Rb Protein is Essential to the Senescence-Associated Heterochromatic Foci Formation Induced by HMGA2 in Primary WI38 Cells

Cellular senescence is an irreversible form of cell cycle arrest that provides a barrier to neoplastic transformation. The integrity of the Rb (Retinoblastoma) pathway is necessary for the formation of the senescence-associated heterochromatin foci (SAHF) that offers a molecular basis for the stability of the senescent state. Surprisingly, although high mobility group A2 protein (HMGA2) can promote tumorigenesis and inhibit Rb function in tumor cells, high-level expression of HMGA2 is sufficient to induce SAHF formation in primary cells. It therefore becomes significant to determine whether Rb protein is necessary in HMGA2-induced SAHF formation. In this study, we established the cellular senescence and SAHF assembly WI38 cell model by ectopic expression of HMGA2, in which typical senescent markers were seen, including notable upregulation of p53, p21 and p16, and elevated SA-β-galactosidase staining together with downregulation of E2F target genes. We then showed that the Rb pathway inhibitor E7 protein was able to partly abolish the ability of SAHF formation after HMGA2 expression in WI38 cells, indicating that Rb is a crucial factor for HMGA2-induced SAHF formation. However, Rb depletion did not completely rescue the cell growth arrest induced by HMGA2, suggesting that Rb is not an exclusive pathway for HMGA2-induced senescence in WI38 cells.