Prothymosin Alpha and Immune Responses: Are We Close to Potential Clinical Applications?

Abstract

The thymus gland produces soluble molecules, which mediate significant immune functions. The first biologically active thymic extract was thymosin fraction V, the fractionation of which led to the isolation of a series of immunoactive polypeptides, including prothymosin alpha (proTα). ProTα displays a dual role, intracellularly as a survival and proliferation mediator and extracellularly as a biological response modifier. Accordingly, inside the cell, proTα is implicated in crucial intracellular circuits and may serve as a surrogate tumor biomarker, but when found outside the cell, it could be used as a therapeutic agent for treating immune system deficiencies. In fact, proTα possesses pleiotropic adjuvant activity and a series of immunomodulatory effects (eg, anticancer, antiviral, neuroprotective, cardioprotective). Moreover, several reports suggest that the variable activity of proTα might be exerted through different parts of the molecule. We first reported that the main immunoactive region of proTα is the carboxy-terminal decapeptide proTα(100-109). In conjunction with data from others, we also revealed that proTα and proTα(100-109) signal through Toll-like receptor 4. Although their precise molecular mechanism of action is yet not fully elucidated, proTα and proTα(100-109) are viewed as candidate adjuvants for cancer immunotherapy. Here, we present a historical overview on the discovery and isolation of thymosins with emphasis on proTα and data on some immune-related new activities of the polypeptide and smaller immunostimulatory peptides thereof. Finally, we propose a compiled scenario on proTα's mode of action, which could eventually contribute to its clinical application.

Keywords: Adjuvant; Alarmin; Cancer; DAMP; Immune response; Immunoenhancing peptide; ProTα(100–109); Prothymosin α; Thymic peptides.

Fig. 1

The intracellular role of proTα. (A) In the absence of proTα, histone H1 binds to nucleosomes and induces condensation of euchromatin to heterochromatin. ProTα interacts with histone H1, mediates its transfer from and to chromatin, and leads to the formation of CREB–CBP–p300 complex, chromatin remodeling, and gene transcription. (B) Apoptotic stimuli induce the release of cytochrome c, which binds to Apaf-1 and forms the apoptosome. The subsequent activation of caspase 9 results in conversion of procaspase 3 to caspase 3, leading to apoptosis. ProTα hinders binding of cytochrome c to Apaf-1, and the apoptotic cascade is inhibited. (C) Normally, the Nrf2–Keap1–Cul3–Rbx1 complex is ubiquitinated and degraded by proteasomes. Under oxidative stress, proTα binds to the complex via Keap1, and Nrf2 is released, migrates to the nucleus, and promotes gene transcription and antioxidant enzyme production.

Fig. 2

Proposed scenario on proTα’s dual role. In normal cells, proTα regulates gene expression and cell proliferation in the nucleus. Under abnormal conditions, cells die via necrosis or apoptosis. During necrosis, cell membrane is ruptured and intact proTα is released out of the cell. During apoptosis, proTα is relocated to the cytoplasm and is cleaved at its C-terminus by activated caspases, and proTα(100–109) is generated. The decapeptide adopts a β-sheet conformation and is excreted. Extracellularly, both proTα and proTα(100–109) activate innate immune cells expressing TLR4 (macrophages, neutrophils, DCs, and monocytes) and signal through the MyD88 and TRIF pathways. Cytotoxic responses are enhanced through antigen presentation with MHC class II molecules and synapsis with helper T cells which secrete T_H1-type cytokines, and/or with MHC class I molecules, leading to activation of cytotoxic effectors (NK cells and CTLs). In both cases, effector cells upregulate adhesion molecule expression (eg, CD2) and produce lytic molecules (eg, perforin), mediating cell binding and cell destruction, respectively (eg, cancer cell targets, as shown here).