On the Mechanisms Underlying the Secretion and Export of Translation- ally Controlled Tumor Protein/Histamine Releasing Factor (TCTP/HRF)

Numerous studies have demonstrated that TCTP/HRF is a unique cytokine, modulating the release of inflammatory mediators from various cell types including cells involved in allergic phenomena. Despite the absence of a leader sequence in its NH2-terminus, TCTP/HRF is regarded as a secreted protein found outside of cells as well as in fluids from allergic patients and parasitic organisms. Recent studies clarified several potential mechanisms leading to its secretion. For example, these studies showed that TCTP/HRF is exported from cells via a non-classical, endoplasmic reticulum (ER)/Golgi-independent mechanism associated with exosomal transport. TSAP6, a p53-inducible transmembrane protein, has been shown to enhance exosome production, and facilitate the secretion of TCTP/HRF into extracellular milieu. Additionally, H,K-ATPase also appears to play a role in the transport of TCTP/HRF, since inhibitors of H,K-ATPase also inhibit TCTP/HRF exit. The exact mechanisms involved in TCTP/HRF secretion has not yet emerged. Here we attempted to collate the available information in the current understanding of the mechanisms underlying the release of TCTP/HRF and of the factors that seem to influence these mechanisms.