TIS7 and SKMc15 in fat metabolism and adipogenesis

The main subject of this proposal, protein named TPA Induced Sequence 7 (TIS7) is a transcriptional regulator playing a role in the differentiation of cells. TIS7 has an orthologue, named SKMc15 with so far only predicted function of a translational inhibitor. Both genes are conserved in many species, including humans. Our laboratory generated TIS7, SKMc15 and double knockout (dKO) mice that exhibit a decreased fat mass, most pronounced in dKO mice and even more are resistant to the high fat diet-induced obesity. The major goal of the proposed project is to identify the molecular mechanisms behind this phenotype. There are two possible scenarios: 1) a lack in the fat resorption in the guts or 2) diminished fat tissue differentiation ability. Yet another option is the combination of former two mechanisms. On the molecular level, we will focus our attention on the DLK1, an inhibitor of adipogenesis. DLK1 became the prime candidate for our studies since mice having high levels of the DLK1 protein are resistant against diet-induced obesity, the same phenotype as we observe in our genetically modified mice. Based on data from mouse embryonic fibroblasts generated from our mutant mice we ascribe the defect in adipogenic differentiation to an upregulation of Dlk1. Therefore, we aim to explore the DLK1 expression and mainly its regulation by TIS7 and SKMc15. In case of TIS7, we propose that this is due to the inhibition of Wnt signaling, which regulates DLK-1 expression and for SKMc15 we suggest a new mechanism linked to its ability to suppress translation. Furthermore, we want to characterize the expression of known regulators of fat tissue differentiation in both tissues and cell lines of TIS7 and SKMc15 knockout mice. The experimental manipulation of Wnt signaling pathway may clarify the involvement of TIS7 and SKMc15 in the adipocyte differentiation via this pathway. On the whole organism level, we will study the role of TIS7 and SKMc15 in the regulation of fat metabolism by crossing the TIS7 and SKMc15 single and double knockout mice with genetically- and diet-based mouse obesity models. Secondly, we will analyze the effects of TIS7 and SKMc15 knockout on the fat resorption and adipocyte generation by measurement of metabolic parameters in steady state and following the high fat diet challenge. The comparison of numerous parameters in TIS7 and SKMc15 single and double knockout mice will give us the opportunity to identify the role of these two proteins in the regulation fat metabolism and adipocyte differentiation. While the effects of DLK-1 on adipogenesis have been widely documented, the factors controlling DLK-1 expression and function remain poorly understood. We propose a novel mechanism for the regulation of DLK-1, affecting the adipocyte differentiation. This study should therefore be of interest for researchers investigating the molecular control of adipogenesis and cell differentiation in general. Furthermore, the characterization of the function of SKMc15 in the control of translation may be of more general interest. TIS7 and SKMc15 may play an important role in human diseases such as obesity and therefore become possible targets for the future development of novel therapies.