IGFBP2 and keratoconus: To investigate the mechanism by which IGFBP2 protects p21 by proteasome degradation, endogenous p21 was immunoprecipitated from protein lysates of IGFBP2-silenced pso KC or control cells treated with MG132 inhibitor, and then subjected to Western blotting analysis for the detection of its ubiquitination state.