*2.4. Extracellular Functions of TCTP(HRF)*

Since the discovery of the histamine releasing factor (HRF) activity of TCTP in 1995 [55], it is established that, apart from its many intracellular functions, TCTP also acts as an extracellular molecule, typically in the context of immune reactions associated with allergic diseases. However, we will not expand on this, since the review by Kawakami et al. in this Special Issue [56] provided an up-to-date account on HRF and the current understanding of its involvement in various disease settings and its potential as a new therapeutic target. For a historic overview on the 'HRF story', the reader is referred to an earlier review article by Susan MacDonald [57].

### **3. Mechanisms of Regulation of Cellular TCTP Protein Levels**

Being an anti-apoptotic protein involved in cellular stress responses, it is not surprising that the levels of TCTP are highly regulated. In our previous review article [5], we compiled a list of cell physiologic conditions that resulted in alterations of cellular TCTP levels. We also provided an overview on the principal mechanisms involved in TCTP regulation. As the name of the protein suggests, translational regulation plays an important role among these processes; it represents the most e ffective means for rapid de novo-synthesis. However, other modes of protein regulation, such as transcriptional control or regulated protein degradation, are also involved (Table 2, below). Since the publication of our review article [5], several new examples and mechanisms of regulation of TCTP levels have been revealed, and here we will largely focus on these more recent developments in this area.
