Studies on the role of integrins in corticotroph tumorigenesis

Introduction: Integrins are heterodimeric transmembrane proteins composed of alpha and beta subunits that mediate cell-cell and cell-extracellular matrix (ECM) interactions. Several integrins are overexpressed in human cancers and their ECM recognition motif, arginine-glycine-aspartate (RGD), is being utilized for tumor imaging and targeting. Aim: To explore the expression and function of RGD-binding integrins in corticotroph tumors. Methods: We determined the expression of RGD-binding integrins by qPCR in 18 corticotroph tumors and compared transcript levels with gonadotroph tumors (n=16) and normal pituitaries (n=2). To study the role of integrins, we established their expression profile in murine corticotroph tumor AtT-20 cells by RT-PCR and investigated the effect of their inhibition with RNA interference on human POMC promoter activity and cell viability (WST-1 colorimetric assay). We used fluorescence microscopy to assess RGD peptide binding in these cells. Results: Corticotroph tumours express αv (ITGAV), β1 (ITGB1), β5 (ITGB5), β8 (ITGB8), and α8 (ITGA8). Integrins αv, β1, β5 are overexpressed in corticotroph compared to gonadotroph tumors, where expression was almost undetectable (P<0.0001) and human normal pituitary (P<0.001). The expression of β8 was higher in corticotroph only compared to gonadotroph tumors (P=0.04), but not to the normal pituitary. We found that AtT-20 cells express all these four integrins. Knocking down each αv, β1, and β5, decreased human POMC promoter activity compared to scramble control (% suppression 63±22, 54±23, and 69±28 respectively; P<0.05), while β8 had little effect. Knocking down αv and β1 had a small but significant effect on AtT-20 cell viability (% suppression 15.92±1.6 and 27.4±1.4 respectively; P