PNC-28: a p53 peptide for attacking the cancer cells through membrane disruption
I met with Dr Matthew Pincus, my mentor from the clinical pathology days, to discuss his latest publication in the Int. J. Cancer (2006) about a novel method for an anti-cancer therapy. You can read a press release describing this article at the SUNY Downstate Web site, and you can read the abstract which was published online in May of last year at Wiley Interscience.
Essentially, his team found that the peptide PNC-28 killed "a variety of human tumor cell lines," and a ras-transformed rat pancreatic carcinoma cell line, but had no effect on untransformed (normal) cell lines.
Now, allow me to provide the backstory to how this potential cancer therapy was discovered which you won't read in the journal article.
The image shows the molecular model of PNC-28, a peptide derived from p53, the protein which plays several important roles in acting as a tumor suppressor. These roles include activating DNA repair proteins when DNA damage has occurred, arresting the cell cycle at G1/S point so that repair can take place, and/or initiate apoptosis (cell death) in the case where the DNA damage cannot repaired.
In designing this peptide, Dr Pincus told me that the original intent was to target ubiquitin ligase, an enzyme which normally degrades p53. The idea was to do this by producing a peptide that was similar to the mdm-2-binding region of p53. Mdm-2, or hdm-2 in humans, inhibits the action of p53 and allows it to move from the cell's nucleus to the cytosol where it undergoes proteolysis by the ubiquitination. Blocking the mdm-2 interaction with p53 with PNC-28 should prolong the half-life of wild-type p53.
His rational design efforts were aimed at placing p53's penetratin part (this allows it to penetrate cell membranes) at the carboxy-terminal (C-terminal) end of of the PNC-28 peptide. The reason for this site and not the N-terminal of the peptide was to utilize the predominant positive charges at the C-terminal to stabilize the alpha helices. This was to preserve the peptide folding seen in the x-ray crystallography of p53 binding to mdm-2. (PNC-28 with the penetratin part at the N-terminal was also produced and tested, but it was not as effective on tumor cell lines.)
"We had the right answer using the wrong logic," said Dr Pincus when he described how the structure of PNC-28 might contribute to its role as a therapeutic agent. The positive charges at the C-terminal actually stabilized a helix-loop-helix structure which is amphipathic (containing polar and non-polar domains, somewhat in the way soap works), and therefore can exist within the lipid bilayer of the cell membrane. When this agent worked in a human tumor cell line that had a homozygous deletion of p53, this suggested that the action of PNC-28 had nothing to do with blocking the binding of mdm-2 to p53. Plus, when tests were done to check for markers of apoptosis, they weren't there. PNC-28 was causing necrosis, but not apoptosis of the tumor cell lines. This is believed to be accomplished through cell membrane disruption, perhaps analogous to the way the membrane attack complex of complement works. However, again, PNC-28 has no effect on untransformed cells, and no effect on the differentiation of human stem cells.
Why the predilection for cancer cells, then?
Comments