Rheumatoid arthritis (RA) is a chronic autoimmune disease characterized by joint deterioration. The clinical outcomes of patients with active RA can be improved using anti-rheumatic medications, such as methotrexate (MTX). Many patients rely on MTX to limit the destructive joint damage and functional disability typical of RA. Although the drug is a folic acid antagonist, its precise mechanisms in RA patients are largely unknown. Previous research suggests that MTX also affects a type of white blood cell called CD4+ T cells. These cells are believed to play a role in the development of RA, specifically, the balance between the activation of interleukin-17-producing helper T (Th17) and CD4+ regulatory T (Treg) lymphocytes. Researchers suspect that MTX affects CD4+ T cells by suppressing T cell activity and increasing Treg cells, but its specific effects besides folate metabolism remain unclear.
Recently, a group of researchers discovered that MTX targets tumor protein p63 (TP63) in CD4+ T lymphocytes. The joint team from the Chiba University and the Kazusa DNA Research Institute in Tokyo, employed DNA microarray profiling of human CD4+ T cells from RA patients to understand how MTX influences gene expression. They also used gene knockdown and RNA sequencing (RNA-Seq) to validate gene function. The researchers found that TAp63, a protein isoform of TP63, was highly expressed in human and mouse Th17 cells. The RNA-Seq and gene knockdown data revealed that another gene, FOXP3, the master regulator of Treg lymphocytes, was targeted by TAp63. When TAp63 was “knocked down” in Treg cells, Foxp3 protein expression increased. By performing a reporter assay, the researchers confirmed that TAp63 was bound to the FOXP3 enhancer and suppressed it.
Together, these findings suggest that TAp63 is intricately linked to the balance of Th17 and Treg cell differentiation. Thus, inhibiting TAp63 could enhance the suppressive function of Treg cells and limit autoimmune RA. These findings reveal a robust mechanism for MTX action and show how Treg cells can be preserved in RA. They also demonstrate the potential of TAp63 as a new therapeutic target for this disease and potentially for others. Regarding the relationship between TP63 and noncancer human diseases, a genome-wide meta-analysis revealed that TP63 is associated with psoriasis. ΔNp63 is a dominant isoform of the skin and is vital for maintaining epidermal stem cells: TAp63, indeed, is downregulated in psoriatic lesions compared with normal skin. The effort to target p63 pharmacologically could also be directed towards finding a molecule that binds directly to this protein.
Along with its smaller cousin p53 and its larger cousin p73, this form of p63 shares a similar three-dimensional structure. And molecules that activate or suppress the function of p53 have been known for some time, both for the regular form and for the mutated one in certain tumor forms. Computer-aided molecular modeling could be a viable option to discover p63-binding drugs by analogy with the p53 binding pocket.
- Edited by Dr. Gianfrancesco Cormaci, PhD, specialist in Clinical Biochemistry.
Scientific references
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