The insulin-like growth factor-1 receptor inhibitor PPP produces only very limited resistance in tumor cells exposed to long-term selection.

The cyclolignan PPP was recently demonstrated to inhibit the activity of insulin-like growth factor-1 receptor (IGF-1R), without affecting the highly homologous insulin receptor. In addition, PPP caused complete regression of xenografts derived from various types of cancer. These data highlight the use of this compound in cancer treatment. However, a general concern with antitumor agents is development of resistance. In light of this problem, we aimed to investigate whether malignant cells may develop serious resistance to PPP. After trying to select 10 malignant cell lines, with documented IGF-1R expression and apoptotic responsiveness to PPP treatment (IC50s less than 0.1 microM), only two survived an 80-week selection but could only tolerate maximal PPP doses of 0.2 and 0.5 microM, respectively. Any further increase in the PPP dose resulted in massive cell death. These two cell lines were demonstrated not to acquire any essential alteration in responsiveness to PPP regarding IGF-1-induced IGF-1R phosphorylation. Neither did they exhibit any increase in expression of the multidrug resistance proteins MDR1 or MRP1. Consistently, they did not exhibit decreased sensitivity to conventional cytostatic drugs. Rather, the sensitivity was increased. During the first half of the selection period, both cell lines responded with a temporary and moderate increase in IGF-1R expression, which appeared to be because of an increased transcription of the IGF-1R gene. This increase in IGF-1R might be necessary to make cells competent for further selection but only up to a PPP concentration of 0.2 and 0.5 microM. In conclusion, malignant cells develop no or remarkably weak resistance to the IGF-1R inhibitor PPP.

Picropodophyllin induces downregulation of the insulin-like growth factor 1 receptor: potential mechanistic involvement of Mdm2 and beta-arrestin1.

The insulin-like growth factor 1 receptor (IGF-1R) is crucial for growth and survival of malignant cells. Experience in targeting IGF-1R in cancer models has shown that strategies promoting downregulation of the receptor are much more efficient in inducing apoptosis than those inhibiting the IGF-1R activity. Recently, we found that the cyclolignan picropodophyllin (PPP) inhibits phosphorylation of IGF-1R and activation of downstream signaling without interfering with the highly homologous insulin receptor (IR). Furthermore, PPP treatment caused strong regression of tumor grafts and prolonged survival of animals with systemic tumor disease. Here we demonstrate that PPP also downregulates the IGF-1R, whereas the IR and several other receptors were not affected. PPP-induced IGF-1R downregulation required expression of the MDM2 E3 ligase, which recently was found to ubiquitinate and cause degradation of the IGF-1R. In addition knockdown of beta-arrestin1, the adaptor molecule known to bridges MDM2 and IGF-1R, prevented downregulation of the receptor and significantly decreased PPP-induced cell death. All together these data suggest that PPP downregulates IGF-1R by interfering with the action of beta-arrestin1/MDM2 as well as the achieved receptor downregulation contributes to the apoptotic effect of PPP.