Cell cycle-coupled changes in the level of reactive oxygen species support the proliferation of human pluripotent stem cells.

The study of proliferation regulation in human pluripotent stem cells is crucial to gain insights into understanding the physiology of these cells. However, redox regulation of the pluripotent cell cycle remains largely unexplored. Here, using human embryonic stem cells (hESCs) as well as human induced pluripotent stem cells (hiPSCs), we demonstrate that the level of reactive oxygen species (ROS) in pluripotent cells oscillates in accordance with the cell cycle progression with the peak occurring at transition from S to G2 /M phase of the cycle. A decrease of this level by antioxidants leads to hindered S-phase initiation and progression but does not affect the early-G1 -phase or mitosis. Cells exposed to antioxidants in the early-G1 -phase accumulate the phosphorylated retinoblastoma protein and overcome the restriction point but are unable to accumulate the main regulators of the S phase-CYCLIN A and GEMININ. Based on the previous findings that CYCLIN A stability is affected by redox homeostasis disturbances in somatic cells, we compared the responses to antioxidant treatments in hESCs and in their differentiated fibroblast-like progeny cells (difESCs). In difESCs, similar to hESCs, a decrease in ROS level results in the disruption of S-phase initiation accompanied by a deficiency of the CYCLIN A level. Moreover, in antioxidant-treated cells, we revealed the accumulation of DNA breaks, which was accompanied by activation of the apoptosis program in pluripotent cells. Thus, we conclude that maintaining the physiological ROS level is essential for promotion of proliferation and accurate DNA synthesis in pluripotent cells and their differentiated descendants.

Sublethal heat shock induces premature senescence rather than apoptosis in human mesenchymal stem cells.

Stem cells in adult organism are responsible for cell turnover and tissue regeneration. The study of stem cell stress response contributes to our knowledge on the mechanisms of damaged tissue repair. Previously, we demonstrated that sublethal heat shock (HS) induced apoptosis in human embryonic stem cells. This study aimed to investigate HS response of human adult stem cells. Human mesenchymal stem cells (MSCs) cultivated in vitro were challenged with sublethal HS. It was found that sublethal HS did not affect the cell viability assessed by annexin V/propidium staining. However, MSCs subjected to severe HS exhibited features of stress-induced premature senescence (SIPS): irreversible cell cycle arrest, altered morphology, increased expression of senescence-associated ß-galactosidase (SA-ß-gal) activity, and induction of cyclin-dependent kinase inhibitor p21 protein. High level of Hsp70 accumulation induced by sublethal HS did not return to the basal level, at least, after 72 h of the cell recovery when most cells exhibited SIPS hallmarks. MSCs survived sublethal HS, and resumed proliferation sustained the properties of parental MSCs: diploid karyotype, replicative senescence, expression of the cell surface markers, and capacity for multilineage differentiation. Our results showed for the first time that in human MSCs, sublethal HS induced premature senescence rather than apoptosis or necrosis. MSC progeny that survived sublethal HS manifested stem cell properties of the parental cells: limited replicative life span and multilineage capacity.

Heat shock induces apoptosis in human embryonic stem cells but a premature senescence phenotype in their differentiated progeny.

Embryonic stem cells (ESC) are able to self-renew and to differentiate into any cell type. To escape error transmission to future cell progeny, ESC require robust mechanisms to ensure genomic stability. It was stated that stress defense of mouse and human ESC against oxidative stress and irradiation is superior compared with differentiated cells. Here, we investigated heat shock response of human ESC (hESC) and their differentiated progeny. Fibroblast-like cells were generated by spontaneous hESC differentiation via embryoid bodies. Like normal human diploid fibroblasts, these cells have a finite lifespan in culture, undergo replicative senescence and die. We found that sublethal heat shock affected survival of both cell types, but in hESC it induced apoptosis, whereas in differentiated cells it produced cell cycle arrest and premature senescence phenotype. Heat shock survived hESC and differentiated cells restored the properties of initial cells. Heated hESC progeny exhibited pluripotent markers and the capacity to differentiate into the cells of three germ layers. Fibroblast-like cells resisted heat shock, proliferated for a limited number of passages and entered replicative senescence as unheated parental cells. Taken together, these results show for the first time that both hESC and their differentiated derivatives are sensitive to heat shock, but the mechanisms of their stress response are different: hESC undergo apoptosis, whereas differentiated cells under the same conditions exhibit stress-induced premature senescence (SIPS) phenotype. Both cell types that survived sublethal heat shock sustain parental cell properties.

Changes in composition and activities of 26S proteasomes under the action of doxorubicin--apoptosis inductor of erythroleukemic K562 cells.

Changes in the subunit composition, phosphorylation of the subunits, and regulation of the activities of 26S proteasomes in proliferating cells undergoing programmed cell death have not been studied so far. Moreover, there are no reports on phosphorylation of proteasome subunits both in normal and in neoplastic cells during apoptosis. The data of the present study show for the first time that apoptosis inductor doxorubicin regulates subunit composition, enzymatic activities, and phosphorylation state of 26S proteasomes in neoplastic (proerythroleukemic K562) cells or, in other words, induces reprogramming of proteasome population. Furthermore, the phosphorylation state of proteasomes is found to be the mechanism controlling specificity of proteasomal proteolytic and endoribonuclease activities.