Differentiation Therapy – Epigenetics Tackles Solid Tumors
Author-Writer: Stephen J. Williams, Ph.D.
Updated 4/27/2021
Word Cloud By Danielle Smolyar
Genetic and epigenetic events within a cell which promote a block in normal development or differentiation coupled with unregulated proliferation are hallmarks of neoplastic transformation. Differentiation therapy is a chemotherapeutic strategy directed at re-activating endogenous cellular differentiation programs in a tumor cell therefore driving the cancerous cell to a state closer resembling the normal or preneoplastic cell and therefore incurring loss of the tumorigenic phenotype.
This post will deal with:
- Agents such as histone deacetylase inhibitors (HDACi), retinoids, and PPARϒ agonists which have been shown to reactivate terminal differentiation programs in solid tumors
- Clinical trials in solid tumors
- Issues regarding the use of differentiation therapy in solid tumors
This post is a follow-up post to Histone Deacetylase Inhibitors Induce Epithelial-to-Mesenchymal Transition in Prostate Cancer Cells
To put the need for alternate chemotherapeutic strategies in perspective, one is referred to the National Cancer Statistics from http://www.cancer.gov show that 33% of cancer patients, treated with standard cytolytic chemotherapy, will still die within five years (i.e. one in three will die within 5 years). However the addition of the differentiation agent retinoic acid to standard chemotherapy regimen for treatment of acute promyelocytic leukemia (APML) had improved 5 year survival rates from a range of 50-80% up to near 90% complete remission rates while 75% become disease free, an astonishing success story. For a review of APML please be referred to http://en.wikipedia.org/wiki/Acute_promyelocytic_leukemia. Briefly, APML is predominantly a result of the chromosomal translocation producing a fusion gene between the promyelocytic leukemia (PML) and RARα receptor genes. The PML-RARα fusion protein recruits transcriptional repressors, histone deacetylases (HDACs), and DNA methyltransferases. Treatment with pharmacologic doses of retinoic acid dissociates the PML-RARα from HDACs and results in degradation of PML-RARα, eventually resulting in the differentiation of the myeloid cells in APML.
Dr. Igor Matushansky of Columbia University believes such differentiation therapy could be useful in soft tissue sarcomas, due to the existence of a connective tissue (mesenchymal) stem cell, in vitro methods which can differentiate these cells into mature tissues, and, from a gene clustering analysis his group had performed, correlation of expression signatures of each liposarcoma subtype throughout the adipocytic differentiation spectrum, including early differentiated to more mature differentiated cells(1). A parallel study by Riester and colleagues had been able to classify breast tumors and liposarcomas along a phylogenetic tree showing solid tumors can be reclassified based on cell of origin via expression patterns(2). In addition, other solid tumors, such as ovarian cancer are easily classified, based both on pathologic, histologic, and expression analysis into well and poorly differentiated tumors, correlating differentiation status with prognosis.
Compound Classes which have potential in
differentiation therapy for solid tumors
A. Histone Deacetylase Inhibitors (HDACi)
In eukaryotes, epigenetic post-translational modification of histones is critical for regulation of chromatin structure and gene expression. Histone deacetylation leads to chromatin compaction and is associated with transcriptional repression of tumor suppressors, cell growth and differentiation. Therefore, HDACi are promising anti-tumor agents as they may affect the cell cycle, inhibit proliferation, stimulate differentiation and induce apoptotic cell death (3). In a review by Kniptein and Gore, entinostat was found to be a well-tolerated HDACi that demonstrates promising therapeutic potential in both solid and hematologic malignancies(4). The path to the discovery of suberoylanilide hydroxamic acid (SAHA, vorinostat) began over three decades ago with our studies designed to understand why dimethylsulfoxide causes terminal differentiation of the virus-transformed cells, murine erythroleukemia cells. SAHA can cause growth arrest and death of a broad variety of transformed cells both in vitro and in vivo at concentrations that have little or no toxic effects on normal cells (for references see (5). In fact, treatment of MCF-7 breast carcinoma cells with SAHA resulted in morphologic changes resembling epithelial mammary differentiation(6).
