The choice of Critical Effect Size – a retrospective analysis on the risk assessment of PFAS

The choice of CES is a matter of debate and a challenge in the standardization of the BMD approach and has a significant impact on the outcome and conclusions of the risk assessments. Factors that influence the choice of CES include both ethical considerations of severity as well as statistical aspects. Data-driven methods such as the 1 SD-approach or the GTES offer an objective assessment without human bias. However, it was shown in this study that the results may vary considerably and lead to CESs that are considered non-protective, while a default value of 5% may face criticism for being too low for many cases. In summary, the most suitable approach might be a comparative combination of strategies, to derive the guidance values that better reflect the compound’s potential threshold for toxicity. Thus, expert judgment will continue to play an irreplaceable role in chemical risk assessment

Characterisation of cell cycle arrest and terminal differentiation in a maximally proliferative human epithelial tissue: Lessons from the human hair follicle matrix

Human hair follicle (HF) growth and hair shaft formation require terminal differentiation-associated cell cycle arrest of highly proliferative matrix keratinocytes. However, the regulation of this complex event remains unknown. CIP/KIP family member proteins (p21CIP1, p27KIP1 and p57KIP2) regulate cell cycle progression/arrest, endoreplication, differentiation and apoptosis. Since they have not yet been adequately characterized in the human HF, we asked whether and where CIP/KIP proteins localise in the human hair matrix and pre-cortex in relation to cell cycle activity and HF-specific epithelial cell differentiation that is marked by keratin 85 (K85) protein expression. K85 expression coincided with loss or reduction in cell cycle activity markers, including in situ DNA synthesis (EdU incorporation), Ki-67, phospho-histone H3 and cyclins A and B1, affirming a post-mitotic state of pre-cortical HF keratinocytes. Expression of CIP/KIP proteins was found abundantly within the proliferative hair matrix, concomitant with a role in cell cycle checkpoint control. p21CIP1, p27KIP1 and cyclin E persisted within post-mitotic keratinocytes of the pre-cortex, whereas p57KIP2 protein decreased but became nuclear. These data imply a supportive role for CIP/KIP proteins in maintaining proliferative arrest, differentiation and anti-apoptotic pathways, promoting continuous hair bulb growth and hair shaft formation in anagen VI. Moreover, post-mitotic hair matrix regions contained cells with enlarged nuclei, and DNA in situ hybridisation showed cells that were >2N in the pre-cortex. This suggests that CIP/KIP proteins might counterbalance cyclin E to control further rounds of DNA replication in a cell population that has a propensity to become tetraploid. These data shed new light on the in situ-biography of human hair matrix keratinocytes on their path of active cell cycling, arrest and terminal differentiation, and showcase the human HF as an excellent, clinically relevant model system for cell cycle physiology research of human epithelial cells within their natural tissue habitat

CDK4/6 inhibition mitigates stem cell damage in a novel model for taxane-induced alopecia

Taxanes are a leading cause of severe and often permanent chemotherapy‐induced alopecia. As the underlying pathobiology of taxane chemotherapy‐induced alopecia remains poorly understood, we investigated how paclitaxel and docetaxel damage human scalp hair follicles in a clinically relevant ex vivo organ culture model. Paclitaxel and docetaxel induced massive mitotic defects and apoptosis in transit amplifying hair matrix keratinocytes and within epithelial stem/progenitor cell‐rich outer root sheath compartments, including within Keratin 15+ cell populations, thus implicating direct damage to stem/progenitor cells as an explanation for the severity and permanence of taxane chemotherapy‐induced alopecia. Moreover, by administering the CDK4/6 inhibitor palbociclib, we show that transit amplifying and stem/progenitor cells can be protected from paclitaxel cytotoxicity through G1 arrest, without premature catagen induction and additional hair follicle damage. Thus, the current study elucidates the pathobiology of taxane chemotherapy‐induced alopecia, highlights the paramount importance of epithelial stem/progenitor cell‐protective therapy in taxane‐based oncotherapy, and provides preclinical proof‐of‐principle in a healthy human (mini‐) organ that G1 arrest therapy can limit taxane‐induced tissue damage. Synopsis Taxane chemotherapy causes alopecia. This study shows how taxanes are toxic to dividing stem cells and transit amplifying cells in the hair follicle. A CDK4/6 inhibitor, that blocks cell division, can antagonise taxane‐induced damage in the hair follicle. Paclitaxel and docetaxel, chemotherapies that cause alopecia, promote mitotic defects and apoptosis in proliferating hair follicle stem/progenitor and transit amplifying cell compartments. Mitotic defects are marked by profound increases in phospho‐histone H3 immunoreactivity, micronucleation and transcriptional arrest. Pharmacological CDK4/6 inhibition potently induces G1 arrest in the human hair follicle. Pharmacological G1 arrest antagonises the mitosis‐targeting cytotoxicity taxanes, thereby protecting stem/progenitor cells and transit amplifying cells from damage. Taxane chemotherapy causes alopecia. This study shows how taxanes are toxic to dividing stem cells and transit amplifying cells in the hair follicle. A CDK4/6 inhibitor, that blocks cell division, can antagonise taxane‐induced damage in the hair follicle