Linker Histone H1.2 Directs Genome-wide Chromatin Association of the Retinoblastoma Tumor Suppressor Protein and Facilitates Its Function

Comparative time-series multi-omics analyses suggest H1.2 involvement in anoxic adaptation and cancer resistance

The naked mole rat (NMR), Heterocephalus glaber, is known as the longest-lived rodent and is extraordinarily resistant to hypoxia and cancer. Here, both NMR embryonic fibroblasts (NEFs) and their mouse counterparts (MEFs) were subjected to anoxic conditions (0% O2, 5% CO2). A combination of comparative transcriptomics and proteomics was then employed to identify differentially expressed genes (DEGs). Notably, we observed distinct levels of histone H1.2 (encoded by HIST1H1C) accumulation between NEFs and MEFs. Subsequent mechanistic analyses showed that higher H1.2 expression in NEFs was associated with the lower expression of its inhibitor, PARP1. Additionally, we discovered that H1.2 can directly interact with HIF-1α PAS domains, thereby promoting the expression of HIF-1α through facilitating the dimerization with HIF-1β. The overexpression of H1.2 was also found to trigger autophagy and to suppress the migration of cancer cells, as well as the formation of xenograft tumors, via the NRF2/P62 signaling pathway. Moreover, an engineered H1.2 knock-in mouse model exhibited significantly extended survival in hypoxic conditions (4% O2) and showed a reduced rate of tumor formation. Collectively, our results indicate a potential mechanistic link between H1.2 and the dual phenomena of anoxic adaptation and cancer resistance.

An antioxidant feedforward cycle coordinated by linker histone variant H1.2 and NRF2 that drives nonsmall cell lung cancer progression

Nonsmall cell lung cancer (NSCLC) is highly malignant with limited treatment options, platinum-based chemotherapy is a standard treatment for NSCLC with resistance commonly seen. NSCLC cells exploit enhanced antioxidant defense system to counteract excessive reactive oxygen species (ROS), which contributes largely to tumor progression and resistance to chemotherapy, yet the mechanisms are not fully understood. Recent studies have suggested the involvement of histones in tumor progression and cellular antioxidant response; however, whether a major histone variant H1.2 (H1C) plays roles in the development of NSCLC remains unclear. Herein, we demonstrated that H1.2 was increasingly expressed in NSCLC tumors, and its expression was correlated with worse survival. When crossing the H1c knockout allele with a mouse NSCLC model (KrasLSL-G12D/+), H1.2 deletion suppressed NSCLC progression and enhanced oxidative stress and significantly decreased the levels of key antioxidant glutathione (GSH) and GCLC, the catalytic subunit of rate-limiting enzyme for GSH synthesis. Moreover, high H1.2 was correlated with the IC50 of multiple chemotherapeutic drugs and with worse prognosis in NSCLC patients receiving chemotherapy; H1.2-deficient NSCLC cells presented reduced survival and increased ROS levels upon cisplatin treatment, while ROS scavenger eliminated the survival inhibition. Mechanistically, H1.2 interacted with NRF2, a master regulator of antioxidative response; H1.2 enhanced the nuclear level and stability of NRF2 and, thus, promoted NRF2 binding to GCLC promoter and the consequent transcription; while NRF2 also transcriptionally up-regulated H1.2. Collectively, these results uncovered a tumor-driving role of H1.2 in NSCLC and indicate an "H1.2-NRF2" antioxidant feedforward cycle that promotes tumor progression and chemoresistance.

Histone H1.2 promotes hepatocarcinogenesis by regulating STAT3 signaling

Linker histone H1.2 (H1.2), encoded by HIST1H1C (H1C), is a major H1 variant in somatic cells. Among five histone H1 somatic variants, upregulated H1.2 was found in human hepatocellular carcinoma (HCC) samples and in a diethylnitrosamine (DEN)‐induced HCC mouse model. In vitro, H1.2 overexpression accelerated proliferation of HCC cell lines, whereas H1.2 knockdown (KD) had the opposite effect. In vivo, H1.2 insufficiency or deficiency (H1c KD or H1c KO) alleviated inflammatory response and HCC development in DEN‐treated mice. Mechanistically, H1.2 regulated the activation of signal transducer and activator of transcription 3 (STAT3), which in turn positively regulated H1.2 expression by binding to its promoter. Moreover, upregulation of the H1.2/STAT3 axis was observed in human HCC samples, and was confirmed in mouse models of methionine‐choline‐deficient diet induced nonalcoholic steatohepatitis or lipopolysaccharide induced acute inflammatory liver injury. Disrupting this feed‐forward loop by KD of STAT3 or treatment with STAT3 inhibitors rescued H1.2 overexpression‐induced proliferation. Moreover, STAT3 inhibitor treatment‐ameliorated H1.2 overexpression promoted xenograft tumor growth. Therefore, H1.2 plays a novel role in inflammatory response by regulating STAT3 activation in HCC, thus, blockade of the H1.2/STAT3 loop is a potential strategy against HCC.

