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Page 19 of 36 J Cancer Metastasis Treat 2019;5:31 I http://dx.doi.org/10.20517/2394-4722.2019.21

not restricted to the chromatin environment, such as the regulation of the cell cycle/apoptosis and, more
recently, a modulator of immune response. However, much remains unknown about the mechanism of
action of HDACs and their roles in the immune-biology of cancer. The non-specific nature of pan-HDAC
inhibitors results in a narrow therapeutic window of use, limiting the dose and duration due to toxicity.
Our group has focused in one specific HDAC, HDAC6, and shown that both the genetic abrogation and
pharmacological inhibition of this HDAC modulates the expression of a variety of immune-regulatory
proteins in the tumor microenvironment, including PD-L1, PD-L2, MHC class I, B7-H4 and TRAIL-R1.
We have previously demonstrated that both pharmacological inhibition and/or genetic abrogation of
HDAC6 plays a critical role in the immune check point blockade by down-regulating the expression of
PD-L1 and other check-point modulators such as PD-L2, B7-H4, etc. Moreover, we have also observed
that in vivo inhibition of HDAC6 reduces tumor growth in B16 and SM1 murine melanoma models within
syngeneic immunocompetent hosts. Additionally, we have found that the combination of low doses of
the HDAC6i Nexturastat A and checkpoint immune blockade therapies, including anti-PD-1 and anti-
CTLA4, result in an important improvement in anti-tumor responses in our murine model as evidenced
by the reduction of tumor growth when compared to treatment with individual stand-alone agents and the
improved modulation of various immune markers. Our studies have also demonstrated that tumors treated
with stand-alone check point inhibitor treatments, specifically anti-PD-1, results in a substantial increase
in the production of IFNγ and IL-2, resulting in the upregulation of PD-L1 and other critical checkpoints
in the immune blockade. Similar levels of IFNγ and IL-2 were found in the combination subject groups,
however, the levels of PD-L1 and PD-L2 were more comparable to the non-treated group. Overall, our
evidence suggests that HDAC6i has the potential for use as an adjuvant in ongoing therapeutic options
involving the immune check-point blockade.

24. Trisenox disrupts MDM2-DAXX-HAUSP complex, degrades MDM2, and activates p53 in

acute leukemia cells

Paul Bernard Tchounwou, Sanjay Kumar

Cellomics and Toxicogenomics Research Laboratory, NIH/NIMHD-RCMI Center for Environmental Health,
College of Science, Engineering and Technology, Jackson State University, Jackson, MS 39217, USA.

Trisenox (TX) has been successfully used in the treatment of both de novo and relapsed acute
promyelocytic leukemia (APL) patients. It inhibits APL cells growth efficiently through cell cycle arrest and
apoptosis, however exact molecular mechanisms of action poorly understood. In present study, we found
a new target of TX action that involves the activation of p53 and p21 expression through association with
death domain-associated protein (DAXX), disruption of MDM2-DAXX-HAUSP complex and degradation
of MDM2 in APL cells. TX-induced stress signal is transmitted by protein kinase (ATM & ATR) and
phosphorylation of CHK1 & CHK2 at Ser 345 and Thr68 residues leading to complex disruption and
accumulation of p53 in APL cells. TX-induced p53 caused cell cycle arrest by regulating expression of p21,
cyclins and cyclin dependent kinases proteins and forcing cells to undergo apoptosis by apoptotic proteins
expression modulation, mitochondrial membrane depolarization leading to caspase 3 activation. Our
immunoprecipitation studies also showed that the complex molecules were well associated in APL cells,
and TX disrupted their associations leading to accumulation of p53. We further studied the functional role
of p53 in disruption and expression of complex molecules in p53-knock down APL cells using lentiviral
shRNA approach. Taken together, our findings showed that TX activates p53, through association of
DAXX, disruption of MDM2-DAXX-HAUSP complex, MDM2 degradation in APL cells leading to cell
cycle arrest and apoptosis. This novel target of TX activity may be useful for designing new APL drugs.