Aldehyde dehydrogenase 1 family, member A3 (ALDH1a3), also known as retinaldehyde dehydrogenase 3 (RALDH3) or as ALDH6 in earlier published studies, is an enzyme that in humans is encoded by the ALDH1A3 gene.,

Function

Aldehyde dehydrogenase isozymes are NAD(P)-dependent dehydrogenases that catalyze the oxidation of an aldehyde into the corresponding carboxylic acid while reducing NAD+ or NADP+. ALDH1a3 oxidizes all-trans retinaldehyde into all-trans retinoic acid and thus serves as the final catalytic step in the activation of the retinoid nuclear receptor (RAR) pathway. While ALDH1a3 and related isozymes are known to utilize many aldehyde substrates in biochemical experiments, genetic and functional analysis demonstrates that ALDH1a3 functions only to oxidize all-trans retinaldehyde in living systems. ALDH1a3 exists as a homotetramer with cytosolic localization. It is not known to have any function in healthy adult tissues. ALDH1a3 contains a catalytic cysteine residue which is only minimally inhibited by the ALDH2-targeted drug disulfiram. While no specific ALDH1a3 inhibitors have been tested in humans, the pan-ALDH1 inhibitor Win18446 (Fertilysin) was tested in humans for 23 weeks with no observed adverse effects. The function of ALDH1a3 is known to be restricted to early fetal development and is dispensable in either adult mammals or healthy adult humans. ALDH1a3 is a potential therapeutic target in type 2 diabetes. cardiovascular disorders, and cancer where its expression is amplified and it has known pathogenic activity. ALDH1a3 is not necessary for spermatogenesis or the visual cycle.

Clinical significance

Type 2 Diabetes

ALDH1a3 is established as a primary marker of failing beta cells in the pancreas, both in human type 2 diabetes patients and mouse models of diabetes. ALDH1a3 expression has been shown to suppress insulin secretion and increase glucagon production in laboratory experiments. ALDH1a3 was more recently established as a driver of beta cell failure and thus type 2 diabetes in a retinoid-dependent mechanism. Genetic and pharmacologic experiments with recently described ALDH1a3 inhibitors suggest that ALDH1a3 is a potential target to reverse beta cell decline in type 2 diabetes and thus restore insulin independence.

Cardiovascular Disorders

ALDH1a3 is activated in injured or inflamed vascular smooth muscle cells in the context of pulmonary arterial hypertension and neointimal hyperplasia. Activation of ALDH1a3 in these cells causes vascular wall thickening and narrowing of pulmonary arteries, leading to disease progression. Chronic activation of the retinoid nuclear receptor causes increases in mortality due to heart failure.

Cancer

ALDH1a3 is expressed in many cancer types while it is not expressed in the normal cells from which those cancers are derived. There is extensive literature evidence for the selective enrichment of ALDH1a3 across many cancers, including melanoma, glioblastoma, lung cancer, pancreatic cancer, breast cancer, sarcomas and many other cancer types. While the putative role of ALDH1a3 in each of these cancers is via activation of the retinoid pathway, many studies disagree on its mechanism. A unifying theory for its activity in cancer was described through the generation of all-trans retinoic acid that acts in a paracrine manner on immune cells in the tumor microenvironment.