A study published in Aging has shown that inhibiting histone deacetylases (HDACs), an approach used for treating cancer and other diseases, can lead to cellular senescence.
What is a histone, anyway?
In biology, a histone is a protein that binds to DNA in order to control its access. Histone acetyl-transferases (HATs) add acetyl groups to histones. These extra groups take away the histones’ positive charge, thus reducing their binding to DNA and opening the door to transcription: the accessible DNA can be used to make proteins. On the other hand, histone deacetylases (HDACs) remove these acetyl groups, causing histones to tightly bind to DNA once more and closing off access.
A histone deacetylase inhibitor, therefore, holds the door open; genes that would otherwise be prevented from making proteins are allowed to continue doing so. HDAC inhibitors have seen use in treating cancer, cardiovascular diseases, and inflammation-related disorders . We have reported on the use of HDAC inhibitors to improve the gene expression of mitochondria and restore the shape of senescent cells.
This new research has demonstrated a negative aspect of this approach, showing that HDAC expression is decreased in cellular senescence and that inhibiting HDACs can cause cells to become senescent.
Which HDACs are important?
In order to test the generalized inhibition of HDACs, the researchers first used SAHA, which affects all HDACs, in dermal fibroblasts. The results were starkly negative, as four components of the SASP skyrocketed: the inflammatory cytokines IL-6 and IL-8 along with the matrix metalloproteinases MMP-1 and MMP-3. Lamin B1, a part of the nuclear lamin that protects DNA from damage, was dramatically decreased. The proportion of cells that express SA beta-galactose (SA ß-gal), another well-known marker of cellular senescence, was also increased.
Using targeted silencing RNA, researchers then inhibited the two HDACs most reduced in dermal fibroblasts, HDAC2 and HDAC7. While HDAC2 inhibition caused the cells to become senescent, halting growth, only the HDAC7 silencer RNA caused the cells to consistently and heavily express the SASP proteins. The researchers showed that they were able to partially offset this effect by blocking the NF-kB pathway.
Causing HDAC expression
Given the results of inhibiting HDACs, the researchers then set out to discover what would happen if they were to cause the expression of HDACs in dermal fibroblasts rather than inhibit it. For this purpose, they infected pre-senescent cells with HDAC2 and HDAC7 lentiviruses. While HDAC2 had little effect, cells given the HDAC7 lentivirus were able to resume proliferation, and the number of cells expressing SA ß-gal was decreased.
There was just one problem: the SASP. Far from decreasing SASP expression, stimulating cells to produce HDAC7 also caused the SASP factors IL-6, IL-8, MMP-1, and MMP-3 to increase, just as inhibiting HDAC7 had. Apparently, directly targeting this HDAC, in either direction, triggers dermal fibroblasts to express the SASP.
Originally simply reported to be in a stable and irreversible growth arrest in vitro, senescent cells are now clearly associated with normal and pathological ageing in vivo. They are characterized by several biomarkers and changes in gene expression that may depend on epigenetic factors, such as histone acetylation, involving a balance between histone acetyltransferases (HATs) and histone deacetylases (HDACs). In this study, we investigate the expression and the role of HDACs on the senescent phenotype of dermal fibroblasts. We report that during replicative senescence, most canonical HDACs are less expressed. Moreover, treatment with SAHA, a histone deacetylase inhibitor (HDACi) also known as Vorinostat, or the specific downregulation of HDAC2 or HDAC7 by siRNA, induces the appearance of senescence biomarkers of dermal fibroblasts. Conversely, the ectopic re-expression of HDAC7 by lentiviral transduction in pre-senescent dermal fibroblasts extends their proliferative lifespan. These results demonstrate that HDACs expression can modulate the senescent phenotype, highlighting their pharmaceutical interest in the context of healthy ageing.
While illuminating for people interested in the relationship between histones and senescence. this research shows bad news for people who intend to develop HDAC inhibitors as treatments for age-related diseases. While HDAC inhibitors have been used as treatments for some diseases, more research needs to be done into their long-term effects, whether or not other cells act the same way as dermal fibroblasts when exposed to HDAC inhibitors, and a more complete picture of the role that HATs and HDACs have in cellular senescence as a whole.
 Yoon, S., & Eom, G. H. (2016). HDAC and HDAC inhibitor: from cancer to cardiovascular diseases. Chonnam medical journal, 52(1), 1-11.