Secretory leukocyte peptidase inhibitor expression and apoptosis effect in oral leukoplakia and oral squamous cell carcinoma.

Oral leukoplakia (OL) is one of the most common oral precancerous lesions with the possibility of malignant transformation, ranging from 17 to 24% of patients with a median follow-up of >7 years. Previous research has revealed that compared with normal oral epithelial tissues, the expression of secretory leukocyte peptidase inhibitor (SLPI) protein is significantly reduced in oral squamous cell carcinoma (OSCC). Based on the above-mentioned research, it is known that SLPI is a potential predictive and diagnostic tool for the progression of oral carcinogenesis. Therefore, we investigated the correlation between the abundance of SLPI protein and the different histological grades of OL by immunohistochemistry. The results indicated that the level of SLPI was negatively correlated with the histological grades of the oral premalignant lesions, indicating that it may be a potential predictive tool for the malignant transformation presented in oral precancerous patients. Subsequently, we investigated the biological effects of SLPI using Cell Counting Kit (CCK)-8, Annexin V/PI apoptosis assay and Caspase-Glo® 3/7 assay. The findings revealed that SLPI promoted apoptosis in the Leuk1 and WSU-HN4 cell lines. Mechanistic studies indicated that SLPI, at least in part, regulated cell apoptosis by inhibiting the expression of TNF receptor-associated factor 1 (TRAF1), which has a close relationship with the nuclear factor-κB (NF-κB) pathway.