Identification of Novel Keratinocyte Differentiation Modulating Compounds by High-Throughput Screening

Human epidermal stem cell function is regulated by circadian oscillations

Human skin copes with harmful environmental factors that are circadian in nature, yet how circadian rhythms modulate the function of human epidermal stem cells is mostly unknown. Here we show that in human epidermal stem cells and their differentiated counterparts, core clock genes peak in a successive and phased manner, establishing distinct temporal intervals during the 24 hr day period. Each of these successive clock waves is associated with a peak in the expression of subsets of transcripts that temporally segregate the predisposition of epidermal stem cells to respond to cues that regulate their proliferation or differentiation, such as TGFβ and calcium. Accordingly, circadian arrhythmia profoundly affects stem cell function in culture and in vivo. We hypothesize that this intricate mechanism ensures homeostasis by providing epidermal stem cells with environmentally relevant temporal functional cues during the course of the day and that its perturbation may contribute to aging and carcinogenesis.

Dermokine-β impairs ERK signaling through direct binding to GRP78

Dermokine-β is abundant in stratified epithelia and in differentiating cultured keratinocytes. In this study, we investigated the role of dermokine-β in differentiation of keratinocytes. Treatment of keratinocytes or skin tumor cells with dermokine-β attenuated phosphorylation of extracellular-signal-regulated kinase (ERK). Exposure of cells to dermokine-β, as well as its carboxyl-terminus domain peptide, interrupted phosphorylation of ERK and stimulated dermokine gene expression. Inhibition of ERK signaling by its specific inhibitor also increased dermokine expression level. A combination of chemical cross-linking and immunoprecipitation, followed by proteomics analyses, identified glucose-regulated protein 78 (GRP78) as a dermokine-β-associated protein. Blockage of GRP78 expression by a specific siRNA abrogated actions of dermokine-β. These findings provide novel insights into the physiological significance of dermokine-β in the epidermis.

Expanding the available assays: adapting and validating In-Cell Westerns in microfluidic devices for cell-based assays

Microfluidic methods for cellular studies can significantly reduce costs due to reduced reagent and biological specimen requirements compared with many traditional culture techniques. However, current types of readouts are limited and this lack of suitable readouts for microfluidic cultures has significantly hindered the application of microfluidics for cell-based assays. The In-Cell Western (ICW) technique uses quantitative immunocytochemistry and a laser scanner to provide an in situ measure of protein quantities in cells grown in microfluidic channels of arbitrary geometries. The use of ICWs in microfluidic channels was validated by a detailed comparison with current macroscale methods and shown to have excellent correlation. Transforming growth factor-β-induced epithelial-to-mesenchymal transition of an epithelial cell line was used as an example for further validation of the technique as a readout for soluble-factor-based assays performed in high-throughput microfluidic channels. The use of passive pumping for sample delivery and laser scanning for analysis opens the door to high-throughput quantitative microfluidic cell-based assays that integrate seamlessly with existing high-throughput infrastructure.

Automated high-throughput microchannel assays for cell biology: Operational optimization and characterization

Screening biological readouts in cell culture are increasing in frequency and throughput. In such assays, cell types may be rare and reagents or compounds may be expensive often resulting in a reduced number of conditions and/or replicates. "Tubeless" microfluidics offers a method to reduce this burden, as has been previously shown.1 In addition the In Cell Western (ICW) has recently been adapted to microfluidic cultures allowing high throughput analysis of immunocytochemistry (ICC) in microfluidic channels.2 Combining automated liquid handling in tubeless microfluidics with the ICW provides rapid and quantitative high throughput cell-based screens. Here we validate this platform using three parameters: operational robustness (pipetting reliability), cell seeding consistency, and cell staining consistency (both nuclear and antibody). Integration of liquid handling with microfluidics was found to be over 97% operationally robust. Cell seeding consistency between each microchannel and within each microchannel was found to be within a standard deviation of less than 5% and 6% respectively. Finally, through optimization of liquid handling steps, uniformity between all the channels was found for both nuclear and antibody staining. These results lay the foundation to perform most standard ICW assays using automated tubeless microfluidics.

