Insulin-like growth factor 1 receptor, novel functions and future possibilities

Abstract

Insulin like growth factor-1 receptor (IGF-1R) has been shown to be important for cancer cell growth and survival, and is often overexpressed in malignant and premalignant tissues. Ligand binding to IGF-1R induces transphosphorylation and activation of the receptor, leading to subsequent activation of the phosphatidyl inositol-3 kinase (PI3K), the mitogen-activated protein kinase (MAPK) and the 14-3-3 pathways. Most of these pathways are shared by other receptor tyrosine kinases. Inhibition of these pathways using specific IGF-1R antibodies has, however, failed in large trials on cancer patients. This opens the possibilities for alternative signaling pathways. Recently, IGF-1R was shown to be SUMOylated and translocated to the cell nucleus. In the nucleus it binds to enhancer-like regions, and regulates expression of genes including CCND1 and AXIN2, and phosphorylates Histone3. The expression of nuclear IGF-1R has also been linked to increased cell growth and aggressive phenotype in cancer.

In paper I the effects of picropodophyllin (PPP), an inhibitor of IGF-1R, on cell cycle progression were studied. Previous studies have indicated that PPP treated cells arrest in G2/M. We found that PPP induced G2/M arrest through interfering with microtubule dynamics causing prolonged mitotic arrest and mitotic catastrophe in an IGF-1R independent manner. This mechanism of PPP may contribute to its efficacy in treatment of cancer patients. In paper II the roles of SUMOylated IGF-1R in regulating cell proliferation and cell cycle progression were investigated. We provided evidence that SUMOylation of IGF-1R increases G1/S phase transition through inducing expression of cyclins (D, A and B) and upregulating CDK2. Cells expressing SUMOylated IGF-1R also proliferated faster and formed more colonies in soft agar compared to cells expressing IGF1R with mutated SUMO-binding sites. In paper III we investigated potential binding partners to nuclear IGF-1R in human embryonic stem cells (hESC). We found that nuclear IGF-1R associates with PCNA and phosphorylates it, not only in hESCs but also in other cell types. The nuclear IGF-1R-induced PCNA phosphorylation was followed by ubiquitination of PCNA, probably through DNA damage tolerance (DDT)-dependent E2/E3 ligases (e.g. Rad18 and UBC13). Our data suggest that IGF-1R may contribute to activation of DDT, as externally induced DNA damage in IGF-1R negative cells led to G1 cell cycle arrest and larger S-phase fork stalling compared to cells expressing IGF-1R.

In summary, the achieved results may contribute in understanding the complexity of IGF-1R’s roles in cell growth and maintenance of genome stability, as well as the shown mitotic block induced by PPP may be a mechanism that favors anti-IGF-1R treatment in cancer.