Thymosin α1 suppresses migration and invasion o f PD-L1 high-expressing non-small-cell lung cancer cells via inhibition of STAT3-MMP2 signaling

Badcground: Thymosin al (T alpha 1) is one of the most commonly used immunomodulators for metastatic non-small-cell lung cancer (NSCLC) patients in many countries. Despite the identification of the direct suppression on cancer cell proliferation, little is known about its effect on metastasis and metastasis-related signaling such as matrix metalloproteinases (MMPs) and programmed cell death ligand 1 (PD-L1). Materials and methods: NSCLC cells with distinguishing PD-L1 expression levels were treated with T alpha 1. siRNAs were used to knockdown PD-L1. Cell migration and invasion abilities were evaluated by wound-healing and transwell assays. The xenograft model by BALB/c nude mice was constructed to test the inhibitory effect of T alpha 1 on metastasis in vivo. The expression levels of metastasis-related signaling pathways and key molecules were assessed by Western blot (WB) and quantitative reverse transcriptase PCR (qRT-PCR). Results: T alpha 1 significantly suppressed cell migration and invasion in PD-L1 high-expressing H1299, NL9980, and L9981 cells but not in PD-L1 low-expressing A549 or SPC-A-1 cells. This difference was demonstrated by mouse model in vivo as well. Knocking down of PD-L1 significantly impaired the inhibition of cell migration and invasion caused by T alpha 1 treating in PD-L1 high-expressing cells. Besides, T alpha 1 inhibited the activation and translocation of STAT3 and the expression of MMP2 in PD-L1 high-expressing NSCLC cells. Moreover, the treatment of STAT3 activator colivelin could partly reverse the T alpha 1-induced MM P2 suppression and the migration phenotype. Conclusion: T alpha 1 significantly suppresses migration and invasion in PD-L1 high-expressing NSCLC cells compared with PD-L1 low-expressing NSCLC cells in vitro and in vivo, through the downregulation of STAT3-MMP2 signaling. These different responses to T alpha 1, together with the depiction of T alpha 1-induced signaling changes, suggest a potential benefit of T alpha 1 for PD-L1-positive NSCLC patients, enlightening the combination of T alpha 1 with target therapy or immune checkpoint inhibitors.