Reduced mitochondrial properties in putative progenitor/stem cells of human keratinocytes

Imaging of metabolic activity adaptations to UV stress, drugs and differentiation at cellular resolution in skin and skin equivalents - Implications for oxidative UV damage

The epidermis is a multi-layered epithelium that consists mainly of keratinocytes which proliferate in its basal layer and then differentiate to form the stratum corneum, the skin's ultimate barrier to the environment. During differentiation keratinocyte function, chemical composition, physical properties, metabolism and secretion are profoundly changed. Extrinsic or intrinsic stressors, like ultraviolet (UV) radiation thus may differently affect the epidermal keratinocytes, depending on differentiation stage. Exposure to UV elicits the DNA damage responses, activation of pathways which detoxify or repair damage or induction of programmed cell death when the damage was irreparable. Recently, rapid diversion of glucose flux into the pentose phosphate pathway (PPP) was discovered as additional mechanism by which cells rapidly generate reduction equivalents and precursors for nucleotides - both being in demand after UV damage. There is however little known about the correlation of such metabolic activity with differentiation state, cell damage and tissue localization of epidermal cells. We developed a method to correlate the activity of G6PD, the first and rate-limiting enzyme of this metabolic UV response, at cellular resolution to cell type, differentiation state, and cell damage in human skin and in organotypic reconstructed epidermis. We thereby could verify rapid activation of G6PD as an immediate UVB response not only in basal but also in differentiating epidermal keratinocytes and found increased activity in cells which initiated DNA damage responses. When keratinocytes had been UVB irradiated before organotypic culture, their distribution within the skin equivalent was abnormal and the G6PD activity was reduced compared to neighboring cells. Finally, we found that the anti-diabetic and potential anti-aging drug metformin strongly induced G6PD activity throughout reconstructed epidermis. Activation of the protective pentose phosphate pathway may be useful to enhance the skin's antioxidant defense systems and DNA damage repair capacity on demand.

Phlorizin, an Active Ingredient of Eleutherococcus senticosus, Increases Proliferative Potential of Keratinocytes with Inhibition of MiR135b and Increased Expression of Type IV Collagen

E. senticosus extract (ESE), known as antioxidant, has diverse pharmacologic effects. It is also used as an antiaging agent for the skin and phlorizin (PZ) is identified as a main ingredient. In this study, the effects of PZ on epidermal stem cells were investigated. Cultured normal human keratinocytes and skin equivalents are used to test whether PZ affects proliferative potential of keratinocytes and how it regulates these effects. Skin equivalents (SEs) were treated with ESE and the results showed that the epidermis became slightly thickened on addition of 0.002% ESE. The staining intensity of p63 as well as proliferating cell nuclear antigen (PCNA) is increased, and integrin α6 was upregulated. Analysis of ESE confirmed that PZ is the main ingredient. When SEs were treated with PZ, similar findings were observed. In particular, the expression of integrin α6, integrin β1, and type IV collagen was increased. Levels of mRNA for type IV collagen were increased and levels of miR135b were downregulated. All these findings suggested that PZ can affect the proliferative potential of epidermal cells in part by microenvironment changes via miR135b downregulation and following increased expression of type IV collagen.

Human oral cancer cells with increasing tumorigenic abilities exhibit higher effective membrane capacitance

OBJECTIVE

Although cells with tumorigenic/stem cell-like properties have been identified in many cancers, including oral squamous cell carcinoma (OSCC), their isolation and characterisation is still at early stages. The aim of this study is to characterise the electrophysiological properties of OSCC cells with different tumorigenic properties in order to establish if a correlation exists between tumorigenicity and cellular electrical characteristics.

MATERIALS AND METHODS

Rapid adherence to collagen IV was used as a non-invasive, functional method to isolate subsets of cells with different tumorigenic abilities from one oral dysplastic and three OSCC-derived cell lines. The cell subsets identified and isolated using this method were further investigated using dielectrophoresis, a label-free method to determine their electrophysiological parameters. Cell membrane morphology was investigated using scanning electron microscopy (SEM) and modulated by use of 4-methylumbelliferone (4-MU).

RESULTS

Rapid adherent cells (RAC) to collagen IV, enriched for increased tumorigenic ability, had significantly higher effective membrane capacitance than middle (MAC) and late (LAC) adherent cells. SEM showed that, in contrast to MAC and LAC, RAC displayed a rough surface, extremely rich in cellular protrusions. Treatment with 4-MU dramatically altered RAC membrane morphology by causing loss of filopodia, and significantly decreased their membrane capacitance, indicating that the highest membrane capacitance found in RAC was due to their cell membrane morphology.

CONCLUSION

This is the first study showing that OSCC cells with higher tumour formation ability exhibit higher effective membrane capacitance than cells that are less tumorigenic. OSSC cells with different tumorigenic ability possessed different electrophysiological properties mostly due to their differences in the cell membrane morphology. These results suggest that dielectrophoresis could potentially used in the future for reliable, label-free isolation of putative tumorigenic cells.

The mitochondrial electron transport chain is dispensable for proliferation and differentiation of epidermal progenitor cells

Tissue stem cells and germ line or embryonic stem cells were shown to have reduced oxidative metabolism, which was proposed to be an adaptive mechanism to reduce damage accumulation caused by reactive oxygen species. However, an alternate explanation is that stem cells are less dependent on specialized cytoplasmic functions compared with differentiated cells, therefore, having a high nuclear-to-cytoplasmic volume ratio and consequently a low mitochondrial content. To determine whether stem cells rely or not on mitochondrial respiration, we selectively ablated the electron transport chain in the basal layer of the epidermis, which includes the epidermal progenitor/stem cells (EPSCs). This was achieved using a loxP-flanked mitochondrial transcription factor A (Tfam) allele in conjunction with a keratin 14 Cre transgene. The epidermis of these animals (Tfam(EKO)) showed a profound depletion of mitochondrial DNA and complete absence of respiratory chain complexes. However, despite a short lifespan due to malnutrition, epidermal development and skin barrier function were not impaired. Differentiation of epidermal layers was normal and no proliferation defect or major increase of apoptosis could be observed. In contrast, mice with an epidermal ablation of prohibitin-2, a scaffold protein in the inner mitochondrial membrane, displayed a dramatic phenotype observable already in utero, with severely impaired skin architecture and barrier function, ultimately causing death from dehydration shortly after birth. In conclusion, we here provide unequivocal evidence that EPSCs, and probably tissue stem cells in general, are independent of the mitochondrial respiratory chain, but still require a functional dynamic mitochondrial compartment.