Immunoblotting Procedure for Protein Analysis

Immunoblotting was performed as described previously^{39 (link)}. In short, cells were lysed and protein extracts were boiled and loaded on 10% polyacrylamide gels. After electrophoretic separation, the proteins were transferred to PVDF membranes, which were blocked with 5% milk powder in Tris-buffered saline + 0.1% Tween 20 (TBS-T) for 1 h. Incubation of primary antibody in TBS-T was performed at 4 °C overnight. Membranes were then washed and stained with secondary antibody. Chemiluminescence was elicited using Western Lightning Ultra from PerkinElmer (Waltham, MA, USA) or Clarity Western ECL Substrate from Bio-Rad Laboratories (Hercules, CA, USA), respectively, according to the manufacturers’ instructions. The following primary antibodies were used: anti-caspase 3, anti-cleaved caspase 3 (Asp175), anti-PARP, and anti-pCHK1(Ser345) (133D3) from Cell Signaling (Cambridge, UK), anti-CHK1 (G-4) and anti-POLD1 (A9) from Santa Cruz Biotechnology (Dallas, TX, USA), and peroxidase-conjugated anti-β-Actin (AC-15) from Sigma-Aldrich (Hamburg, Germany). HRP-conjugated anti-rabbit, anti-goat and anti-mouse antibodies from Santa Cruz Biotechnology were used as secondary antibodies.