Molecular Biology Reports 27: 81–86, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 81 STAT3 involvement in the acute phase-related expression of the rat haptoglobin gene Ilijana Grigorov, Tanja Lazi´ c, Ivana Cvetkovi´ c, Tanja Milosavljevi´ c & Miodrag Petrovi´ c Institute for Biological Research, Department for Molecular Biology, Belgrade, Yugoslav Federal Republic (Fax: (38111)761-433; E-mail: pemi@ibiss.bg.ac.yu) Received 25 February 2000; accepted 23 April 2000 Key words: acute-phase reaction, haptoglobin gene, STAT3, trans-acting proteins Abstract Turpentine-induced acute-phase (AP) response in rats is followed by transcriptional activation of the haptoglobin (Hp) gene in liver. Analysing the promoter sequence of the rat Hp gene we postulated an involvement of Signal Transducer and Activator of Transcription 3 (STAT3) in the regulation of this process. Results obtained by using a combination of Western immunoblot and DNA-binding assays revealed AP-induced binding of constitutive 86kD- and inducible 91kD-STAT3 isoforms to the rat Hp gene inducible promoter element. On the basis of these data we assumed that AP-related interactions of these two STAT3 isoforms correlates with an activated transcriptional status of the rat Hp gene. Introduction The AP protein response is a part of the systemic re- sponse to inflammatory processes. It is characterized by a release of AP mediators that are able to interact with hepatocytes and induce a dramatic change in pro- teins secreted by these cells, which in turn results in a marked increase in the serum levels of AP proteins [4]. Of the numerous cytokines and growth factors that are involved in the upregulation of AP proteins gene ex- pression, interleukin-1 (IL-1) and IL-6 are considered to be the major mediators. Also, there is evidence that glucocorticoids or dexamethasone, alone or synergis- tically with inflammatory cytokines, can enhance the production of many AP proteins [1, 2]. The rat Hp gene is an example of AP proteins genes that are strongly induced by IL-1, IL-6 and dexamethasone [3]. These mediators promote the full expression of the rat Hp gene via the hormone re- sponsive element also known as ABC element, located in the promoter proximal region at position −170 to −56. This element is composed of three cooper- atively interacting cis-acting elements A, B and C, each of which is capable to bind a distinct set of liver specific trans-acting nucleoproteins (NPs) under basal and activated transcriptional status [4]. Exam- ination of the sequences within the ABC region re- vealed an existence of multiple functional elements that implicated interactions with several families of transcription factors [5]. According to sequence spe- cific protein binding sites some of them could belong to the IL-6 regulated transcription factors [6]. Our previous results demonstrated that elements A and C, which contain the type I of the IL-6 responsive elements (IL-6REs) that conform to the consensus se- quence T(T/G)NNGNAA(T/G), bind constitutive and AP-inducible NPs that belong to the CCAAT/enhancer binding protein (C/EBP) family of transcription fac- tors [6]. Sequence analysis of element B revealed that it contains the type II of the IL-6REs called acute phase response elements (APREs) [7]. The binding of two proteins was demonstrated, with the first being described as the factor termed IL-6 response element binding protein (IL-6REBP), which was shown to bind to the rat α2-macroglobulin (α2MG) IL-6RE after IL- 6 stimulation of various human cell lines [8, 9]. The second APRE binding protein, acute phase response factor (APRE), was purified to homogeneity from IL- 6 stimulated rat [10] and mouse liver [11] and it was