HDAC6 Regulates Radiosensitivity of Non-Small Cell Lung Cancer by Promoting Degradation of Chk1.

We have previously discovered that HDAC6 regulates the DNA damage response (DDR) via modulating the homeostasis of a DNA mismatch repair protein, MSH2, through HDAC6's ubiquitin E3 ligase activity. Here, we have reported HDAC6's second potential E3 ligase substrate, a critical cell cycle checkpoint protein, Chk1. We have found that HDAC6 and Chk1 directly interact, and that HDAC6 ubiquitinates Chk1 in vivo and in vitro. Specifically, HDAC6 interacts with Chk1 via the DAC1 domain, which contains its ubiquitin E3 ligase activity. During the cell cycle, Chk1 protein levels fluctuate, peaking at the G2 phase, subsequently resolving via the ubiquitin-proteasome pathway, and thereby allowing cells to progress to the M phase. However, in HDAC6 knockdown non-small cell lung cancer (NSCLC) cells, Chk1 is constitutively active and fails to resolve post-ionizing radiation (IR), and this enhanced Chk1 activity leads to preferential G2 arrest in HDAC6 knockdown cells accompanied by a reduction in colony formation capacity and viability. Depletion or pharmacological inhibition of Chk1 in HDAC6 knockdown cells reverses this radiosensitive phenotype, suggesting that the radiosensitivity of HDAC6 knockdown cells is dependent on increased Chk1 kinase activity. Overall, our results highlight a novel mechanism of Chk1 regulation at the post-translational level, and a possible strategy for sensitizing NSCLC to radiation via inhibiting HDAC6's E3 ligase activity.

The USP10-HDAC6 axis confers cisplatin resistance in non-small cell lung cancer lacking wild-type p53.

Ubiquitin-specific peptidase 10 (USP10) stabilizes both tumor suppressors and oncogenes in a context-dependent manner. However, the nature of USP10's role in non-small cell lung cancer (NSCLC) remains unclear. By analyzing The Cancer Genome Atlas (TCGA) database, we have shown that high levels of USP10 are associated with poor overall survival in NSCLC with mutant p53, but not with wild-type p53. Consistently, genetic depletion or pharmacological inhibition of USP10 dramatically reduces the growth of lung cancer xenografts lacking wild-type p53 and sensitizes them to cisplatin. Mechanistically, USP10 interacts with, deubiquitinates, and stabilizes oncogenic protein histone deacetylase 6 (HDAC6). Furthermore, reintroducing either USP10 or HDAC6 into a USP10-knockdown NSCLC H1299 cell line with null-p53 renders cisplatin resistance. This result suggests the existence of a "USP10-HDAC6-cisplatin resistance" axis. Clinically, we have found a positive correlation between USP10 and HDAC6 expression in a cohort of NSCLC patient samples. Moreover, we have shown that high levels of USP10 mRNA correlate with poor overall survival in a cohort of advanced NSCLC patients who received platinum-based chemotherapy. Overall, our studies suggest that USP10 could be a potential biomarker for predicting patient response to platinum, and that targeting USP10 could sensitize lung cancer patients lacking wild-type p53 to platinum-based therapy.

HDAC6 regulates DNA damage response via deacetylating MLH1.

MutL homolog 1 (MLH1) is a key DNA mismatch repair protein, which plays an important role in maintenance of genomic stability and the DNA damage response. Here, we report that MLH1 is a novel substrate of histone deacetylase 6 (HDAC6). HDAC6 interacts with and deacetylates MLH1 both in vitro and in vivo Interestingly, deacetylation of MLH1 blocks the assembly of the MutSα-MutLα complex. Moreover, we have identified four novel acetylation sites in MLH1 by MS analysis. The deacetylation mimetic mutant, but not the WT and the acetylation mimetic mutant, of MLH1 confers resistance to 6-thioguanine. Overall, our findings suggest that the MutSα-MutLα complex serves as a sensor for DNA damage response and that HDAC6 disrupts the MutSα-MutLα complex by deacetylation of MLH1, leading to the tolerance of DNA damage.

Implications of the HDAC6-ERK1 feed-forward loop in immunotherapy.

The oncogene HDAC6 controls numerous cell processes that are related to tumorigenesis and metastasis, and has recently arisen as a target to treat malignancies. The ERK cascade is a classic pathway driving oncogenesis, and the components of this pathway are either highly mutated in cancers or are vital in cancer's pathological activity. The interactions between these important components of tumor proliferation have been examined, and our research has demonstrated that they regulate each other as evidenced by different posttranslational modifications. Preclinical evidence also supports clinical trials cotargeting these two pathways, which may provide better efficacy than single treatment. Furthermore, HDAC6 and ERK both participate in the regulation of T cell maturation and may have implications on the functions of immune cells. This leads to the possibility of connecting HDAC6 and ERK to immunotherapy. In this review, we summarize the published studies about the interaction of HDAC6 and ERK cascade and their relationship to cancers. We also include the association of HDAC6 and ERK to immune system and discuss the plausibility of linking these to immunotherapy.