Ppia, also known as peptidylprolyl isomerase A or cyclophilin A, is a central enzyme in the protein-folding response pathway [3]. It plays diverse roles in various biological processes. For instance, it is involved in promoting protein folding, which is crucial for normal cellular function. It also participates in multiple pathways, such as those related to cell growth, differentiation, and metabolism.

In lung cancer, ablation of Ppia promotes NRF2 protein degradation and blocks NRF2-driven growth in NSCLC cells, as Ppia binds to NRF2 and prevents its ubiquitin-mediated degradation [1]. In adipogenesis, PPIA-/-mice had less fat and smaller adipocytes, and silencing PPIA in 3T3L1 cells impaired adipocyte differentiation, indicating its role as an adipogenic factor [2]. In multiple myeloma, CRISPR-Cas9 deletion of PPIA or its inhibition with a small molecule inhibitor sensitizes MM tumor cells to proteasome inhibitors [3]. In oral cancer, reducing PPIA expression and secretion decreased cell invasion and migration, while recombinant PPIA increased these processes, suggesting its role in promoting cancer cell metastasis [4].

In conclusion, Ppia is essential for protein-folding and is involved in numerous biological processes. Its functions are particularly evident in diseases such as lung cancer, obesity, multiple myeloma, and oral cancer. Gene knockout (KO) mouse models have been instrumental in revealing these functions, helping us understand the role of Ppia in specific biological processes and disease conditions.