25 Molecular and Cellular Biochemistry 228: 25–31, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. Corticosteroid-binding globulin synthesis and distribution in rat white adipose tissue Maria del Mar Grasa, 1 Cristina Cabot, 1 Cristina Adán, 1 Rita de Matteis, 2 Montserrat Esteve, 1 Saverio Cinti, 2 José Antonio Fernández- López, 1 Xavier Remesar 1 and Marià Alemany 1 1 Departament de Bioquímica i Biologia Molecular, Facultat de Biologia, Universitat de Barcelona, Barcelona, Spain; 2 Institute of Normal Morphology-Anatomy, University of Ancona, Ancona, Italy Received 27 April 2001; accepted 31 July 2001 Abstract Corticosterone binding (CB) capacity was determined in visceral and subcutaneous white adipose tissue (WAT), as well as in plasma of lean Zucker rats. Perfusion of rats with saline eliminated most liver and kidney corticosterone binding but did not affect CB in WAT. The cytosol extracts of isolated cells, however, did not bind corticosterone in detectable amounts. By means of a RT-PCR procedure it was found that corticosterone-binding globulin (CBG) was expressed in WAT. By immunohisto- chemical detection in WAT sections, CBG was seen in a thin layer surrounding the cells near the plasma membrane. These data suggest that the CBG layer surrounding the cells may act as a protective barrier limiting the access of glucocorticoids to adi- pocytes. (Mol Cell Biochem 228: 25–31, 2001) Key words: CBG, corticosterone-binding globulin, white adipose tissue, glucocorticoids Introduction Glucocorticoid action is mediated through at least two types of specific receptor, present in most tissues [1, 2]: mineralo- corticoid (type I) and glucocorticoid (type II) [3]; their stimu- lation with specific agonists and other factors elicits a range of responses in specialized tissues [4, 5]. Synthetic agonists such as dexamethasone [6] are used for studies of hormone binding to tissues, since the ‘physiological’ glucocorticoids, corticosterone and cortisol, bind both types of corticosteroid receptors, and in addition are bound by a plasma protein: CBG or corticosterone-binding globulin [7]. CBG is synthe- sized by liver [8], under glucocorticoid regulation [9]. Other tissues have been found to synthesize CBG, such as endo- metrium and lung [10, 11]; the reasons for this widespread distribution remain to be determined. It is usually assumed that the primary role of CBG is to bind and carry glucocorti- coids in the bloodstream [7], although the finding of specific CBG membrane receptors in several tissues [12] and the lo- calized release of cortisol by CBG cleavage as a consequence of inflammation [13] have considerably widened the regu- latory possibilities of this protein [14]. In most types of obesity, glucocorticoid activity is enhanced [15]. The obese show a more marked response to stress [16], and their hypothalamic-hypophysis-adrenal axis function is altered [17]. Unchecked high levels of glucocorticoids are considered one of the main causes of obesity and a key rea- son for its maintenance [18, 19]. Glucocorticoids are an im- portant (but not strictly necessary) factor for the development and maintenance of obesity [19]. Corticosterone binding by white adipose tissue is due only in a small portion to glucocorticoid and mineralocorticoid receptors, since most of the hormone is bound by CBG [20]. This ability to bind the most important physiological gluco- corticoid of the rat is affected by obesity [20, 21] and fatty acid binding [22]. Since adipose tissue is both a key target Address for offprints: M. Alemany, Departament de Bioquímica i Biologia Molecular, Facultat de Biologia, Universitat de Barcelona, Av. Diagonal 645, 08028 Barcelona, Spain (E-mail: alemany@bio.ub.es)