IGF-1R : Studies on the expression and role in transformation

Abstract: Transformation is a complex nonlinear multistep process during which normal cell becomes cancerous. Main characteristics of transformed cells include uncontrolled growth, invasion and metastasis. Extensive research over recent decades on mechanisms behind transformation has established the tyrosine kinase receptor, Insulin-like Growth Factor 1 Receptor (IGF-1R) as the major factor involved. Consequently, IGF-1R has gained ever increasing attention as a promising target in cancer therapy. The current opinion is that inhibition of IGF-1R activity is not enough to cause massive apoptosis and tumor regression. To obtain these responses the receptor must be downregulated. The small molecule picropodophyllin (PPP), discovered by our group, inhibits IGF-1R signaling. PPP induces massive apoptosis in tumor cells and causes tumor regression in various animal models. PPP is well tolerated in vivo. This thesis focuses on mechanisms of IGF-1R expression and their role in transformation. Paper I shows that IGF-1R knockout cells (R-) cultured over long time express the kinase subunit (beta-subunit) of the receptor. This aberrant receptor is demonstrated to be important for survival of these cells. Paper II shows that the aberrant IGF-1R beta-subunit in R- cells represents not only a beneficial factor for cell survival but is crucial for transformation of these cells. Knockdown by siRNA targeting IGF-1R abrogates oncogenic transformation by H-RasV12 and/or polyoma middle T-antigen. The beta-subunit in R- cells is shown to be intracellular and does not interfere with ERK and Akt phosphorylation. These findings may suggest involvement of a non canonical pathway of signaling in the IGF-1R dependent transformation. Paper III shows that PPP induces partial downregulation of the IGF-1R and that this action may be important for its apoptotic effect in tumor cells. beta-arrestin1, adaptor protein involved in IGF-1R signaling seems to be important for transduction of this effect. Paper IV reveals that beta-arrestin1 mediated IGF-1R signaling is important for Ras induced transformation. Signal transduction of activated Ras is impaired in the absence of beta-arrestin1. Incomplete Akt and ERK activation after IGF-1 stimulation cannot sustain growth and proliferation under anchorage independent conditions. In conclusion, this thesis suggests that the holo-IGF-1 receptor is not necessary for transformation, but action of the intracellular beta-subunit is sufficient. This finding may have patho-physiological relevance since the occurrence of intracellular IGF-1R in malignant cells has been widely reported. PPP-induced IGF-1R downregulation may contribute to its strong anti-tumor effect. Finally, beta-arrestin appears to be important for Ras-induced transformation.