The Role of C5b-9 Terminal Complement Complex in Activation of the Cell Cycle and Transcription

Abstract

Activation of the complement system plays an important role in innate and acquired immunity. Activation of complement and subsequent formation of C5b-9 channels on the surface of cellular membranes leads to cell lysis. When the number of channels assembled on the surface of nucleated cells is limited, C5b-9 does not cause lysis, but instead can induce cell-cycle progression by activating signal transduction pathways, transcription factors, and key components of the cell-cycle machinery. Cell-cycle induction by C5b-9 is dependent on the activation of phosphatidylinositol 3-kinase and the ERK1 pathway in a Gi protein-dependent manner. Cell-cycle activation is regulated, in part, by activation of proto-oncogene c-jun and AP1 DNA binding activity. C5b-9 induces sequential activation of CDK4 and CDK2, leading to G1/S-phase transition and cellular proliferation. RGC-32 is a novel gene whose expression is induced by C5b-9. RGC-32 may play a key role in cell-cycle activation by increasing cyclin B1-CDC2 activity. C5b-9-mediated cell-cycle activation plays an important role in cellular proliferation and protection from apoptosis.