Molecular and Cellular Functions of the Linker Histone H1.2

Linker histone H1.2, which belongs to the linker histone family H1, plays a crucial role in the maintenance of the stable higher-order structures of chromatin and nucleosomes. As a critical part of chromatin structure, H1.2 has an important function in regulating chromatin dynamics and participates in multiple other cellular processes as well. Recent work has also shown that linker histone H1.2 regulates the transcription levels of certain target genes and affects different processes as well, such as cancer cell growth and migration, DNA duplication and DNA repair. The present work briefly summarizes the current knowledge of linker histone H1.2 modifications. Further, we also discuss the roles of linker histone H1.2 in the maintenance of genome stability, apoptosis, cell cycle regulation, and its association with disease.

The missing linker: emerging trends for H1 variant-specific functions

In this review, Prendergast and Reinberg discuss the likelihood that the family of histone H1 variants may be key to understanding several fundamental processes in chromatin biology and underscore their particular contributions to distinctly significant chromatin-related processes.

Major advances in the chromatin and epigenetics fields have uncovered the importance of core histones, histone variants and their post-translational modifications (PTMs) in modulating chromatin structure. However, an acutely understudied related feature of chromatin structure is the role of linker histone H1. Previous assumptions of the functional redundancy of the 11 nonallelic H1 variants are contrasted by their strong evolutionary conservation, variability in their potential PTMs, and increased reports of their disparate functions, sub-nuclear localizations and unique expression patterns in different cell types. The commonly accepted notion that histone H1 functions solely in chromatin compaction and transcription repression is now being challenged by work from multiple groups. These studies highlight histone H1 variants as underappreciated facets of chromatin dynamics that function independently in various chromatin-based processes. In this review, we present notable findings involving the individual somatic H1 variants of which there are seven, underscoring their particular contributions to distinctly significant chromatin-related processes.

Hydroxyurea and Caffeine Impact pRb-like Protein-Dependent Chromatin Architecture Profiles in Interphase Cells of Vicia faba

The survival of cells depends on their ability to replicate correctly genetic material. Cells exposed to replication stress can experience a number of problems that may lead to deregulated proliferation, the development of cancer, and/or programmed cell death. In this article, we have induced prolonged replication arrest via hydroxyurea (HU) treatment and also premature chromosome condensation (PCC) by co-treatment with HU and caffeine (CF) in the root meristem cells of Vicia faba. We have analyzed the changes in the activities of retinoblastoma-like protein (RbS807/811ph). Results obtained from the immunocytochemical detection of RbS807/811ph allowed us to distinguish five unique activity profiles of pRb. We have also performed detailed 3D modeling using Blender 2.9.1., based on the original data and some final conclusions. 3D models helped us to visualize better the events occurring within the nuclei and acted as a high-resolution aid for presenting the results. We have found that, despite the decrease in pRb activity, its activity profiles were mostly intact and clearly recognizable, with some local alterations that may correspond to the increased demand in transcriptional activity. Our findings suggest that Vicia faba’s ability to withstand harsh environments may come from its well-developed and highly effective response to replication stress.

Capn3 depletion causes Chk1 and Wee1 accumulation and disrupts synchronization of cell cycle reentry during liver regeneration after partial hepatectomy

Recovery of liver mass to a healthy liver donor by compensatory regeneration after partial hepatectomy (PH) is a prerequisite for liver transplantation. Synchronized cell cycle reentry of the existing hepatocytes after PH is seemingly a hallmark of liver compensatory regeneration. Although the molecular control of the PH-triggered cell cycle reentry has been extensively studied, little is known about how the synchronization is achieved after PH. The nucleolus-localized protein cleavage complex formed by the nucleolar protein Digestive-organ expansion factor (Def) and cysteine proteinase Calpain 3 (Capn3) has been implicated to control wounding healing during liver regeneration through selectively cleaving the tumor suppressor p53 in the nucleolus. However, whether the Def-Capn3 complex participates in regulating the synchronization of cell cycle reentry after PH is unknown. In this report, we generated a zebrafish capn3b null mutant (capn3b^∆19∆14). The homozygous mutant was viable and fertile, but suffered from a delayed liver regeneration after PH. Delayed liver regeneration in capn3b^∆19∆14 was due to disruption of synchronized cell proliferation after PH. Mass spectrometry (MS) analysis of nuclear proteins revealed that a number of negative regulators of cell cycle are accumulated in the capn3b^∆19∆14 liver after PH. Moreover, we demonstrated that Check-point kinase 1 (Chk1) and Wee1, two key negative regulators of G2 to M transition, are substrates of Capn3. We also demonstrated that Chk1 and Wee1 were abnormally accumulated in the nucleoli of amputated capn3b^∆19∆14 liver. In conclusion, our findings suggest that the nucleolar-localized Def-Capn3 complex acts as a novel regulatory pathway for the synchronization of cell cycle reentry, at least partially, through inactivating Chk1 and Wee1 during liver regeneration after PH.

