Polarized targeting of epithelial cell proteins in thyrocytes and MDCK cells

Polarized trafficking signals may be interpreted differently in different c ell types. In this study, we have compared the polarized trafficking of dif ferent proteins expressed endogenously in primary porcine thyroid epithelia l cells to similar proteins expressed in MDCK cells, As in MDCK cells, NH4C l treatment of filter-grown thyrocytes caused mis-sorted soluble proteins t o exhibit enhanced secretion to the apical medium, In independent studies, thrombospondin 1 (a thyroid basolaterally secreted protein) was secreted ba solaterally from MDCK cells. Likewise, the 5'-deiodinase (a thyroid basolat eral membrane protein) encoded by the D101 gene was also distributed basola terally in transfected MDCK cells. Consistent with previous reports, when t he secretion of human growth hormone tan unglycosylated regulated secretory protein was examined from transfected MDCK cells, the release was nonpolar ized, However, transfected studies, secreted thyrocytes secreted growth hor mone apically in a manner dependent upon zinc addition. Moreover, two addit ional regulated secretory proteins expressed in thyrocytes, thyroglobulin ( the major endogenous glycoprotein) and parathyroid hormone tan unglycosylat ed protein expressed transiently, were secreted apically even in the absenc e of zinc. We hypothesize that while cellular mechanisms for interpreting p olarity signals are generally similar between thyrocytes and MDCK cells, th yrocytes allow for specialized packaging of regulated secretory proteins fo r apical delivery, which does not require glycosylation but may involve ava ilability of certain ions as well as appropriate intracellular compartmenta tion.