PROTEINASE-INHIBITOR-6 CANNOT BE SECRETED, WHICH SUGGESTS IT IS A NEW-TYPE OF CELLULAR SERPIN

We have recently described a new serine proteinase inhibitor, proteina se inhibitor 6 (PI-6). This serpin has features that suggest it may fu nction intracellularly, but its close resemblance to ovalbumin serpins like plasminogen activator inhibitor 2 (PAI-2) raises the possibility that it is secreted to regulate an extracellular proteinase. To deter mine whether PI-6 is secreted, we have examined its cellular distribut ion by immunohistochemistry and have attempted to induce its release f rom platelets and from cultured cells, We find that PI-6 is present in endothelial and epithelial cells, but it is apparently cytoplasmic an d it is not released from cells in response to phorbol ester, dibutyry l cAMP or tumor necrosis factor alpha treatment. It is also not releas ed from activated platelets. The addition of a conventional signal pep tide to the amino terminus of PI-6 directed its translocation into the endoplasmic reticulum (ER), resulting in glycosylation but not secret ion of the molecule. By contrast, the addition of the same signal pept ide to PAI-2 markedly enhanced its translocation and secretion. Glycos ylated PI-6 was sequestered in the ER and was incapable of interacting with thrombin. The failure of PI-6 to move along the secretory pathwa y, and the loss of inhibitory function of ER-localized PI-6, demon str ates that unlike PAI-2, PI-6 is not naturally secreted, Taken together , these results suggest that PI-6 has evolved to fulfil an intracellul ar role and that it represents a new type of cellular serpin.