Figure. Structures of some HDACi used in clinical trials for cancer (see section below)
Figure. HDAC with SAHA
B. Retinoids
Vitamin A and retinoids play significant roles in basic physiological processes such as vision, reproduction, growth, development, hematopoiesis and immunity (7). Retinoids are the natural derivatives and synthetic analogs of vitamin A. They have been shown to prevent mammary carcinogenesis in rodents (8), to inhibit the growth of human cancer cells in vitro (9,10) and be effective chemopreventive and chemotherapeutic agents in a variety of human epithelial and hematopoietic tumors (11-14).
Retinoids cannot be synthesized de novo by higher animals and consequently must be consumed in the diet. The two sources of retinoids are animal products that contain retinol and retinyl esters, and plant-derived carotenoids (provitamin A). b-carotene is the most potent vitamin A precursor and has been shown to be an active inhibitor of both tumor initiation and promotion (15).
A major function of retinol, relevant to cancer, is its function as an antioxidant. The antioxidant properties of vitamin A have been shown both in vitro and in vivo (16,17). Retinol deficiency causes oxidative damage to liver mitochondria in rats that can be reversed by vitamin A supplementation (18). A caveat to this is in vitro and in vivo evidence of chronic hypervitaminosis A inducing oxidative DNA damage, as well (19-21). Therefore, it is evident that maintaining the vitamin A concentration within a physiological range is critical to normal cell function because either a deficiency or an excess of vitamin A induces oxidative stress (22). Retinoic acids (RA) (all-trans, 9-cis and 13-cis) are the major biologically active retinoids and exert their effects by regulation of gene expression by binding two families of ligand-activated nuclear retinoid receptors (23). Retinoic acid receptors (RARs) and retinoid X receptors (RXRs) regulate the transcription of a large number of target genes that contain retinoic acid response elements (RAREs) in their promoters. Many of these genes are involved in cancer (13,24) and differentiation (24-26).
Several lines of evidence suggest involvement of defects in retinol signaling in cancer, from the observation that a vitamin A-deficient (VAD) diet leads to an increase in the number of spontaneous and chemically induced tumors in animals (27-29) to the observation that RA itself can induce differentiation and inhibit the growth of many tumor cells (30-32), as well as the identification that components of the RA signaling pathway are absent in cancer cells (33). Vitamin A and its metabolites have been proposed to have a dual effect in cancer prevention, as antioxidants (16,17,19,34) and differentiating agents (35-37). as it is well accepted that retinoid signaling is integral in maintaining the differentiated state of many cell types (13,38). Additionally, current rationale for chemoprevention with retinoids is based, in part, on the hypothesis that some tumors, may arise due to loss of normal somatic differentiation during tissue repair.
C. PPARϒ Agonists
Peroxisome proliferator-activated receptor ϒ (PPARϒ) is a member of the steroid hormone receptor superfamily that responds to changes in lipid and glucose homeostasis but has increasing roles in differentiation and tumorigenesis. The first PPAR (PPARα) was discovered during the search of a molecular target for a group of agents then referred to as peroxisome proliferators, as they increased peroxisomal numbers in rodent liver tissue, apart from improving insulin sensitivity. One of the first agents, developed in the early 80’s for treatment of hyperlipidemia and hperlipoproteinemia, was clofibrate. All PPAR subtypes heterodimerize with the retinoid-x-receptor (RXR) and, upon binding of ATRA, activate target genes.
PPARϒ agonists have shown potential as a therapeutic in a variety of cancer types including bladder cancer (39), colon cancer(40), breast cancer(41), prostate cancer(42). There are numerous studies showing that PPARϒ agonists have anti-tumorigenic activity via anti-proliferative, pro-differentiation and anti-angiogenic mechanisms of action(43). For example, Papi et al. observed that agonists for the retinoid X receptor (6-OH-11-O-hydroxyphenanthrene), retinoic acid receptor (all-trans retinoic acid (RA)) and peroxisome proliferator-activated receptor (PPAR)-γ (pioglitazone (PGZ)), reduce the survival of MS generated from breast cancer tissues and MCF7 cells, but not from normal mammary gland or MCF10 cells(44) with concomitant upregulation of differentiation markers.
