YKL‐40 augments the proliferation of HaCaT cells via the extracellular regulatory kinase pathway

YKL-40, one of the chitinase-like proteins, functions as both cytokine and growth factor and is produced by multiple cell types including stromal cells and immune cells. YKL-40 has diverse roles in cell proliferation, differentiation, survival, inflammation and tissue remodeling, and its aberrant expression is associated with the pathogenesis of a wide variety of human diseases. Concerning skin diseases, serum YKL-40 levels are increased in patients with psoriasis or atopic dermatitis (AD) and correlated with disease severity, suggesting that YKL-40 is involved in skin inflammatory diseases. However, there have been few reports on YKL-40 function in skin inflammation. As YKL-40 is known to promote proliferation or survival of epithelial cells, such as colonic epithelial cells and airway epithelial cells, we assessed the proliferative effect of YKL-40 on HaCaT cells, one of the cell lines of epidermal keratinocytes in this study. HaCaT cells were cultured in 75-cm cell culture flasks (Corning, Corning, NY, USA) at 37°C, 5% CO2 in Eagle’s minimum essential medium (Sigma, St Louis, MO, USA) containing 10% fetal bovine serum. When the cells reached confluence, they were trypsinized, washed and resuspended in the medium at 1 9 10 cells/mL, and 1 mL of cells was added to each well of the six-well plates (Becton Dickinson, Franklin Lakes, NJ, USA). When the cells reached approximately 50% of confluence, recombinant human YKL-40 (30 or 100 ng/mL; ATGen, Seongnam, Korea) was added. In some experiments, 10 lmol/ L U0126 (MEK1/2 inhibitor; Cell Signaling Technology, Danvers, MA, USA) was added 1 h before YKL-40 administration. After