The ␤-amyloid precursor protein (APP) represents a type I transmembrane glycoprotein that is ubiquitously expressed. In the brain, it is a key player in the molecular pathogenesis of Alzheimer disease. Its physiological function is however less well understood. Previous studies showed that APP is up-regulated in prostate, colon, pancreatic tumor, and oral squamous cell carcinoma. In this study, we show that APP has an essential role in growth control of pancreatic and colon cancer. Abundant APP staining was found in human pancreatic adenocarcinoma and colon cancer tissue. Interestingly, treating pancreatic and colon cancer cells with valproic acid (VPA, 2-propylpentanoic acid), a known histone deacetylase (HDAC) inhibitor, leads to up-regulation of GRP78, an endoplasmic reticulum chaperone immunoglobulin-binding protein. GRP78 is involved in APP maturation and inhibition of tumor cell growth by down-regulation of APP and secreted soluble APP␣. Trichostatin A, a pan-HDAC inhibitor, also lowered APP and increased GRP78 levels. In contrast, treating cells with valpromide, a VPA derivative lacking HDAC inhibitory properties, had no effect on APP levels. VPA did not modify the level of epidermal growth factor receptor, another type I transmembrane protein, and APLP2, a member of the APP family, demonstrating the specificity of the VPA effect on APP. Small interfering RNA-mediated knockdown of APP also resulted in significantly decreased cell growth. Based on these observations, the data suggest that APP downregulation via HDAC inhibition provides a novel mechanism for pancreatic and colon cancer therapy. ␤-Amyloid precursor protein (APP)2 is a highly conserved single transmembrane protein (type I) with a receptor-like structure and consists of a heterogeneous group of proteins migrating between 110 and 135 kDa (1, 2). The heterogeneity is due to alternative splicing, leading to eight distinct isoforms (namely APP677, APP695, APP696, APP714, APP733, APP751, APP752, and APP770), as well as by a variety of post-translational modifications, including O-and N-glycosylation, sulfation, and phosphorylation. APP isoforms exist as immature (N-glycosylated) and mature (N-and O-glycosylated, tyrosylsulfated) species. Immature APP localizes in the endoplasmic reticulum and cis-Golgi, and the mature APP form preferentially localizes in the trans-Golgi network, secretory and endocytic vesicles, and at the plasma membrane (3, 4).APP695 is the most common isoform in the central nervous system, whereas APP751 and APP770 are predominantly expressed in non-neuronal cells (5). The key event in the pathogenic cascade in Alzheimer disease is the amyloidogenic pathway characterized by subsequent cleavage of APP by the enzyme ␤-secretase and further processing by ␥-secretase, which finally leads to the generation of A␤ peptides. However, the predominant route of APP processing consists of successive cleavages by ␣-and ␥-secretases in non-neuronal cells (6, 7). The cleavage of APP at Lys 16 -Leu 17 bond by ␣-secretase within the A␤ sequence ...