Artem K. Velichko

Mechanisms of heat shock response in mammals

Heat shock (HS) is one of the best-studied exogenous cellular stresses. The cellular response to HS utilizes ancient molecular networks that are based primarily on the action of stress-induced heat shock proteins and HS factors. However, in one way or another, all cellular compartments and metabolic processes are involved in such a response. In this review, we aimed to summarize the experimental data concerning all aspects of the HS response in mammalian cells, such as HS-induced structural and functional alterations of cell membranes, the cytoskeleton and cellular organelles; the associated pathways that result in different modes of cell death and cell cycle arrest; and the effects of HS on transcription, splicing, translation, DNA repair, and replication.

Dual effect of heat shock on DNA replication and genome integrity

The data presented here suggest that in an asynchronous cell culture, heat shock might affect DNA integrity both directly and via arrest of replication fork progression and that the phosphorylation of histone H2AX has a protective effect on the arrested replication forks in addition to its known DNA damage signaling function.

Small molecule compounds that induce cellular senescence.

To date, dozens of stress‐induced cellular senescence phenotypes have been reported. These cellular senescence states may differ substantially from each other, as well as from replicative senescence through the presence of specific senescence features. Here, we attempted to catalog virtually all of the cellular senescence‐like states that can be induced by low molecular weight compounds. We summarized biological markers, molecular pathways involved in senescence establishment, and specific traits of cellular senescence states induced by more than fifty small molecule compounds.

Mechanism of heat stress-induced cellular senescence elucidates the exclusive vulnerability of early S-phase cells to mild genotoxic stress

Heat stress is one of the best-studied cellular stress factors; however, little is known about its delayed effects. Here, we demonstrate that heat stress induces p21-dependent cellular senescence-like cell cycle arrest. Notably, only early S-phase cells undergo such an arrest in response to heat stress. The encounter of DNA replication forks with topoisomerase I-generated single-stranded DNA breaks resulted in the generation of persistent double-stranded DNA breaks was found to be a primary cause of heat stress-induced cellular senescence in these cells. This investigation of heat stress-induced cellular senescence elucidates the mechanisms underlying the exclusive sensitivity of early S-phase cells to ultra-low doses of agents that induce single-stranded DNA breaks.

HP1α is not necessary for the structural maintenance of centromeric heterochromatin

Heterochromatin protein 1 (HP1) was discovered as a protein essential for maintaining the silent transcriptional status of genes located within or close to centromeric regions of Drosophila chromosomes. Mammals express three variants of HP1; of these, HP1α is a direct homolog of Drosophila HP1. The prevailing view states that HP1 is a structural component of heterochromatin and is essential for compact DNA packaging. HP1 contains a chromodomain that binds to di- and- tri-methylated lysine 9 of histone H3. Additionally, it contains a chromoshadow domain that allows HP1 to dimerize and interact with other proteins. HP1 is thought to form “bridges” between neighboring rows of nucleosomes in heterochromatin. In mammalian cells, a significant portion of HP1α is located in the centromeric regions of chromosomes. In this study, we show that the majority of HP1α is removed from centromeres upon heat shock. This occurs without a loss of H3K9 trimethylation and does not correlate with a decompaction of centromeres. Furthermore, HP1α is not degraded and remains bound to chromatin. Therefore, it is likely that HP1α is simply redistributed to euchromatic regions. We propose that this redistribution is essential for reversal of the transcriptional status of euchromatic and heterochromatic compartments.

Heat stress-induced transcriptional repression

Heat stress is one of the most popular models for studying the regulation of gene expression. For decades, researchers’ attention was focused on the study of the mechanisms of transcriptional activation of stress-induced genes. Although the phenomenon of heat stress-induced global transcriptional repression is known for a long time, the exact molecular mechanisms of such a repression are poorly explored. In this mini-review, we attempt to summarize the existing experimental data on heat stress-induced transcriptional repression.

Early S-phase cell hypersensitivity to heat stress

ABSTRACT Heat stress is one of the best-studied exogenous stress factors; however little is known about its delayed effects. Recently, we have shown that heat stress induces cellular senescence-like G2 arrest exclusively in early S-phase cells. The mechanism of this arrest includes the generation of heat stress-induced single-stranded DNA breaks, the collision of replication forks with these breaks and the formation of difficult-to-repair double-stranded DNA breaks. However, the early S phase-specific effects of heat stress are not limited to the induction of single-stranded DNA breaks. Here, we report that HS induces partial DNA re-replication and centrosome amplification. We suggest that HS-induced alterations in the expression levels of the genes encoding the replication licensing factors are the primary source of such perturbations. Notably, these processes do not contribute to acquisition of a senescence-like phenotype, although they do elicit postponed effects. Specifically, we found that the HeLa cells can escape from the heat stress-induced cellular senescence-like G2 arrest, and the mitosis they enter is multipolar due to the amplified centrosomes.

Inducing cellular senescence in vitro by using genetically encoded photosensitizers

Cellular senescence, a form of cell cycle arrest, is one of the cellular responses to different types of exogenous and endogenous damage. The senescence phenotype can be induced in vitro by oncogene overexpression and/or DNA damage. Recently, we have reported a novel mechanism of cellular senescence induction by mild genotoxic stress. Specifically, we have shown that the formation of a small number of DNA lesions in normal and cancer cells during S phase leads to cellular senescence-like arrest within the same cell cycle. Here, based on this mechanism, we suggest an approach to remotely induce premature senescence in human cell cultures using short-term light irradiation. We used the genetically encoded photosensitizers, tandem KillerRed and miniSOG, targeted to chromatin by fusion to core histone H2B to induce moderate levels of DNA damage by light in S phase cells. We showed that the cells that express the H2B-fused photosensitizers acquire a senescence phenotype upon illumination with the appropriate light source. Furthermore, we demonstrated that both chromatin-targeted tandem KillerRed (produces O2−) and miniSOG (produces 1O2) induce single-stranded DNA breaks upon light illumination. Interestingly, miniSOG was also able to induce double-stranded DNA breaks.

Heat stress induces formation of cytoplasmic granules containing HSC70 protein

Using indirect immunofluorescence, in this study we showed that the constitutive heat shock protein HSC70 forms granule-like structures in the cytoplasm of human cells several days after the exposure to heat stress. It was shown that this effect is not the result of HSC70 overexpression under heat stress and is not due to the formation of hyperthermia-induced translational stress granules in the cytoplasm.

Heat shock-induced dissociation of TRF2 from telomeres does not initiate a telomere-dependent DNA damage response

Telomeric repeat binding factor 2 (TRF2) is a well‐studied shelterin complex subunit that plays a major role in the protection of chomosome ends and the prevention of the telomere‐associated DNA damage response. We show that heat shock induces the dissociation of TRF2 from telomeres in human primary and cancer cell cultures. TRF2 is not simply degraded in response to heat shock, but redistributed thoughout the nucleoplasm. This TRF2 depletion/redistribution does not initiate the DNA damage response at chomosome termini.