POLARIZED SECRETION OF THROMBOSPONDIN IS OPPOSITE TO THYROGLOBULIN INTHYROID EPITHELIAL-CELLS

In addition to thyroglobulin, primary thyrocytes secrete into the cult ure medium significant quantities of p500, a protein so named because of its M(r) greater-than-or-equal-to 500,000. Epithelial monolayers cu ltured on porous filters serve as a useful model system in which to st udy protein secretion. From these monolayers, thyroglobulin, the precu rsor in thyroid hormonogenesis, was released with apical predominance, while p500 was found mostly in the basolateral medium. Thyrocyte expo sure to thyrotropin augmented selectively thyroglobulin but not p500 p roduction. By contrast, exposure to cycloheximide actually augmented p 500 production, a response observed for immediate-early proto-oncogene s. Using thyrocyte conditioned medium, the p500 protein was purified t o homogeneity. Peptide sequencing of tryptic fragments of purified p50 0 showed identity to thrombospondin. Immunoprecipitation of thrombospo ndin from media bathing primary thyrocytes and the FRTL5 cell line qua ntitatively recovered p500, confirming its identity and indicating an epithelial origin. Gel filtration of secreted thrombospondin eluted at a high molecular weight, suggesting complexation with components of t he extracellular matrix. Further, immunofluorescence showed cellular c odistribution of thrombospondin and thyroglobulin, although thrombospo ndin exhibited predominantly an extracellular, basolateral deposition. It seems likely that thrombospondin production by thyrocytes plays a role in the growth or development of the thyroid epithelium.