The secret life of nuclear IGF-1R : functions beyond traditional signaling pathways

Abstract: The implications of IGF-1R in normal development and disease are well established. IGF-1R plays pivotal roles in cell proliferation and cell survival and aberrant activation result in tumorigenesis and cancer progression. Extensive studies have concluded that much of the biological effects of IGF-1R are mediated by a plethora of cytoplasmic signaling pathways originating from the cell surface. However, findings describing IGF-1R in the cell nucleus of cancer cells have recently begun to emerge. The high prevalence of IGF-1R in the nucleus of cancer cells suggests a functional role for nuclear IGF-1R in cancer biology. The first study (paper I) in this thesis aimed to elucidate the role of nuclear IGF-1R in gene transcription. We show that nuclear IGF-1R associates with β-catenin and LEF-1, key components of the Wnt signaling pathway. Nuclear IGF-1R is enriched at the cyclin D1 promoter and elevates cyclin D1 and axin2 protein levels. In paper II we propose a model for IGF-1R nuclear transportation in cancer cells. We present data showing that the transportation is dependent on microtubules and the retrograde transport protein complex dynactin. IGF-1R was also found to associate with EEA1 in both the cytoplasm and the nucleus, suggesting that IGF-1R is membrane bound during the transport. By utilizing siRNA and mutant constructs we show that passage of IGF-1R across the nuclear envelope is dependent on importin-β, RanBP2 and Ran GTPase. The nuclear pore complex protein and SUMO E3 ligase, RanBP2, SUMOylates IGF-1R at the nuclear periphery and as a consequence stabilize IGF-1R. Stabilized receptor is able to enter the nucleus. In the third paper we identified histone H3 as an interacting partner for nuclear IGF-1R. Furthermore, we show that IGF-1R phosphorylates histone H3Y41. By utilizing wild type and mutant histone H3 constructs we demonstrate that phosphorylated H3Y41 stabilizes the association of Brg1 chromatin remodeling protein to chromatin. We also identified SNAI2 to be a target gene for nuclear IGF-1R and its expression was reduced when H3Y41 phosphorylation was impaired. Both IGF-1R and Brg1 was found to associate with SNAI2 promoter. Taken together, these studies provide novel findings about IGF-1R function and trafficking. We show that nuclear IGF-1R takes a more direct part in gene transcription in addition to its classical role as a cell surface receptor in cancer cells. We also propose a mechanism by which IGF-1R is transported into the nucleus. As IGF-1R is highly implicated in cell proliferation and cell survival, our findings provide a regulatory role for nuclear IGF-1R in tumorigenesis and cancer progression.