STAT3 and STAT5b expression during rat liver development and the acutephase response

family of transcription factors are present in a functionally latent, monomeric form in the cytoplasm of resting cells (D a r n e l l, 1997). Following stimulation with cytokines or growth factors and their binding to a cell-surface receptor, STAT factors are rapidly tyrosine-phosphorylated, dimerized, and translocated to the nucleus, where they bind to specific hormone responsive elements of the target genes. There are several members of the STAT family (STAT1, STAT2, STAT3, STAT4, STAT5, and STAT6) with different roles in regulating cell growth, proliferation, and differentiation (M u i, 1999). The liver is a multifunctional organ which emerges from the primitive foregut at day 9 of rat embryonic development (L e m i r e et al., 1991). By the 16th day of gestation, the liver is made up of hepatoblasts (S h i o j i r i et al., 1991), which differentiate into mature hepatocytes. Full liver activity is reached around the third week of postnatal development. Among other functions, the liver plays an important role during the acute-phase (AP) response, a complex systemic reaction of an organism in response to various mechanical, chemical, or biological insults (K o j et al., 1982). As STAT3 and STAT5b were described to be involved in AP response regulation in adults, the aim of this work was to determine their expression throughout rat liver development under normal and AP conditions.