Secretory versus degradative autophagy: unconventional secretion of inflammatory mediators

Autophagy (macroautophagy) is often defined as a degradative process and a tributary of the lysosomal pathway. In this context, autophagy carries out cytoplasmic quality control and nutritional functions by removing defunct or disused organelles, particulate targets and invading microbes, and by bulk digestion of the cytoplasm. However, recent studies indicate that autophagy surprisingly affects multiple secretory pathways. Autophagy participates in extracellular delivery of a number of cytosolic proteins that do not enter the conventional secretory pathway via the Golgi apparatus but are instead unconventionally secreted directly from the cytosol. In mammalian cells, a prototypical example of this manifestation of autophagy is the unconventional secretion of a major proinflammatory cytokine, IL-1β. This review examines the concept of secretory autophagy and compares and contrasts the role of autophagy in the secretion of IL-1α and IL-1β. Although IL-1α and IL-1β have closely related extracellular inflammatory functions, they differ in intracellular activation, secretory mechanisms and how they are affected by autophagy. This example indicates that the role of autophagy in secretion is more complex, at least in mammalian cells, than the simplistic view that autophagosomes provide carriers for unconventional secretion of cytosolic proteins.