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Page 2 of 11 Jiang et al. Hepatoma Res 2019;5:5 I http://dx.doi.org/10.20517/2394-5079.2018.97

as PMI. More importantly, this stapling functionally rescued PMI that, on its own, failed to activate p53 because of its
poor membrane permeability and susceptibility to proteolytic degradation.

Conclusion: Taken together, this work not only illustrates that the restoration of p53 is a potentially feasible program for
HCC therapy, but promises an important new tool for peptide drug discovery and development for a variety of human
diseases.

Keywords: Hepatocellular carcinoma, p53, stapled peptide, dimethylbromobenzene-cysteine

INTRODUCTION
p53 acts as a tumor suppressor by initiating cell-cycle arrest, apoptosis, and senescence in response to
[1]
cellular stress to maintain the integrity of the genome . In a substantial proportion of cancers, p53 is
wild type but the protein is inactivated; this offers an attractive strategy for cancer therapy based on p53
[2,3]
[2]
reactivation . Recent studies in cancer patients have provided proof-of-concept for this approach . Such
activators are the product of basic research conducted over the past 20 years that has led to the appreciation
of MDM2 and MDMX as the two major negative regulators of p53, which now seem to be “druggable” using
[4]
a variety of strategies .

Of all human cancers, hepatocellular carcinoma (HCC) is the fifth most frequently diagnosed cancer
[5]
worldwide and is the third leading cause of cancer death globally . Yet, the current most common treatment
for HCC is surgical resection, which is less effective than those used against other cancers . Because the loss
[1]
of p53 function plays a critical role in multistage hepatocarcinogenetic, the p53 gene has been regarded as
[6]
a good candidate for modulating HCC risk . Furthermore, the top two risk factors of HCC are metabolic
disease (such as fatty liver) and viral infection (such as hepatitis B and C), both of which cause cirrhosis
before HCC . As one of the hallmarks of cancer, the changes observed in cancer cell metabolism and
[7,8]
bioenergetics are also regulated by p53 [9,10] . Therefore, the connection between p53 stress response and the
disordered metabolic process leading to HCC is a potential avenue for HCC therapies.

Several classes of molecules that inhibit this interaction between p53/MDM2 (MDMX) have been developed
(e.g., Nutlin and MI-219) [11,12] . They mimic the conserved residues from a region of the p53 N-terminal that
are functional for the interaction with the N-terminal p53 binding domain of MDMX or MDMX . This
[4]
region forms an α-helix upon binding, enabling the three conserved hydrophobic residues of the MDM2
binding motif (F19, W23, and L26) to optimally embed into the hydrophobic binding groove located on
MDM2 and its homologous MDMX protein [13,14] . Except for small molecules, it has been proved that the p53
peptide is appropriate as a biological tool and prototype therapeutic by enforcing its R-helical structure while
[15]
preserving the key interacting residues that enable specific MDM2 and MDMX engagement .

As the wild-type p53 peptide (ETFSDLWKLLPE) has a low affinity for MDM2/MDMX and comes from a
region of p53 that interacts with many other proteins , we explored the effects of stapling a peptide derived
[15]
[14]
from phage selection experiments . Phage display and rational design methods have been used to isolate
[13]
linear peptides that bind MDM2 with high affinity . The most avid of these published peptides, described
[14]
by Pazgier et al. , named p53-MDM2/MDMX inhibitor (PMI), was used as the template for this study.
Besides, when PMI helices are taken out of protein context and placed into aqueous buffer in isolation, it
usually adopts random coil conformations, leading to a drastic reduction in biological activity and thus
diminishing therapeutic potential [14,16] . To overcome it, numerous strategies have been developed to stabilize
or mimic peptide helices [17-19] . Among these, the most straightforward, yet effective, strategy is sidechain
[16]
cross-linking (‘‘peptide stapling’’) . Since peptide stapling necessitates macrocyclization, an entropically
unfavorable process, very few reactions are known to date that give rise to good yields along with the