PI3-kinase-dependent activation of apoptotic machinery occurs on commitment of epidermal keratinocytes to terminal differentiation

We have investigated the earliest events in commitment of human epidermal keratinocytes to terminal differentiation. Phosphorylated Akt and caspase activation were detected in cells exiting the basal layer of the epidermis. Activation of Akt by retroviral transduction of primary cultures of human keratinocytes resulted in an increase in abortive clones founded by transit amplifying cells, while inhibition of the upstream kinase, PI3-kinase, inhibited suspension-induced terminal differentiation. Caspase inhibition also blocked differentiation, the primary mediator being caspase 8. Caspase activation was initiated by 2 h in suspension, preceding the onset of expression of the terminal differentiation marker involucrin by several hours. Incubation of suspended cells with fibronectin or inhibition of PI3-kinase prevented caspase induction. At 2 h in suspension, keratinocytes that had become committed to terminal differentiation had increased side scatter, were 7-aminoactinomycin D (7-AAD) positive and annexin V negative; they exhibited loss of mitochondrial membrane potential and increased cardiolipin oxidation, but with no increase in reactive oxygen species. These properties indicate that the onset of terminal differentiation, while regulated by PI3-kinase and caspases, is not a classical apoptotic process.

Cell and molecular biology of the novel protein tyrosine-phosphatase-interacting protein 51

This chapter examines the current state of knowledge about the expression profile, as well as biochemical properties and biological functions of the evolutionarily conserved protein PTPIP51. PTPIP51 is apparently expressed in splice variants and shows a particularly high expression in epithelia, skeletal muscle, placenta, and germ cells, as well as during mammalian development and in cancer. PTPIP51 is an in vitro substrate of Src- and protein kinase A, the PTP1B/TCPTP protein tyrosine phosphatases and interacts with 14-3-3 proteins, the Nuf2 kinetochore protein, the ninein-interacting CGI-99 protein, diacylglycerol kinase alpha, and also with itself forming dimers and trimers. Although the precise cellular function remains to be elucidated, the current data implicate PTPIP51 in signaling cascades mediating proliferation, differentiation, apoptosis, and motility.

Identification and characterization of small-molecule inducers of epidermal keratinocyte differentiation

An essential function of the human epidermis is the maintenance of a protective barrier against the environment. As a consequence, keratinocytes, which make up this layer of the skin, undergo an elaborate process of self-renewal, terminal differentiation, and cell death. Misregulation of these processes can lead to several human diseases, including psoriasis and basal cell and squamous cell carcinomas. To identify novel regulators of keratinocyte differentiation, a cell-based screen of small-molecule libraries was carried out for molecules that induce terminal differentiation of normal human epidermal keratinocytes. One class of molecules was identified, the 2-(3,4,5-trimethoxyphenylamino)-pyrrolo[2,3-d]pyrimidines, which were shown to induce differentiation of epidermal progenitor cells to terminally differentiated keratinocytes. These molecules serve as useful mechanistic probes of the cellular differentiation programs that regulate the formation and homeostasis of the epidermis and may lead to novel therapeutic approaches for the treatment of skin hyperproliferative disorders.

In vitro Biological Characterization of a Novel, Synthetic Diaryl Pyrazole Resorcinol Class of Heat Shock Protein 90 Inhibitors

The molecular chaperone heat shock protein 90 (HSP90) has emerged as an exciting molecular target. Derivatives of the natural product geldanamycin, such as 17-allylamino-17-demethoxy-geldanamycin (17-AAG), were the first HSP90 ATPase inhibitors to enter clinical trial. Synthetic small-molecule HSP90 inhibitors have potential advantages. Here, we describe the biological properties of the lead compound of a new class of 3,4-diaryl pyrazole resorcinol HSP90 inhibitor (CCT018159), which we identified by high-throughput screening. CCT018159 inhibited human HSP90beta with comparable potency to 17-AAG and with similar ATP-competitive kinetics. X-ray crystallographic structures of the NH(2)-terminal domain of yeast Hsp90 complexed with CCT018159 or its analogues showed binding properties similar to radicicol. The mean cellular GI(50) value of CCT018159 across a panel of human cancer cell lines, including melanoma, was 5.3 mumol/L. Unlike 17-AAG, the in vitro antitumor activity of the pyrazole resorcinol analogues is independent of NQO1/DT-diaphorase and P-glycoprotein expression. The molecular signature of HSP90 inhibition, comprising increased expression of HSP72 protein and depletion of ERBB2, CDK4, C-RAF, and mutant B-RAF, was shown by Western blotting and quantified by time-resolved fluorescent-Cellisa in human cancer cell lines treated with CCT018159. CCT018159 caused cell cytostasis associated with a G(1) arrest and induced apoptosis. CCT018159 also inhibited key endothelial and tumor cell functions implicated in invasion and angiogenesis. Overall, we have shown that diaryl pyrazole resorcinols exhibited similar cellular properties to 17-AAG with potential advantages (e.g., aqueous solubility, independence from NQO1 and P-glycoprotein). These compounds form the basis for further structure-based optimization to identify more potent inhibitors suitable for clinical development.