TSAP6 Facilitates the Secretion of Translationally Controlled Tumor Protein/Histamine-releasing Factor via a Nonclassical Pathway

Abstract

Translationally controlled tumor protein (TCTP) is cytoplasmic and structurally related to guanine-nucleotide free chaperones. TCTP (also called histamine-releasing factor) has been described previously as a secreted protein that participates in inflammatory responses by promoting the release of histamine. How TCTP is eventually exported out of the cell to promote such activities is unknown. Here we show that TCTP secretion was insensitive to either brefeldin A or monensin, suggesting that it proceeds via an endoplasmic reticulum/Golgi-independent or nonclassical pathway. Moreover, our analyses also suggest that secreted TCTP originates from pre-existing pools. TSAP6, a p53-inducible 5-6 transmembrane protein, was found to interact with TCTP in a yeast two-hybrid hunt. GST pull down assays confirmed their direct interaction, and immunofluorescence analysis revealed their partial co-distribution to vesicular-like structures at the plasma membrane and around the nucleus. Functionally, the overexpression of TSAP6 consistently leads to enhanced secretion of both endogenously and exogenously expressed TCTP. Finally, we found TCTP in preparations of small secreted vesicles called exosomes, which have been suggested as a possible pathway for nonclassical secretion. Overexpression of TSAP6 also increased TCTP levels in exosome preparations. Altogether, these data identify a novel role for TSAP6 in the export of TCTP and indicate that this multipass membrane protein could have a general role in the regulation of vesicular trafficking and secretion.

The Translational Controlled Tumour Protein TCTP: Biological Functions and Regulation

“…First, the protein TCTP does not have a signal sequence (Chitpatima et al 1988;Gross et al 1989), and no precursor protein for TCTP was detected, meaning that TCTP is not secreted through the classical secretory pathway, via the ER and Golgi apparatus. This was confirmed by the finding that its secretion was insensitive to brefeldin A or monensin, two inhibitors of this pathway (Amzallag et al 2004). These authors proposed a non-classical pathway for the secretion of TCTP.…”

“…Overexpression of TSAP6 did indeed result in increased TCTP levels in exosome preparations and in enhanced secretion of TCTP (Amzallag et al 2004), whereas cells derived from TSAP6-deficient mice are severely compromised in the DNA damage-induced p53-dependent exosomal secretory pathway . Another, also exosome-related pathway for TCTP secretion, was proposed by the Kyunglim Lee's group.…”

The P53 pathway: what questions remain to be explored?

“…More recently the p53-regulated TSAP-6 gene was shown to enhance the rate of exosome production from cells undergoing a p53 response to stress. 56 Exosomes also can communicate with other cells both in the immune system and cells in its own vicinity. Cells in stress will want to coordinate their responses with the surrounding cell layer in vivo and this is a new field opening up to future research efforts.…”

“…In addition p53 activation can enhance the rate of exosome production by the damaged cell by inducing the p53-regulated TSAP-6 gene. 56,61 Exosomes can communicate with adjacent cells by cell fusion and with T cells by presenting antigens to the immune system. Could p53-mediated apoptosis enhance immunization of the host in this fashion?…”

Silencing of human papillomavirus (HPV) E6/E7 oncogene expression affects both the contents and the amounts of extracellular microvesicles released from HPV‐positive cancer cells

“…38-41 Therefore, we tested whether the reconstitution of endogenous p53 amounts on E6/E7 repression (Fig. 2a) was linked to the stimulation of these genes in HeLa cells.…”