IGF2R: a new player in the IGF2 loop sustaining adrenocortical carcinogenesis

Most of adrenocortical carcinomas (ACC) are characterized by IGF2 overexpression; therefore, several studies have focused on its two oncogenic effectors, the tyrosine-kinase receptors IGF1R and IR. However, the specific IGF2 receptor, IGF2R, due to its scavenging activity, was considered a tumour suppressor and has never been fully investigated in this context. Nevertheless, recent evidence from other tumours identified IGF2R as a pro-tumorigenic actor able to exert its function through the downstream activation of the pro-mitotic sphingosine kinases (SphK) enzymes responsible for sphingosine phosphorylation. Hence, the main aims of this study were to elucidate the role of IGF2R in ACC cells, investigate its action mechanism, and test IGF2R and SphK inhibitors as possible novel therapies for ACC. The present study was conducted in vitro in 4 different ACC cell lines and in 3 ACC primary cultures, to reflect the heterogeneity of ACC. Target protein and transcript expression were evaluated on normal and tumoral adrenal tissues and cell lines, while the effects of IGF2R downregulation and overexpression, along with IGF2R and SphK inhibition, were tested on cell viability, proliferation, and apoptosis as well as cortisol secretion. ACC tissue analysis demonstrated the IGF2R overexpression compared to normal adrenal cortex, which was also positively correlated with IGF2 and IGF1R expression in ACC. In vitro assays proved the pro-mitotic involvement of IGF2R and its role in the downstream activation of SphK. Finally, we observed the anti-tumoral efficacy of three different inhibitors of this pathway on ACC cell lines and primary cultures: a specific neutralizing anti-IGF2R antibody was tested for its anti-proliferative action, while two SphK inhibitors, safingol (SAF) and fingolimod (FTY), were able to decrease cell proliferation, viability, and cortisol secretion, while promoting cell apoptosis. Overall, this comprehensive in vitro evaluation of the role of IGF2R in ACC demonstrates its tumorigenic effect through SphK activation. Moreover, the three pharmacological strategies here employed successfully controlled in vitro ACC growth, suggesting IGF2R/SphK pathway represents a novel therapeutic target for ACC.