Phenylhexyl isothiocyanate has dual function as histone deacetylase inhibitor and hypomethylating agent and can inhibit myeloma cell growth by targeting critical pathways
Journal of Hematology & Oncology
Phenylhexyl isothiocyanate has dual function as histone deacetylase inhibitor and hypomethylating agent and can inhibit myeloma cell growth by targeting critical pathways
Quanyi Lu 1 2
Xianghua Lin 2
Jean Feng 2
Xiangmin Zhao 2
Ruth Gallagher 2
Marietta Y Lee 0
Jen-Wei Chiao 2
Delong Liu 2
0 Department of Biochemistry and Molecular Biology, New York Medical College , Valhalla, NY 10595 , USA
1 Department of Hematology, Zhongshan Hospital of Xiamen University , Xiamen, Fujian Province , PR China
2 Division of Hematology/Oncology, New York Medical College , Valhalla, NY 10595 , USA
Histone deacetylase (HDAC) inhibitors are a new class of chemotherapeutic agents. Our laboratory has recently reported that phenylhexyl isothiocyanate (PHI), a synthetic isothiocyanate, is an inhibitor of HDAC. In this study we examined whether PHI is a hypomethylating agent and its effects on myeloma cells. RPMI8226, a myeloma cell line, was treated with PHI. PHI inhibited the proliferation of the myeloma cells and induced apoptosis in a concentration as low as 0.5 M. Cell proliferation was reduced to 50% of control with PHI concentration of 0.5 M. Cell cycle analysis revealed that PHI caused G1-phase arrest of RPMI8226 cells. PHI induced p16 hypomethylation in a concentration- dependent manner. PHI was further shown to induce histone H3 hyperacetylation in a concentration-dependent manner. It was also demonstrated that PHI inhibited IL-6 receptor expression and VEGF production in the RPMI8226 cells, and reactivated p21 expression. It was found that PHI induced apoptosis through disruption of mitochondrial membrane potential. For the first time we show that PHI can induce both p16 hypomethylation and histone H3 hyperacetylation. We conclude that PHI has dual epigenetic effects on p16 hypomethylation and histone hyperacetylation in myeloma cells and targets several critical processes of myeloma proliferation.
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Background
Despite many recent advances in treatment, multiple
myeloma (MM) remains as an incurable disease without
an allogeneic hematopoietic cell transplantation. The
emergence of drug resistance and incomplete responses
have been the major obstacles for improving the
treatment results [1,2]. The new treatment strategies have been
based largely upon targeting specific molecules or
pathways, such as proteosome inhibitors and thalidomide
analogs. Aberrant methylation of gene promoter regions
is a widely studied epigenetic process in malignant
disorders. Cell cycle inhibitors of p15 and p16 are the tumor
suppressor genes frequently affected by this epigenetic
change [3,4]. The aberrant methylation of gene promoter
regions is associated with loss of gene function. In
addition to gene deletions and mutations, quantitative
changes in gene methylation status play a significant role
in tumorigenesis [5]. Hypermethylation of p15 and p16
promoter CpG islands has been reported in MM clinical
specimens and myeloma cell lines [4,6,7]. The
methylation status of p15 and p16 genes were not significantly
different between MM and MGUS (monoclonal
gammopathy of unknown significance) nor in pre-treated
and post-treated patients with MM [6-8]. It was further
demonstrated in MM patients that p16 hypermethylation
is associated with high plasma cell proliferation, higher
2-microglobulin concentration, and shorter survival,
whereas no such clear correlation was found with p15
CpG island hypermethylation [4,7,9].
The proliferation and survival of myeloma cells are also
potentiated by IL-6 and IL-6 receptor signal transduction
through autocrine and paracrine stimulation [10,11].
Exogenous IL-6 was able to block the apoptosis induced
by the chemotherapeutic agent dexamethasone [10,12].
Increased angiogenesis and microvascular density in the
bone marrow microenvironment correlate with poor
prognosis and drug resistance of myeloma cells [13-15].
Cytokines that augment angiogenesis are known to be
present at elevated levels in the bone marrow. The
vascular endothelial growth factor (VEGF) is one of those
elevated cytokines associated with angiogenesis.
Thalidomide and its derivative, lenalidomide (CC-5013,
Revlimid; Celgene), are inhibitors of angiogenesis and are
widely used for MM therapy [1].
In the search for novel molecular targets, histone
deacetylases (HDACs) that affect epigenetic processes have
emerged as one of the potential targets [16,17]. Recent
studies have indicated that the expression of various genes
that regulate differentiation, proliferation, and apoptosis
are also influenced by HDACs. Aberrant histone
acetylation appears to play an important role in the development
of numerous malignancies [18,19]. Agents that modify
histone acetylation thus show great promise against
various malignancies [20-26]. Vorinostat (Suberoylanilide
hydroxamic acid, SAHA, Zolinza; Merck) is among the
first HDAC inhibitors approved for clinical treatment of
cutaneous T cell lymphoma [27,28]. Our laboratory has
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