like Growth Factor-1 Receptor and Malignant Cell Growth

Abstract

The insulin-like growth factor-1 receptor (IGF-1R) plays a pivotal role in transformation, growth, and survival of malignant cells, and has emerged as a general and promising target for cancer treatment. However, no fully selective IGF-1R inhibitors have thus far been found. This is explained by the fact that IGF-1R is highly homologous to the insulin receptor, coinhibition of which may cause diabetic response. The receptors are both tyrosine kinases, and their ATP binding sites are identical, implying that ATP inhibitors cannot discriminate between them. Therefore, the current strategy has been to identify compounds interfering with receptor autophosphorylation at the substrate level. In this study we investigated the effects of cyclolignans and related molecules on IGF-1R activity. We report that certain cyclolignans are potent and selective inhibitors of tyrosine phosphorylation of the IGF-1R. Of particular interest was picropodophyllin (PPP), which is almost nontoxic (LD 50 >500 mg/kg in rodents). PPP efficiently blocked IGF-1R activity, reduced pAkt and phosphorylated extracellular signal regulated kinase 1 and 2 (pErk1/2), induced apoptosis in cultured IGF-1R-positive tumor cells, and caused complete tumor regression in xenografted and allografted mice. PPP did not affect the insulin receptor or compete with ATP in an in vitro kinase assay, suggesting that it may inhibit IGF-1R autophosphorylation at the substrate level. This is also in agreement with our molecular model of how the cyclolignans may act on the IGF-1R kinase. Our results open the possibility to use PPP or related compounds with inhibitory effects on IGF-1R as lead compounds in development of anticancer agents.

Insulin-like growth factor-I receptor activity is essential for Kaposi's sarcoma growth and survival

“…Treatment of KSIMM cells with PPP induced a dosedependent cell death with an IC 50 at 95 nM ( Figure 5A), in the range reported for other IGF-IR-positive tumours (Girnita et al, 2004). Treatment of KSIMM cells with 100 nM PPP induced apoptosis as documented by Annexin V positivity ( Figure 5B) and by typical apoptotic morphological cell appearance and positive immunostaining for TUNEL ( Figure 5C) (22.171.5 vs 1.370.5% in controltreated cells) (Po0.001).…”

“…Moreover, we demonstrated that both KSIMM cells as well as KS tumours express IGF-1R. Even though hybrid receptors with insulin cannot be excluded (Pandini et al, 2002), the effect of PPP (specific blocker of the beta subunit of IGF-IR (Girnita et al, 2004)) on KSIMM apoptosis suggests that the IGF-IR is the main mediator of IGF system on these cells. The decrease of 3 H-thymidine incorporation in KSIMM cells after treatment with the IGF-IRblocking antibody a IR3 powerfully suggested the presence of an autocrine loop in these cells.…”

“…Mouse monoclonal antibody against the a subunit of IGF-IR (2C8), used for immunohistochemistry, and mouse monoclonal-anti-CD34 (endothelial antigen) were from Santa Cruz Biotechnology, Inc. (USA); anti-mouse IgG and peroxidase-linked whole antibody were from Amersham (Uppsala, Sweden), and mouse monoclonal anti-IGF-IR antibody (aIR 3), used for cell culture, was obtained from Oncogene Research Products (Boston, USA). Picropodophyllin was prepared as described previously (Girnita et al, 2004). All other reagents used for immunohistochemistry were from DAKO (Denmark).…”

“…The presence of IGF-IR in the tumour specimens from patients with AIDS-KS coupled with the essential functional role of the IGF-IR in KSIMM biology determined us to investigate the effect of PPP, a new, specific IGF-IR tyrosine kinase inhibitor, which has been shown to have antitumoral proprieties in vivo (Girnita et al, 2004). Treatment of KSIMM cells with PPP induced a dosedependent cell death with an IC 50 at 95 nM ( Figure 5A), in the range reported for other IGF-IR-positive tumours (Girnita et al, 2004).…”

“…The presence of IGF-IR on the surface of AIDS-KS tumour cells in situ suggests that our experimental observation can be used as a basis for the pharmacological IGF-IR interference for the treatment of patients with this disease. In this respect, a novel specific IGF-IR tyrosine phosphorylation blocker, picropodophyllin (PPP), which has shown antitumoral properties in vivo (Girnita et al, 2004), induced apoptosis in KSIMM cells.…”