The Secretory Route of the Leaderless Protein Interleukin 1β Involves Exocytosis of Endolysosome-related Vesicles

Abstract

Interleukin 1␤ (IL-1␤), a secretory protein lacking a signal peptide, does not follow the classical endoplasmic reticulum-to-Golgi pathway of secretion. Here we provide the evidence for a "leaderless" secretory route that uses regulated exocytosis of preterminal endocytic vesicles to transport cytosolic IL-1␤ out of the cell. Indeed, although most of the IL-1␤ precursor (proIL-1␤) localizes in the cytosol of activated human monocytes, a fraction is contained within vesicles that cofractionate with late endosomes and early lysosomes on Percoll density gradients and display ultrastructural features and markers typical of these organelles. The observation of organelles positive for both IL-1␤ and the endolysosomal hydrolase cathepsin D or for both IL-1␤ and the lysosomal marker Lamp-1 further suggests that they belong to the preterminal endocytic compartment. In addition, similarly to lysosomal hydrolases, secretion of IL-1␤ is induced by acidotropic drugs. Treatment of monocytes with the sulfonylurea glibenclamide inhibits both IL-1␤ secretion and vesicular accumulation, suggesting that this drug prevents the translocation of proIL-1␤ from the cytosol into the vesicles. A high concentration of extracellular ATP and hypotonic medium increase secretion of IL-1␤ but deplete the vesicular proIL-1␤ content, indicating that exocytosis of proIL-1␤-containing vesicles is regulated by ATP and osmotic conditions. INTRODUCTIONInterleukin 1 (IL-1) 1 is a multifunctional cytokine and a major soluble mediator of inflammation (Dinarello, 1991). Although its biological activity is extracellular, this protein lacks a secretory signal sequence (Rubartelli and Sitia, 1997), raising the question of how it can be transported out of the cell. Two forms of IL-1 exist, ␣ and ␤; however, studies on IL-1 secretion mostly focused on IL-1␤, which is the major extracellular form in humans (Dinarello, 1991). IL-1␤ is synthesized by monocytes upon activation as a 35-kDa precursor, which accumulates in the cytosol (Singer et al., 1988) and is proteolytically processed to the mature form of 17 kDa by the caspase IL-1␤-converting enzyme (ICE) (Cerretti et al., 1992;Thornberry et al., 1992). ICE is present in the cell cytosol (Ayala et al., 1994;Singer et al., 1995) as a p45 inactive polypeptide. Where and how maturation of ICE to the active (p10/p20) form and the processing of IL-1␤ take place are unclear; however, the mature form of IL-1␤ seems to be either ¶ Corresponding author. E-mail address: annarub@hp380.ist. et al., 1990). Although the levels of basal release of IL-1␤ are quite low (Rubartelli et al., 1990), secretion is dramatically induced by extracellular ATP (Hogquist et al., 1991;Rubartelli et al., 1993;Perregaux and Gabel, 1994), which can be autocrinally produced by activated monocytes (Ferrari et al., 1997) and interact with P2Z purinoreceptors expressed on their membrane (Hickman et al., 1994;Di Virgilio, 1995). The cellular pathway underlying this ATP-driven secretion is, however, unknown. We have previously shown that IL-1␤ is ...

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