Preserving energy homeostasis in the presence of stressors such as proinflammatory cytokines and nutrient overload is crucial to maintaining normal cellular function. Six-transmembrane epithelial antigen of the prostate 4 (STEAP4), a metalloreductase involved in iron and copper homeostasis, is thought to play a potentially important role in the cellular response to inflammatory stress. Genome-wide association studies have linked various mutations in STEAP4 with the development of metabolic disorders such as obesity, metabolic syndrome, and type 2 diabetes. Several studies have shown that expression of Steap4 is modulated by inflammatory cytokines, hormones, and other indicators of cellular stress, and that STEAP4 may protect cells from damage, helping to maintain normal metabolic function. STEAP4 appears to be particularly relevant in metabolically oriented cells, such as adipocytes, hepatocytes, and pancreatic islet cells. These cells struggle to maintain their function in iron or copper overloaded states, presumably due to increased oxidative stress, suggesting STEAP4’s role in metal homeostasisis critical to the maintenance of cellular homeostasis in general, and in preventing the onset of metabolic disease. In this review, we explore genetic associations of STEAP4 with metabolic disorders, and we examine STEAP4 tissue expression, subcellular localization, regulation, structure, and function as it relates to metabolic diseases. We then examine how STEAP4’s role as a regulator of cellular iron and copper may relate to type 2 diabetes.

Six-transmembrane epithelial antigen of the prostate 4 (STEAP4) is a metalloreductase that has been shown previously to protect cells from inflammatory damage. Genetic variants in STEAP4 have been associated with numerous metabolic disorders related to obesity, including putative defects in the acute insulin response to glucose in type 2 diabetes (T2D). Purpose We examined whether obesity and/or T2D altered STEAP4 expression in human pancreatic islets. Methods Human islets were isolated from deceased donors at two medical centers and processed for quantitative PCR. Organ donors were selected by status as non-diabetic or having T2D. Site1 (Edmonton): N=13 T2D donors (7M, 6F), N=20 non-diabetic donors (7M, 13F). Site2 (Virginia): N=6 T2D donors (6F), N=6 non-diabetic donors (3M, 3F). Results STEAP4 showed reduced islet expression with increasing BMI among all donors (P<0.10) and non-diabetic donors (P<0.05) from Site1; STEAP4 showed reduced islet expression among T2D donors with increasing HbA1c. Islet STEAP4 expression was also marginally higher in female donors (p<0.10). Among T2D donors from Site2, islet insulin expression was reduced, STEAP4 expression was increased, and white blood cell counts were increased compared to non-diabetic donors. Islets from non-diabetic donors that were exposed overnight to 5ng/ml IL-1β displayed increased STEAP4 expression, consistent with STEAP4 upregulation by inflammatory signaling. Conclusions These findings suggest that increased STEAP4 mRNA expression is associated with inflammatory stimuli, whereas lower STEAP4 expression is associated with obesity in human islets. Given its known protective role, downregulation of STEAP4 by chronic obesity suggests a mechanism for reduced islet protection against cellular damage.