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

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.

Studies on new events involved in IGF-1R downregulation and intracellular signaling constitute the subject of the present thesis.

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.

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.

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.

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.

List of scientific papers

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
https://pubmed.ncbi.nlm.nih.gov/15878855

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
https://pubmed.ncbi.nlm.nih.gov/17303558

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)

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)