title: 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.
authors.affiliation: Section of Oncology, Department of Oncology, Radiology and Clinical Immunology, Uppsala University Hospital, Uppsala, Sweden. Xuping_wu@yahoo.com
authors.forename: Xuping
authors.initials: X
authors.lastname: Wu
authors.forename: Linda
authors.initials: L
authors.lastname: Sooman
authors.forename: Malin
authors.initials: M
authors.lastname: Wickström
authors.forename: Mårten
authors.initials: M
authors.lastname: Fryknäs
authors.forename: Christine
authors.initials: C
authors.lastname: Dyrager
authors.forename: Johan
authors.initials: J
authors.lastname: Lennartsson
authors.forename: Joachim
authors.initials: J
authors.lastname: Gullbo
mesh_terms: D000970:Antineoplastic Agents / Q000633:toxicity*; D002294:Carcinoma, Squamous Cell / Q000378:metabolism; D045744:Cell Line, Tumor; D004305:Dose-Response Relationship, Drug; D066246:ErbB Receptors / Q000037:antagonists & inhibitors / Q000378:metabolism; D004938:Esophageal Neoplasms / Q000378:metabolism; D006801:Humans; D008870:Microtubules / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D011034:Podophyllotoxin / Q000031:analogs & derivatives* / Q000633:toxicity; D047428:Protein Kinase Inhibitors / Q000494:pharmacology; D017526:Receptor, IGF Type 1 / Q000037:antagonists & inhibitors* / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects; D050257:Tubulin Modulators / Q000494:pharmacology
chemical_list: D000970:Antineoplastic Agents; D047428:Protein Kinase Inhibitors; D050257:Tubulin Modulators; C415032:picropodophyllin; D066246:ErbB Receptors; D017526:Receptor, IGF Type 1; D011034:Podophyllotoxin