The Barrier Function of the Skin Protects the Body from the External Environment

The skin is the largest organ of the body and as the outer covering of the body, has a significant protective role. The outermost layer of the skin is called the epidermis, and is composed of multiple layers of keratinocytes. Keratinocytes proliferate in the innermost layer of the epidermis, and as they migrate outward, they differentiate, finally becoming anucleated corneocytes, which are sloughed off the surface of the skin. The epidermis also contains melanocytes, the pigmented cells of the skin that are the cell-of-origin of melanoma. The inner layer of the skin is called the dermis, which contains the sweat glands, hair follicles, and mechanoreceptors of the skin.

As the first layer of protection against the external environment, the skin serves a barrier function. Like most epithelial cell types, adjacent keratinocytes are connected via tight junctions, protein complexes that form a barrier between cells, allowing only selective transport of molecules. Since it is constantly exposed, the skin receives continual stressors and damage, and conditions such as inflammation (eczema, psoriasis, etc.), can disrupt homeostasis in the skin and compromise its barrier function.

Recent Studies Using Lifeline® Keratinocytes and Melanocytes

Atopic dermatitis is caused by chronic skin inflammation and is typically treated with steroids. However, steroids can cause various undesirable effects in humans, prompting a search for alternative treatment options. Using in vivo and in vitro techniques, Yamada and colleagues set out to determine whether high doses of topical glucose could treat atopic dermatitis. First, the researchers treated a mouse model of dermatitis (induced by topical application of mite antigen) with tacrolimus (an immunosuppressive agent), prednisolone (a steroid), or high doses of glucose. They found that like tacrolimus and prednisolone, glucose alleviated dermatitis in mice, assessed by improved histopathology, decreased inflammatory infiltrate, and decreased levels of inflammatory chemokines and cytokines, including TARC, IgE, IL-12, IFN-γ, IL-4, IL-5, and IL-13. Importantly, high doses of topical glucose also rescued expression of filaggrin and the tight junction protein claudin-1; loss of these proteins is associated with development of atopic dermatitis.

Finally, using Lifeline® normal human keratinocytes, the researchers performed in vitro time course experiments to determine the dynamics of the claudin-1 and filaggrin responses to inflammation. They found that gene expression of claudin-1 and filaggrin peaked at 0.5 and 2 h, respectively, following glucose treatment, confirming that glucose induces expression of these molecules. Importantly, following treatment with thymic stromal lymphoprotein (TSLP) to induce inflammation, glucose treatment was able to increase filaggrin expression and reduce STAT3 signaling, suggesting it has a protective effect against inflammatory stimulation. Together, the results of this study demonstrate that glucose might be a viable alternative to steroids for treatment of atopic dermatitis.

In order to maintain a survival advantage, cancer cells disrupt normal metabolic processes to sustain rapid and continual proliferation. Recently, it has become clear that cancer cells have an increased requirement for cholesterol. In a study from the British Journal of Cancer, Kuzu et al. investigated whether targeting cholesterol transport using acid sphingomyelin (ASM) inhibitors could be effective in inducing cell death in melanoma cells.

First, the researchers knocked down SMPD1, the gene for ASM, which normally functions to promote cholesterol transport out of lysosomes. They found that knockdown of ASM in melanoma cells caused lysosomal cholesterol accumulation and cell death. To evaluate the efficacy of ASM inhibitors against melanoma cells, they screened 42 inhibitors for the ability to kill melanoma cells and normal skin fibroblasts. Interestingly, they found that melanoma cells were more sensitive to ASM inhibitors than normal fibroblasts. Using five of the 42 ASM inhibitors and an inhibitor of the v-ATPase (a molecule that allows transport of the drugs into the lysosome), the researchers confirmed that cell death was caused by lysosomal accumulation of cholesterol. Importantly, following treatment with excess cholesterol, the authors demonstrated that lysosomal accumulation of cholesterol was due to inhibition of cholesterol transport out of the lysosome.

To evaluate how ASM inhibitors affect cholesterol homeostasis on a more global level, the authors performed genome-wide RNA Seq analysis on melanoma cells treated with ASM inhibitors. As expected, they found that a number of genes involved in cholesterol homeostasis were dysregulated, including an increase in genes responsible for cholesterol biosynthesis; they found a similar effect was true following analysis of gene expression data from breast cancer cells, mined from an available online database.

The authors next examined whether ASM inhibitors were effective against a murine melanoma xenograft model. Indeed, treatment of mice bearing tumors with ASM inhibitors resulted in decreased tumor volume, however, the doses had to be limited due to toxicity. To improve tolerance, the researchers encapsulated the drug in nanoliposomes for deliver to mice. Importantly, nanoliposome drug delivery decreased tumor volume, and ameliorated the side effects, indicating that this delivery method may be useful for cancer therapy in humans.

The authors also examined the effects of ASM inhibitors on various cellular processes, and found that ASM inhibitor treatment inhibited autophagy, endocytosis, intracellular trafficking, and PI3K/AKT/STAT3 signaling. Interestingly, they found that cells with a mutation in the PI3K/AKT negative regulator, PTEN, were more sensitive to ASM inhibitors than normal cells, including Lifeline® normal human melanocytes, suggesting a potential genetic sensitivity. Finally, by determining the mechanism of cell death in response to ASM inhibitors, the authors concluded that apoptosis was due to caspase-independent processes, but involved BAX translocation to the mitochondrial membrane. Since ASM inhibitors are already in use for certain diseases, including Niemann-Pick disease, this study demonstrates that these agents might also be effective as chemotherapeutic compounds in certain cancers.

Lifeline® offers keratinocytes and melanocytes, each optimized for growth in respective DermaLife® media, including:

Neonatal and adult melanocytes
Neonatal and adult keratinocytes

Please visit our website at https://www.lifelinecelltech.com to see our entire catalog of cells and media. And keep visiting us here on our blog every other week to learn how researchers around the world are using our cells!