Apolipoprotein J (Apo J), also known as clusterin, is a 75 – 80 kDa disulfide-linked heterodimeric protein. It is a molecular chaperone responsible for aiding protein folding of secreted proteins, and its three isoforms have been differentially implicated in pro- or antiapoptotic processes. Overexpression of the secretory Apo J isoform protects the cell from apoptosis induced by cellular stress, such as chemotherapy, radiotherapy, or androgen/estrogen depletion. Apo J promotes cell survival by a number of means, including inhibition of BAX on the mitochondrial membrane, activation of the phosphatidylinositol 3-kinase/protein kinase B pathway, promotion of angiogenesis, and mediation of the nuclear factor kappa B (NF-κB) pathway.
Apo J is involved in many diseases related to oxidative stress, including neurodegenerative diseases, cancers, inflammatory diseases, and aging. Studies have suggested that people who already have Alzheimer’s disease have more Apo J in their blood, and Apo J levels in blood correlate with faster cognitive decline in individuals with Alzheimer’s disease.