A great website for further information on PPAR is Dr. Jack Vanden Heuvel, Professor of Toxicology at Penn State University at http://ppar.cas.psu.edu/general_information.html.
D. Trabectedin
Trabectedin (ecteinascidin-743 (ET-743); Yondelis) is derived from the Caribbean tunicate Ecteinascidia turbinacta has antitumor activity by binding to the DNA minor groove thus disrupting binding of transcription factors and inhibiting DNA synthesis. However, it has also been shown, in myxoid liposarcoma (MLS) cells, to cause dissociation of transcription factor TLS-CHOP from promoter sequences resulting in downregulation of target genes such as CHOP, PTX3 and FN1 and induces an adipogenic differentiation program by enhancing activation of CAAT/enhancer binding protein (C/EBP) family of genes. In MLS, TLS-CHOP sequesters C/EBPβ resulting in block of differentiation programs while trabectedin disrupts this association freeing up C/EBPβ to act as transcriptional activator of genes related to differentiation.
Ongoing Cancer Clinical Trials with HDAC Inhibitors
The following is a listing of some clinical trials using histone deacetylase inhibitors in combination with approved chemotherapeutics in various tumors. This data was taken from the New Medicine Oncology Knowledge Base ( at http://www.nmok.net).
Issues and Future of Differentiation-based Therapy
In the review by Filemon Dela Cruz and Igor Matushansky(1), the authors suggest that, like days of old of cytotoxic monotherapy, differentiation therapy would not evolve as a simplistic one-size-fits –all but mirror an extremely complicated process. Therefore they suggest three theoretical mechanisms in which differentiation therapy may occur:
- Cancer directed differentiation: differentiation pathways are activated without correcting the underlying oncogenic mechanisms which produced the initial differentiation block
- Cancer reverted differentiation: correction of the underlying oncogenic mechanism results in restoration of endogenous differentiation pathways
- Cancer diverted differentiation: cancer cell is redirected to an earlier stage of differentiation
Finally the authors suggest that “the potential for reversion of the malignant cancer phenotype to a more benign, or at the very least a lower grade of biological aggressiveness, may serve as a critical clinical and biologic transition of a uniformly fatal cancer into one more amenable to management or to treatment using conventional therapeutic approaches.”
References:
1. Cruz, F. D., and Matushansky, I. (2012) Oncotarget 3, 559-567
2. Riester, M., Stephan-Otto Attolini, C., Downey, R. J., Singer, S., and Michor, F. (2010) PLoS computational biology 6, e1000777
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5. Marks, P. A. (2007) Oncogene 26, 1351-1356
6. Munster, P. N., Troso-Sandoval, T., Rosen, N., Rifkind, R., Marks, P. A., and Richon, V. M. (2001) Cancer research 61, 8492-8497
7. Napoli, J. L. (1999) Biochim Biophys Acta 1440, 139-162
8. Moon, R., Metha, R., and Rao, K. (1994) Retinoids and cancer in experimental animals. in The Retinoids: Biology, Chemistry, and Medicine (Sporn, M., Roberts, A., and Goodman, D. eds.), 2 Ed., Raven Press, New York. pp 573-596
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Updated 4/27/2021
Inhibiting EZH2 either genetically or with a chemical inhibitor signaled the immune system to respond to PD-1 inhibition in prostate cancer. (NIH)
The protein EZH2 has long been known as a major driver of prostate cancer because of its ability to inactivate genes that would normally suppress tumor growth. Now, a team at Cedars-Sinai Cancer has shown in preclinical models of the disease that blocking EZH2 reduces resistance to immune-boosting checkpoint inhibitors—and they did it with the help of Epizyme, which won FDA approval for the first EZH2 blocker last year.
The Cedars-Sinai team inhibited EZH2 in preclinical prostate cancer models, activating interferon-stimulated genes in the immune system. The interferons then boosted the immune response and reversed resistance to drugs that inhibit the checkpoint PD-1, they reported in the journal Nature Cancer.
By inhibiting EZH2 either genetically or with a chemical inhibitor donated by Epizyme, the researchers used a technique called “viral mimicry” to “reopen” parts of the genome that are typically inactive, they explained in a statement. That signaled the immune system to respond to PD-1 inhibition.
