Characterization of FAP-1 expression and function in thyroid follicular cells

Human thyrocytes are resistant to Fas-mediated programmed cell death (PCD). It has been reported that a labile protein inhibitor is involved in the pr otection of thyrocytes from PCD, and its action can be reversed by incubati on of thyrocytes with cycloheximide (CHX) during treatment with agonist ant i-Fas Ab. Fas-associated phosphatase-l (FAP-1) is a protein that has been s hown to interact with the negative regulatory domain of Fas and block Fas-m ediated apoptosis in FAP-I transfected Jurkat cells. We investigated the po ssibility that FAP-1 might be involved in protection against Fas-mediated P CD in human thyrocytes. FAP-I mRNA was detected in primary thyrocytes using a ribonuclease protection assay. The presence of FAP-1 protein was confirm ed by immunohistochemical staining and flow cytometry using a polyclonal an ti-FAP-l Ab. FAP-1 protein also disappeared from thyroid cells in response to CHX. To determine whether FAP-1 is a functional inhibitor of PCD in thyr ocytes, we incubated thyrocytes with synthetic SLV (Ac-SLV) tripeptide to c ompete with Fas for interaction with FAP-I. Thyrocytes treated with Ac-SLV tripeptide showed significantly increased cell death as compared to cells t reated with control tripeptide. In addition, in the presence of a suboptima l concentration of CHX, the Ac-SLV tripeptide yielded a strong, synergistic increase in Fas-mediated PCD as compared to thyrocytes treated with contro l tripeptide. These results implicate FAP-1 as a regulator of Fas-induced P CD in thyrocytes.