From membrane to nucleus: New roles and functions of SUMOylated IGF-1R and EGFR

Abstract: Cell surface receptor tyrosine kinases (RTKs) role in cell signaling have been studied for decades and their role in cancer progression are undisputable. The insulin-like growth factor 1 receptor, IGF-1R, has been demonstrated to play a critical part in tumorigenesis; downregulation of the IGF-1R in tumor xenografts results in complete tumor regression. Previously, RTK research has focused on the canonical signaling pathways activated by ligand binding at the plasma membrane. However, strong evidence keeps emerging that several RTKs have a second functionally mechanism, inside the cell nucleus, where the receptors reside after ligand stimulation. The aim of this thesis was to elucidate the function of recently discovered nuclear IGF-1R as well as to investigate its nuclear translocation pathway. Since it was previously shown that SUMOylation of the IGF-1R is essential for its nuclear translocation we also set out to investigate SUMO modification of the epidermal growth factor receptor (EGFR). In paper I, we present a functional role for nuclear IGF-1R in gene transcription. Inside the nucleus, IGF-1R functions as a co-activator to LEF-1/TCF transcription factor. Nuclear IGF-1R enhances transcription of cyclin D1 and axin2, and we show that it is enriched in the cyclin D1 promoter region. In the following study, paper II, we propose a pathway by which IGF-1R is transported into the nucleus. IGF-1R is transported along microtubules via the dynactin transportation complex, to the nuclear pore where it is transferred to importin-β which guides the receptor to the nuclear pore complex protein RanBP2, which further assists the receptor into the cell nucleus in a RanGTPase dependent manner. Inhibition or obstruction of any of these components results in a reduction in nuclear IGF-1R. Further, we suggest that RanBP2 is the SUMO E3 ligase in IGF-1R SUMOylation and we show that SUMO-1 modification of the receptor is also important for its stability. In paper III, we demonstrate that the EGFR is SUMOylated and propose five lysine residues as SUMO-1 targets which were identified by two different mass spectrometry strategies. One of these residues, lysine 37, came up as a suggested target in both mass spectrometry methods. EGFR mutated in this site – EGFR-K37R – causes a decrease in protein levels as well as transcriptional activity of cyclin D1 and c-myc, two target genes of nuclear EGFR. To summarize, our data shows (I) a pathway by which nuclear IGF-1R is being transported and the functional importance of nuclear IGF-1R as a co-activator in transcription and (II) that the EGFR is also SUMOylated and might play a role in its transcriptional activity. Together these results may unravel new mechanisms for IGF-1R and EGFR that have implications in carcinogenesis.