Background & aims: Colon cancers with high-frequency microsatellite instability (MSI-H) develop frameshift mutations in tumor suppressors as part of their pathogenesis. ACVR2 is mutated at its exon 10 polyadenine tract in >80% of MSI-H colon cancers, coinciding with loss of protein. ACVR2 transmits the growth effects of activin via phosphorylation of SMAD proteins to affect gene transcription. The functional effect of activin in colon cancers has not been studied. We developed and characterized a cell model in which we studied how activin signaling affects growth.
Methods: hMLH1 and ACVR2 mutant HCT116 cells were previously stably transferred with chromosome 2 (HCT116+chr2), restoring a single regulated copy of wild-type ACVR2 but not hMLH1. Both HCT116+chr2 and parental HCT116 cells (as well as HEC59 and ACVR2 and hMSH2 complemented HEC59+chr2 cells) were assessed for genetic complementation and biologic function.
Results: HCT116+chr2 cells and HEC59+chr2 cells, but not ACVR2-mutant HCT116 or HEC59 cells, acquired wild-type ACVR2 as well as expression of ACVR2 wild-type messenger RNA. Complemented ACVR2 protein complexed with ACVR1 with activin treatment, generating nuclear phosphoSMAD2 and activin-specific gene transcription. ACVR2-restored cells showed decreased growth and reduced S phase but increased cellular migration following activin treatment. ACVR2 small interfering RNA reversed these effects in complemented cells.
Conclusions: ACVR2-complemented MSI-H colon cancers restore activin-SMAD signaling, decrease growth, and slow their cell cycle following ligand stimulation but show increased cellular migration. Activin is growth suppressive and enhances migration similar to transforming growth factor beta in colon cancer, indicating that abrogation of the effects of activin contribute to the pathogenesis of MSI-H colon cancers.