Checkpoint inhibitors have been approved to treat several cancer types, but they’ve been largely disappointing in prostate cancer. Hence several research groups have been exploring combination strategies. They include the University of Texas MD Anderson Cancer Center, which published research in 2019 showing early evidence that combining checkpoint inhibition with anti-TGF-beta drug could be effective in prostate cancer.
More recently, bispecific antibodies have shown early promise in prostate cancer. Last September, Amgen presented data from a phase 1 study of AMG 160, a bispecific targeting PSMA and CD3 on T cells. The company said that 68.6% of patients experienced a decline in PSA, and eight out of 15 patients evaluated showed stable disease.
Regeneron is also developing a bispecific antibody for prostate cancer, targeting PSMA and CD28. The drug is being tested as a solo therapy and in combination with Regeneron’s PD-1 inhibitor Libtayo in a phase 1/2 clinical trial enrolling men with metastatic castration-resistant prostate cancer.
As for Epizyme’s EZH2 inhibitor, Tazverik, its path to market hasn’t been perfectly smooth. An advisory committee to the FDA questioned its efficacy and safety in its initial indication, metastatic or locally advanced epithelioid sarcoma. Still, the company got the go-ahead to market the drug in adult patients with the rare cancer last January. Then the FDA added follicular lymphoma to the label in June. The drug’s takeoff has been slower than expected, however, largely because the pandemic has prevented face-to-face interactions between the sales force and physicians.
The company is currently testing Tazverik in several other cancer types, including as a combination with standard-of-care treatments in castration-resistant prostate cancer.
Other research papers on Cancer and Cancer Therapeutics were published on this Scientific Web site as follows:
Histone Deacetylase Inhibitors Induce Epithelial-to-Mesenchymal Transition in Prostate Cancer Cells
PIK3CA mutation in Colorectal Cancer may serve as a Predictive Molecular Biomarker for adjuvant Aspirin therapy
Nanotechnology Tackles Brain Cancer
Response to Multiple Cancer Drugs through Regulation of TGF-β Receptor Signaling: a MED12 Control
Personalized medicine-based cure for cancer might not be far away
GSK for Personalized Medicine using Cancer Drugs needs Alacris systems biology model to determine the in silico effect of the inhibitor in its “virtual clinical trial”
Lung Cancer (NSCLC), drug administration and nanotechnology
Non-small Cell Lung Cancer drugs – where does the Future lie?
Cancer Innovations from across the Web
arrayMap: Genomic Feature Mining of Cancer Entities of Copy Number Abnormalities (CNAs) Data
How mobile elements in “Junk” DNA promote cancer. Part 1: Transposon-mediated tumorigenesis.
Cancer Genomics – Leading the Way by Cancer Genomics Program at UC Santa Cruz
Closing the gap towards real-time, imaging-guided treatment of cancer patients.
Closing the gap towards real-time, imaging-guided treatment of cancer patients.
mRNA interference with cancer expression
Search Results for ‘cancer’ on this web site
Cancer Genomics – Leading the Way by Cancer Genomics Program at UC Santa Cruz
Closing the gap towards real-time, imaging-guided treatment of cancer patients.
Lipid Profile, Saturated Fats, Raman Spectrosopy, Cancer Cytology
mRNA interference with cancer expression
Pancreatic cancer genomes: Axon guidance pathway genes – aberrations revealed
Biomarker tool development for Early Diagnosis of Pancreatic Cancer: Van Andel Institute and Emory University
Is the Warburg Effect the cause or the effect of cancer: A 21st Century View?
Crucial role of Nitric Oxide in Cancer
Targeting Glucose Deprived Network Along with Targeted Cancer Therapy Can be a Possible Method of Treatment
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Prostate cancer is the second most common cancer in American men, killing nearly 30,000 per year. In 2004, I attended a conference where one of the nation’s leading researchers in the field declared that the gold-standard test for this disease was not successful at identifying dangerous invasive tumors. That triggered my interest in how to address the challenge of developing a blood test to detect the deadly form of prostate cancer.
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