Apolipoprotein serum amyloid A4 (SAA4): Expression and signal-transduction activities of human SAA4 protein

The serum amyloid A (SAA) family comprises a number of differentially expressed apolipoproteins, acute-phase SAA isoproteins and constitutive SAAs isoproteins. Acute-phase-SAA isoforms are liver-derived inflammatory markers whose in vivo concentrations increase by as much as 1000-fold during inflammation. Besides that fact, acute phase SAAs are precursor proteins of secondary reactive amyloidosis and do represent the major apolipoproteins of high-density lipoprotein during inflammation drastically altering the biological properties of its physiological carrier. In contrast, constitutive SAA is induced minimally, if at all, during the the acute-phase response and has only been found in human and mouse. Human constitutive SAA (also named SAA4), a protein of hepatic origin, is a molecule of unknown function. In line with the proposed aims, we first established suitable expression models for SAA4. In contrast to pichia pastoris (a yeast strain), expression in Escherichia coli has turned out a promising expression system for further isolation of respectible protein amounts. One approach was devoted to the physicochemical characterization (CD spectroscopy and cristallization trials) of SAA4. Another approach was directed studying signal-transduction activities of human SAA4. Preliminary attempts doing expression cloning in oocytes were unsuccesful to identify a single candidate receptor for human SAA4. However, an increase of intracellular calcium in various cellular system following addition of exogenous SAA4 was found likely to be mediated by different pathways including calcium channels, G-protein coupled receptors, and recruitment of calcium from intracellular storage pools. Classical pathways including activation of phospholipase C and subsequent activation of protein kinase C(s) lead to phospholipase A2-mediated induction of the eicosanoid pathway resulting in increased generation of prostaglandin biosynthesis. We believe that the outcome of the proposed experiments will provide a platform for further biological activites of the human SAA4 protein.