Regulation of insulin-like growth factor-1 receptor expression and signaling

<p>Insulin-like growth factor-1 receptor (IGF-1R), a member of the insulin receptor tyrosine kinase family is a broadly expressed transmembrane receptor that plays a key role in malignant cell growth. IGF-1R transmits information provided by extracellular stimuli into intracellular signaling pathways resulting in the subsequent regulation of various effector systems. Under normal cellular conditions IGF-1R signaling network is tightly regulated. The most prominent regulator of IGF-1R signal termination is desensitization of receptors by the removal of activated receptors from the cell surface mediated by accelerated endocytosis. For some membrane receptors the signal mediating receptor internalization/downregulation is constituted by ubiquitination. Recently, we showed that IGF-1R undergoes ubiquitination following ligand stimulation. The proto-oncogene MDM2 was identified as an E3 ligase involved in IGF-1R ubiquitination.</p><p>Studies on new events involved in IGF-1R downregulation and intracellular signaling constitute the subject of the present thesis.</p><p>â-arrestins are ubiquitously expressed cytosolic proteins generally known to be involved in the regulation of endocytosis and signaling elicited by G protein-coupled receptors (GCPRs). We provide evidence that the two widely co-expressed isoforms of â-arrestin, bind to the IGF-1R and, by serving as adaptor proteins bring the oncoprotein E3 ligase MDM2 to the receptor. Thus, â-arrestins promote ubiquitination but also degradation of the receptor. In this respect, â-arrestin 1 is more potent then isoform 2. Actually, â-arrestins are an absolute requirement for interaction between MDM2 and IGF-1R, indicating their relevance for cell growth and cancer.</p><p>We also investigated the role of â-arrestin 1 and MDM2 in intracellular signaling. We found that both MDM2 and â-arrestin 1 also are necessary for IGF-1 stimulated phosphorylation of ERK1/2 but not of Akt. In addition, the modulating effect of MDM2 and â-arrestin 1 on ERK activation has consequences on cell cycle progression. Thus, MDM2 and â-arrestin 1 do not only induce ubiquitination and degradation of IGF-1R but also influence cell growth by modulating the activity of ERKs.</p><p>The cyclolignan PPP is an inhibitor of phosphorylation of IGF-1R and activation of downstream molecules, without interfering with the highly homologous insulin receptor (IR). Further, PPP has well established anti-tumor effects on several in vivo tumor models. We could demonstrate that PPP also causes downregulation of IGF-1R. Furthermore, the PPP-induced downregulation of IGF-1R required the expression of wild type MDM2 E3 ligase, indicating that MDM2-dependent ubiquitination and degradation of IGF-1R represents an important mechanism in this respect. Our data also suggest that this effect of PPP plays a role in induction of apoptosis.</p><p>Finally, we demonstrated that PPP in fact induces IGF-1R ubiquitination, but also temporarily activates ERK1/2. This effect is IGF-1R-specific since PPP does not affect ERK phosphorylation in IGF-1R negative cells. Moreover, in the absence of MDM2, PPP-induced activation of ERK did not occur. The temporary MDM2-dependent ERK phosphorylation induced by PPP may contribute to the apoptotic effect of this compound.</p><h3>List of scientific papers</h3><p>I. Girnita L, Shenoy SK, Sehat B, Vasilcanu R, Girnita A, Lefkowitz RJ, Larsson O (2005). "{beta}-Arrestin is crucial for ubiquitination and down-regulation of the insulin-like growth factor-1 receptor by acting as adaptor for the MDM2 E3 ligase." J Biol Chem 280(26): 24412-9. Epub 2005 May 3 <br><a href="https://pubmed.ncbi.nlm.nih.gov/15878855">https://pubmed.ncbi.nlm.nih.gov/15878855</a><br><br></p><p>II. Girnita L, Shenoy SK, Sehat B, Vasilcanu R, Vasilcanu D, Girnita A, Lefkowitz RJ, Larsson O (2007). "Beta-arrestin and Mdm2 mediate IGF-1 receptor-stimulated ERK activation and cell cycle progression." J Biol Chem 282(15): 11329-38. Epub 2007 Feb 15 <br><a href="https://pubmed.ncbi.nlm.nih.gov/17303558">https://pubmed.ncbi.nlm.nih.gov/17303558</a><br><br></p><p>III. Vasilcanu R, Vasilcanu D, rosengren L, Natalishvili N, Sehat B, Yin S, Girnita A, Axelson M, Girnita L, Larsson O (2007). "Picropodophyllin induces downregulation of the insulin-like growth factor 1 receptor. Potential mechanistic involvement of MDM2 and beta-arrestin1." (Submitted)</p><p>IV. Vasilcanu R, Vasilcanu D, Sehat B, Yin S, Girnita A, Axelson M, Larsson O, Girnita I (2007). "Insulin-like growth factor 1 receptor (IGF-1R) dependent phosphorylation of ERK1/2 but not Akt (PKB) can be induced without receptor autophosphorylation." (Submitted)</p>