Rac GTPase regulation of 3D invasion in neuroblastomas lacking MYCN amplification

Neuroblastomas are highly invasive tumors that occur in pediatric patients and treatment of invasive disease remains a challenge. The study of cells invading in 3-dimensional (3D) hydrogels has revealed morphologically distinct modes of invasion by which cancer cells adapt to the local tissue environment in order to invade local tissue. Specifically, the small G protein Rac GTPase has been implicated as regulating the elongated/mesenchymal mode of cell invasion. In the present study we demonstrate an inverse association between Rac expression and amplification of MYCN, a well-established prognostic indicator in neuroblastoma. Moreover, the association further tracks with previously described morphological variants of neuroblastoma. Importantly, while MYCN amplification is associated with universally poor prognosis, the clinical course of patients whose tumors lack MYCN amplification are more difficult to predict. Therefore, we analyzed the role that Rac plays in regulating the invasive behavior of neuroblastoma cells lacking MYCN amplification. Using siRNA targeting Rac in single cell suspensions in 3D collagen gels and Rac inhibition of multicellular spheroids (MCS) embedded in collagen gels, we find that the high Rac-expressing lines differ in their morphological response to Rac depletion and inhibition. Live cell imaging of embedded MCS reveals distinct individual and collective modes of invasion between the cell lines. Critically, Rac inhibition blocked both individual and collective invasion in 2 of the 3 high Rac expressing cell lines. Our study suggests that Rac activity may be an important determinant of metastatic capability in subsets of neuroblastoma cells lacking MYCN amplification.