Relevance of Tristetraprolin in Inflammation and Disease

Inflammation is of central importance in the response to injury to protect an organism from harmful triggers, however, it can also entail chronic diseases and favour cancer development when incorrectly regulated. In endothelial cells, the basis for its complex molecular regulation is the activation of the transcription factor NF- kappaB and its coordinated crosstalk with mitogen activated protein kinases (MAPKs). These signaling pathways result in a variety of biological responses including not only inflammatory gene regulation but also the balance of cell survival and death. Whereas the coordination of proinflammatory effectors during the initiation phase of inflammation has been studied extensively, mechanisms of its resolution are less well understood. However, molecular mechanisms leading to the shut-down of proinflammatory responses are of central importance. One such promising candidate regulator is the mRNA-binding and -degrading protein Tristetraprolin (TTP): it is involved in the AU-rich element (ARE) mediated degradation of proinflammatory mRNAs, including TNFalpha and various interleukins. We have shown recently that TTP has a second activity that is independent from mRNA destabilization, namely the impairment of NF-kappaB activation through restricting TNFalpha induced NF- kappaB/p65 nuclear translocation. Further preliminary analyses suggest that TTP function(s) is (are) controlled by alternating MAPK-activities that differ from those described in the context of ARE mediated decay. In addition, we find TTP involved in the TNFalpha-mediated onset of JNK-signaling resulting in apoptosis. The aim of this project is to investigate this novel activity of TTP in more detail. This includes the kinase(s) that phosphorylate TTP to differentially regulate its NF-kappaB inhibiting vs. ARE mediated decay activities; Second, we will study the role of phosphorylation-induced ubiquitination, for which we have preliminary evidence, and which may serve not only degradative but also regulatory functions. Third, the influence of the novel TTP activity on gene expression that is independent from the effects on ARE mediated decay will be studied in vitro and in vivo. For the latter approach we will generate a "knock-in" mouse carrying a mutant TTP that retains the NF- kappaB-inhibitory-, but not the ARE-decay activity. Together, these experiments will elucidate the biological function of TTP as a "bifunctional regulator" in the activation of NF-kappaB and MAPK cascades and as a feedback regulator in the inflammatory episode of the endothelium.

Inflammation appears as indispensable process that guarantees a rapid response to tissue injury or infections but necessitates tight control in order to minimize collateral tissue damage, maintain self-tolerance and prevent cancer development. Our goal was to investigate the role of the zinc finger protein Tristetraprolin (TTP) in the context of inflammation and disease and our investigations brought about stunning new details concerning its mode of function. Briefly, we uncovered TTP as a novel regulatory component of tumor necrosis factor (TNF)- signaling where it functions as inducible regulator between signals promoting cell survival and death.In an inflammatory background, binding of TNF to its receptor (TNFR) concomitantly activates the so called NF-?B and JNK signaling cascades which is the key for a balanced regulation of cell survival and death. Our biochemical approaches evidence TTP as timer in this context: we find TTP inducibly hypermodified by signaling components oft the NF-?B/JNK cascades. At first, different, heavily phosphorylated TTP-forms were introduced as high-moleclar-weight (HMW)-, middle-molecular-weight (MMW)- and low-molecular weight (LMW)-TTP (Schichl et al., 2011). Second, we could correlate the appearance of HMW-TTP with protein ubiquitination, and third we find this ubiquitination responsible for TTPs specific regulatory function towards JNK activation, operating both cell survival and apoptotic death.More precise analyses revealed that recruitment of HMW-TTP to the TNFR facilitates the formation of a protein complex with TRAF2, thereby promoting JNK activation and the regulation of corresponding transcription factors. Concomitantly, these HMW-forms of TTP protect cells from TNF-induced apoptotic death. Importantly, we observed that the growth and survival of cancer cells can be controlled by TTP in a TNF dependent manner (Resch et al., 2014) which opens a new therapeutic window for the treatment of inflammatory driven malignant cell growth.