All HDAC Inhibitors Are Not Equal for Treating Anaplastic Thyroid Cancer

John Copland, PhD

John Copland, PhD

A Mayo Clinic Laboratory, which discovered that upregulation of ras homolog gene family member B (RhoB), in combination with paclitaxel results in enhanced antitumor activity, has uncovered the molecular mechanisms underlying combination therapy for anaplastic thyroid cancer (ATC).^1-3

Senior author John Copland, PhD, associate professor of biochemistry and molecular biology, and professor of cancer biology at the Mayo Clinic in Jacksonville, Florida, et al stress the urgent need for a new interventional therapy to manage the disease.³“Although ATC is rare, accounting for only 1% to 2% of thyroid cancers, it is responsible for up to 50% of thyroid cancer deaths,”²said Copland in a Mayo Clinic news release.

In an earlier paper, using cell lines, researchers showed that a histone deacetylase inhibitor (HDACi), romidepsin, inhibited ATC proliferation via cyclin-dependent kinase inhibitor 1A (p21), and this was RhoB dependent.¹There are 11 HDACs that are closely associated with DNA to suppress gene expression, and two classes of HDAC inhibitors have been approved for the treatment of malignancy: a class I romidepsin (Istodax), and class II, vorinostat (Zolinza) and belinostat (Beleodaq).^2-6

“HDAC inhibitors can increase activity of RhoB, a tumor suppressor that we had discovered earlier, which is switched off in ATC,” said Copland. “We also know that this RhoB pathway, once turned on, can stimulate the expression of two proteins: p21, which prevents cells from replicating, and BIM, which induces cell death — both of which can help stop cancer growth.”²

The present research focused on finding which class of HDACi would best synergize with paclitaxel to reduce the survival of ATC cells, and the mechanisms involved.^2,3

Researchers used patient derived ATC cell lines created in their laboratory and studied the effects of treatment with romidepsin (class I HDACi), vorinostat and belinostat (class II HDACi).^2,3

Findings

During their work Copland et al discovered that RhoB downstream signaling partners differed depending on which HDACi was used, and consequently the effects on the ATC cells differed. If romidepsin (class I HDACi) was applied, the resultant increase of RhoB upregulated p21, and the tumor cells exhibited cytostasis. If a class II inhibitor was applied (vorinostat or belinostat), another protein, BIM_ELwas upregulated, and the tumor cells underwent apoptosis. Furthermore, they found that only when RhoB upregulated BIM_EL,did antitumor synergy occur with paclitaxel.³

“Only vorinostat and belinostat always induced the BIM protein, always induced cell death, and always showed synergy with paclitaxel,” explained Laura A Marlow, MS, the laboratory manager for Copland.²

To find out which HDAC was suppressing RhoB the researchers made small hairpin RNA constructs targeting HDACs 1 to 11, and discovered that HDAC6 repressed the RhoB promoter. This was a completely new finding, not previously reported.³

Commenting on the implications, the press release stated “She (Marlow), also found that class I HDAC inhibitors suppress HDAC1, which mostly upregulates p21 and sometimes BIM_EL, but to a lesser degree. When p21 is produced, no synergy occurs with paclitaxel; therefore, there is no benefit of adding the HDAC inhibitor to treatment.”²

In contrast, belinostat and verinostat both suppressed HDAC6, which led to an increase in RhoB. BIM_ELwas preferentially expressed due to RhoB’s reexpression. Immunohistochemistry on normal and ATC tissue from patients found HDAC6 was overexpressed in ATC tissue, indicating class II HDAC inhibitors will be effective in patients.^2,3

Overall Implications

“We now know which of the many HDAC inhibitors to try with ATC. It makes sense that similar investigations would help other researchers who want to use an HDAC inhibitor to treat specific cancers,”²said Copland.

“Physicians and researchers should know that just using any of the many approved HDAC inhibitors for a patient’s cancer may not offer the results one wants to see,” said coauthor and endocrinologist Robert Smallridge, MD, professor of medicine and the Alfred D and Audrey M professor of cancer research at the Mayo Clinic in Jacksonville Florida, in the Mayo Clinic news release. “That appears to be the case in ATC, and it is probably likely true in a number of cancers.”²

References:

1. Marlow LA, Reynolds LA, Cleland AS, et al. Reactivation of suppressed RhoB is a critical step for the inhibition of anaplastic thyroid cancer growth.Cancer Res. 2009;69:1536-1544.

2. Mayo Clinic News Network. Researchers decode molecular action of combination therapy for a deadly thyroid cancer. http://newsnetwork.mayoclinic.org/discussion/mayo-clinic-researchers-decode-molecular-action-of-combination-therapy-for-a-deadly-thyroid-cancer/. Accessed August 15, 2015.

3. Marlow LA, Bok I, Smallridge R, et al. RhoB uporegulation leads to either apoptosis or cytostasis through differential target selection.Endocr Relat Cancer. 2015 Jul 23. doi: 10.1530/ERC-14-0302.

4. Isodex [package insert]. Summit, NJ: Celgene Corporation.http://www.istodax.com/pdfs/istodax_packageinsert.pdf. Accessed August 17, 2015.

5. Zolinza [package insert]. Whitehouse Station, NJ: Merk & Co Inc.https://www.merck.com/product/usa/pi_circulars/z/zolinza/zolinza_pi.pdf[accessed 08 17 2015]

6.http://www.accessdata.fda.gov/drugsatfda_docs/label/2014/206256lbl.pdf[accessed 08 17 2015}