Lamina-associated polypeptide 2α regulates cell cycle progression and differentiation via the retinoblastoma-E2F pathway

Lamina-associated polypeptide (LAP) 2 alpha is a non-membrane-bound LAP2 isoform that forms complexes with nucleoplasmic A-type lamins. In this study, we show that the overexpression of LAP2 alpha. in fibroblasts reduced proliferation and delayed entry into the cell cycle from a GO arrest. In contrast, stable downregulation of LAP2 alpha by RNA interference accelerated proliferation and interfered with cell cycle exit upon serum starvation. The LAP2 alpha-linked cell cycle phenotype is mediated by the retinoblastoma (Rb) protein because the LAP2 alpha COOH terminus directly bound Rb, and overexpressed LAP2 alpha inhibited E2F/Rb-dependent reporter gene activity in G1 phase in an Rb-dependent manner. Furthermore, LAP2 alpha associated with promoter sequences in endogenous E2F/Rb-dependent target genes in vivo and negatively affected their expression. In addition, the expression of LAP2 alpha in proliferating preadipocytes caused the accumulation of hypophosphorylated Rb, which is reminiscent of noncycling cells, and initiated partial differentiation into adipocytes. The effects of LAP2 alpha on cell cycle progression and differentiation may be highly relevant for the cell- and tissue-specific phenotypes observed in laminopathic diseases.