Hepatocyte growth factor activator inhibitor type 1 maintains the assembly of keratin into desmosomes in keratinocytes by regulating protease-activated receptor 2-dependent p38 signaling.

Hepatocyte growth factor activator inhibitor type 1 (HAI-1; official symbol SPINT1) is a membrane-associated serine proteinase inhibitor abundantly expressed in epithelial tissues. Genetically engineered mouse models demonstrated that HAI-1 is critical for epidermal function, possibly through direct and indirect regulation of cell surface proteases, such as matriptase and prostasin. To obtain a better understanding of the role of HAI-1 in maintaining epidermal integrity, we performed ultrastructural analysis of Spint1-deleted mouse epidermis and organotypic culture of an HAI-1 knockdown (KD) human keratinocyte cell line, HaCaT. We found that the aggregation of tonofilaments to desmosomes was significantly reduced in HAI-1-deficient mouse epidermis with decreased desmosome number. Similar findings were observed in HAI-1 KD HaCaT organotypic cultures. Immunoblot and immunohistochemical analyses revealed that p38 mitogen-activated protein kinase was activated in response to HAI-1 insufficiency. Treatment of HAI-1 KD HaCaT cells with a p38 inhibitor abrogated the above-observed ultrastructural abnormalities. The activation of p38 induced by the loss of HAI-1 likely resulted from enhanced signaling of protease-activated receptor-2 (PAR-2), because its silencing abrogated the enhanced activation of p38. Consequently, treatment of HAI-1 KD HaCaT cells with a serine protease inhibitor, aprotinin, or PAR-2 antagonist alleviated the abnormal ultrastructural phenotype in organotypic culture. These results suggest that HAI-1 may have a critical role in maintaining normal keratinocyte morphology through regulation of PAR-2-dependent p38 mitogen-activated protein kinase signaling.