Alternative Cytotoxic Effects of the Postulated IGF-IR Inhibitor Picropodophyllin In Vitro

Abstract

The insulin-like growth factor-1 (IGF-I) and its receptors play an important role in transformation and progression of several malignancies. Inhibitors of this pathway have been developed and evaluated but generally performed poorly in clinical trials, and several drug candidates have been abandoned. The cyclolignan picropodophyllin (PPP) has been described as a potent and selective IGF-IR inhibitor and is currently undergoing clinical trials. We investigated PPP's activity in panels of human cancer cell lines (e.g., esophageal squamous carcinoma cell lines) but found no effects on the phosphorylation or expression of IGF-IR. Nor was the cytotoxic activity of PPP related to the presence or spontaneous phosphorylation of IGF-IR. However, its activity correlated with that of known tubulin inhibitors, and it destabilized microtubule assembly at cytotoxic concentrations also achievable in patients. PPP is a stereoisomer of podophyllotoxin (PPT), a potent tubulin inhibitor, and an equilibrium between the two has previously been described. PPP could thus potentially act as a reservoir for the continuous generation of low doses of PPT. Interestingly, PPP also inhibited downstream signaling from tyrosine kinase receptors, including the serine/threonine kinase Akt. This effect is associated with microtubule-related downregulation of the EGF receptor, rather than the IGF-IR. These results suggest that the cytotoxicity and pAkt inhibition observed following treatment with the cyclolignan PPP in vitro result from microtubule inhibition (directly or indirectly by spontaneous PPT formation), rather than any effect on IGF-IR. It is also suggested that PPT should be used as a reference compound in all future studies on PPP. Mol Cancer Ther; 12(8); 1526-36. Ó2013 AACR.

Picropodophyllin inhibits proliferation and survival of diffuse large B-cell lymphoma cells

“…Neither did the activity of PPP correlate with the expression level of IGF-1R in the different cell lines. Instead, the inhibitory effect of PPP correlated with the activities of the microtubule inhibitors PPT and colchicine, indicating that the highly dynamic microtubule network may constitute a target for PPP as recently suggested [27,28]. Furthermore, treatment of four DLBCL cell lines with either PPT or colchicine at 0.5 lM closely mimicked the alterations in cell cycle phase distribution resulting from incubation with PPP at the same concentration.…”

Picropodophyllin causes mitotic arrest and catastrophe by depolymerizing microtubules via Insulin-like growth factor-1 receptor-independent mechanism

“…10 μM), has an effect on microtubules, leading to blurry tubular staining in micrographs taken after 24 or 48 hours [35]. The authors suggested that this effect is due to PPP binding weakly to b-tubulin at the same location as colchicine and the PPP diastereomer podophyllotoxin (PPT).…”

Inhibition of type I insulin-like growth factor receptor tyrosine kinase by picropodophyllin induces apoptosis and cell cycle arrest in T lymphoblastic leukemia/lymphoma

“…Wu et al recently reported that, in human esophageal squamous cell carcinoma cell lines, the cytotoxicity and downregulation of pAkt observed after treatment with PPP result from microtubule inhibition (directly or indirectly through spontaneous PPT formation), rather than any effect of PPP on IGF-IR [51]. From our results, PPP efficiently blocked IGF-IR activity, reduced the phosphorylation of Akt (pAkt) and Erk1/2 (pErk1/2), induced apoptosis and G2/M cell cycle arrest in Jurkat and Molt-3 cells.…”