You are studying expression of the pro-apoptotic protein p53 in different cells lines.  You lyse a sample of each cell line, separate the proteins by SDS-PAGE and then probe the gel with an anti-p53 antibody; an anti-alpha actin antibody is also used to probe the membrane.  The results from the Western blot are shown in the figure below:

Explain what is shown by these results and be sure to address both p53 and alpha-actin.