Identifying, Modeling and Targeting IGF-II Overexpression in the Adrenocortical Carcinoma.

Abstract

Insulin-like growth factor-II (IGF-II) is a mitogenic growth factor that elicits cellular effects primarily through its downstream signaling pathway. The IGF signaling pathway is a critical cellular system for stimulating growth and preventing apoptosis and is implicated in development and progression of various cancers, including adrenocortical carcinoma (ACC). ACC is a rare endocrine malignancy that carries a poor clinical prognosis, and lacks effective treatment options. Recently, there is emerging data regarding the molecular determinants involved in ACC initiation and/or progression through examination of the gene expression profiles and of the familial ACC syndromes. From these data, one particular molecular alteration linked to ACC is the loss of imprinting (LOI) within the 11p15.5 chromosomal region, which results in overexpression of the gene encoding IGF-II (named IGF2). We hypothesize that IGF2 upregulation plays a critical role in ACC pathogenesis. We tested our hypothesis first by validating that our ACC samples possessed LOI-mediated IGF2 upregulation with concomitant active IGF signaling and then identified which regulatory elements in the IGF2 gene locus affect its expression. Next, we commenced modeling LOI to upregulate IGF2 expression specifically in the mouse adrenal cortex. Although, LOI did not result in ACC, we identified IGF2-mediated activation of canonical Wnt signaling, another molecular perturbation commonly associated with ACC pathogenesis. The activation of IGF and Wnt signaling pathways simultaneously in the mouse adrenal cortex resulted in a more dramatic dysplastic phenotype compared to both LOI adrenals and constitutive Wnt adrenals. Finally, we tested our hypothesis by targeting the IGF-1R (the receptor through which IGF-II elicits its effects) in H295 cells, a cell line derived from a human ACC and possessing constitutive IGF signaling. Targeting resulted in specific growth inhibition in vitro and also in vivo using mouse xenografts. We observed targeting IGF signaling inhibition in combination with mitotane, the only FDA approved drug for ACC treatment in patients, resulted in greater growth inhibition in vitro and in vivo when compared either agent alone. Taken together, this findings support our hypothesis that IGF2 upregulation plays a critical role in ACC pathogenesis.