Carcinogenic role of K-Ras-ERK1/2 signaling in bladder cancer via inhibition of H1.2 phosphorylation at T146

It has been reported that Ras-ERK signaling regulated tumor suppressive genes via epigenetic mechanisms. Herein, we set out to investigate the correlation between K-Ras-ERK1/2 signaling and H1.2 phosphorylation, to provide a better understanding of K-Ras-ERK signaling in cancer. A plasmid for expression of mutated K-Ras was transfected into human bladder carcinoma HT1197 cells. Western blot was carried out for testing the expression changes of ERK1/2 and H1.2. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay, soft-agar colony formation assay, and transwell assay were used to test the effects of H1.2 phosphorylation at T146 (H1.2 ^T146ph ) on HT1197 cells growth and migration. Quantitative reverse transcription polymerase chain reaction (qRT-PCR) and chromatin immunoprecipitation (ChIP) were performed to test whether H1.2 ^T146ph regulated K-Ras-ERK1/2 downstream genes. Furthermore, how K-Ras-ERK1/2 regulated H1.2 ^T146ph expression was studied. We found that the ERK1/2 was activated when K-Ras was mutated, and H1.2 ^T146ph expression was significantly downregulated by K-Ras mutation. H1.2 ^T146E for mimicking H1.2 ^T146ph significantly attenuated K-Ras mutation induced increases in HT1197 cells viability, colony formation, and relative migration. Besides, H1.2 ^T146ph regulated the transcription of K-Ras-ERK1/2 downstream genes, including NT5E, GDF15, CARD16, CYR61, IGFBP3, and WNT16B. Furthermore, K-Ras-ERK1/2 signaling inhibited H1.2 phosphorylation at T146 through degradation of DNA-PK, and the degraded DNA-PK by K-Ras-ERK1/2 possibly via modulation of MDM2. In conclusion, the activation of K-Ras-ERK1/2 signaling will repress the phosphorylation of H1.2 at T146, and thereby, promoted the growth and migration of bladder cancer cells. K-Ras-ERK1/2 signaling repressed H1.2 phosphorylation possibly by MDM2-mediated degradation of DNA-PK.

Molecular interactions underpinning the phenotype of hibernation in mammals

Mammals maintain a constant warm body temperature, facilitating a wide variety of metabolic reactions. Mammals that hibernate have the ability to slow their metabolism, which in turn reduces their body temperature and leads to a state of hypothermic torpor. For this metabolic rate reduction to occur on a whole-body scale, molecular interactions that change the physiology of cells, tissues and organs are required, resulting in a major departure from normal mammalian homeostasis. The aim of this Review is to cover recent advances in the molecular biology of mammalian hibernation, including the role of small molecules, seasonal changes in gene expression, cold-inducible RNA-binding proteins, the somatosensory system and emerging information on hibernating primates. To underscore the importance of differential gene expression across the hibernation cycle, mRNA levels for 14,261 ground squirrel genes during periods of activity and torpor are made available for several tissues via an interactive transcriptome browser. This Review also addresses recent findings on molecular interactions responsible for multi-day survival of near-freezing body temperatures, single-digit heart rates and a slowed metabolism that greatly reduces oxygen consumption. A better understanding of how natural hibernators survive these physiological extremes is beginning to lead to innovations in human medicine.

Non-canonical functions of the RB protein in cancer

The canonical model of RB-mediated tumour suppression developed over the past 30 years is based on the regulation of E2F transcription factors to restrict cell cycle progression. Several additional functions have been proposed for RB, on the basis of which a non-canonical RB pathway can be described. Mechanistically, the non-canonical RB pathway promotes histone modification and regulates chromosome structure in a manner distinct from cell cycle regulation. These functions have implications for chemotherapy response and resistance to targeted anticancer agents. This Opinion offers a framework to guide future studies of RB in basic and clinical research.

Linking H1 with chromatin and growth control

The retinoblastoma protein (pRb) is considered to be one of the key regulators of cell proliferation. Here we describe our recent findings that linker histone H1.2 is an interaction partner for pRb and impacts upon the genome-wide chromatin binding properties of pRb. Consequently, H1.2 influences transcriptional repression and cell cycle control.