The protumorigenic insulin-like growth factor (IGF)-II is highly expressed in a significant fraction of human hepatocellular carcinomas (HCC). However, a functional dissection that clarifies the contribution of IGF-II-binding receptors in tumor progression and a respective molecular characterization of IGF-II signaling has not been performed. Therefore, expression of IGF-II and its receptors IGF-receptor type I (IGF-IR) and insulin receptor (IR) was efficiently blocked using small interfering RNA (siRNA) in HCC cells. Despite functional IR-signaling, oncogenic IGF-II effects such as tumor cell viability, proliferation, and anti-apoptosis were solely transmitted by IGF-IR. Although IGF-II signaling was previously not described in the context of HCC cell migration, the IGF-II-dependent expression profile displayed a high percentage of genes involved in cell motility and adhesion. Indeed, IGF-II overexpression promoted HCC cell migration, especially in synergy with hepatocyte growth factor (HGF). The therapeutic relevance of IGF-II/IGF-IR signaling was tested in vitro and in a murine xenograft transplantation model using the IGF-IR inhibitor picropodophyllin (PPP). IGF-IR inhibition by small molecule treatment efficiently reduced IGF-II-dependent signaling and all protumorigenic properties of the IGF-II/IGF-IR pathway.
Conclusion: In human HCC cells, IGF-IR but not IR is involved in oncogenic IGF-II signaling. Autocrine stimulation of IGF-II induces HCC motility by integration of paracrine signals for full malignant competence. Thus, activation of IGF-II/IGF-IR signaling is likely a progression switch selected by function that promotes tumor cell dissemination and aggressive tumor behavior.