Post-translational modifications of Hepatoma-derived growth factor

Abstract

Post-translational modifications (PTM's) are modifications that occur during or after protein translation. Nascent or folded protein can be subjected to an array of specific enzyme-catalyzed modifications on the amino acid side chains or the peptide backbone. Two broad categories of protein PTM's occur; the first includes all enzyme catalyzed covalent additions of different lower molecular chemical groups up to complex proteins to amino acid side chains in the target protein, whereas the second category comprises structural changes and the cleavage of peptide backbones in proteins either by action of proteases or, less commonly, by autocatalytic cleavage. PTM's can modulate the function of proteins by altering their activity state, localization, turnover, and interactions with other proteins.Hepatoma-derived growth factor (HDGF) is the prototype of a family of six proteins comprising HDGF, the four HDGF-related proteins (HRP-1-4), and the lens epithelium-derived growth factor (LEDGF). HDGF exhibits growth factor properties and has been implicated in organ development and tissue differentiation of the intestine, kidney, liver, and cardiovascular system. Recently, the role of HDGF in cancer biology has become a main focus of its research. HDGF was found to be over-expressed in a large number of different tumor types. Although a direct influence of HDGF on tumor biology is still unclear, its expression is correlated with metastasis and tumor recurrence in multiple studies. HDGF appears to be a novel prognostic marker for different types of cancer. Growth promoting as well as other activities of HDGF, like the suppression of differentiation; possible role in apoptotic processes; or its angiogenic properties have been suggested to play a role in tumor induction and/or cancer progression.Interestingly, until now, very little is known about how PTM's are involved in modulating HDGF function. Therefore, the main aim of the thesis was the identification of HDGF PTM's and their consequence on its function. At first, we have identified that HDGF is post-translationally modified by SUMO-1 at a non consensus site and SUMOylated HDGF does not associate with chromatin in contrast to the unSUMOylated form. Further, we show that HDGF secretion is regulated by the presence or absence of a serine phosphorylation site and loss in secretion is attributed to N-terminal processing of the protein. Additionally, the two cysteine residues in HDGF are involved in formation of intra and inter molecular disulfide bonds and N terminal processing favors dimer formation. Furthermore, we demonstrated the presence of HRP-2 isoforms with their developmentally regulated expression in different rat brain regions and were able to show that HDGF can interact with HRP-2. Moreover, a new HRP 2 isoform can exclusively interact and enrich a processed form of HDGF. Finally, we identified the presence of a functional caspase cleavage site in the C terminal region of murine HDGF. The results complied provide substantial new knowledge concerning PTM's of HDGF and affected mechanisms, and will help to add new perspectives for research in the field of HDGF and its related proteins.