Results presented in the study show that TCTP is a heat inducible protein that can protect cells from thermal shock by potentially acting as a molecular chaperone. Our studies also show that overexpression of TCTP in cells can protect the cells from heat shock. These findings collectively suggest that TCTP may be a novel small molecular weight heat shock protein.
Expression of TCTP is known to be highly regulated, especially in response to a wide range of stress conditions such as ammonium starvation in fission yeast [9
], in response to environmental pollutants such as dioxin in mouse embryonic cells [19
], hypoxia conditions [20
], DTT-induced oxidative stress [9
], calcium perturbation [21
], heavy metal toxicity [10
] and heat shock [12
]. In addition, one of the TCTP sequences deposited in the Genbank designates the protein as a pO2 related protein in the hepato-carcinoma cells (Genbank accession number: AAM51565). These reports collectively support the notion that TCTP may be a stress-related protein.
Recently, we cloned TCTP homologues from several human parasites including B. malayi, W. bancrofti
], S. mansoni
] and S. haematobium
(Genbank accession number: AY157847). All these parasites have life cycle stages inside an invertebrate host before infecting human. Analysis of the TCTP expression levels show that larval stages that enter the warm-blooded vertebrate host has significantly higher levels of TCTP compared to the stages in the cold-blooded invertebrate hosts. This observation did not surprise us because TCTP has been documented as a heat-induced protein [12
]. In fact, following heat shock, TCTP was one of the three genes that were up regulated in the infective larvae of another human parasite T. spiralis
]. In our study, larval stages of S. mansoni
and human PBMCs subjected to thermal shock in vitro
also showed higher TCTP expression. These findings thus confirmed previous finding that TCTP is a heat-inducible protein [12
Heat shock proteins are a family of highly conserved molecules that are up regulated during heat or oxidative stress and protect the cellular proteins from denaturation [22
]. Based on their molecular size, the heat shock proteins can be grouped into small molecular weight heat shock proteins (14–33 kDa) and large heat shock proteins (60–100 kDa). Given the molecular size of TCTP and the fact that TCTP is up regulated during heat stress, it is possible that TCTP is a small heat shock-like protein. One of the primary functions of heat shock proteins is to protect cells from stress-induced damages including inhibiting both apoptotic and necrotic pathways [23
]. Several recent reports suggest that TCTP is an anti-apoptotic protein [6
]. Heat shock proteins also function as molecular chaperon by modulating other protein functions by changing their conformation, promoting protein-protein interaction and disassembly, regulating protein degradation, facilitating protein translocation across membranes, and ensuring proper folding of newly synthesized proteins during translation [13
]. A typical characteristics of molecular chaperones is that they can recognize proteins of non-native structure and prevent them from irreversible intracellular aggregation [26
]. Results from our study show that TCTP can bind to a variety of proteins including CS, luciferase and lysozyme denatured by heat or by chemical. Another interesting finding in our study is that similar to small heat shock proteins [27
], TCTP appears to have the ability to bind to denatured proteins in the absence of ATP. This suggests that TCTP may be a novel class of small heat shock chaperones.
By binding to denatured proteins, molecular chaperones prevent aggregation of proteins during heat stress [26
]. Our study suggests that TCTP behaved similar to other classic molecular chaperones. In addition to binding and preventing thermal aggregation of denatured proteins, rSmTCTP can also reactivate the denatured proteins. These findings suggested that TCTP in addition to protecting the proteins from thermal damage could help reverse their function during or following a heat shock. This property of TCTP may be thus potentially important in rescuing cells from the damaging effects of heat stress.
Further confirmation on the cell protective role of TCTP during heat shock came from studies using E. coli cells over expressed with TCTP. This implies that SmTCTP may have an important role in protecting S. mansoni from harmful effects of heat shock that they encounter in the vertebrate host. In summary, the observation that the expression of TCTP is increased several fold during a heat shock plus the findings that TCTP can potentially act as a molecular chaperone by protecting the cellular proteins from the damaging effects of thermal shock and are capable of reactivating thermally damaged proteins suggest that TCTP is a novel small molecular weight heat shock protein.