Is Environmental Cadmium Exposure Causally Related to Diabetes and Obesity?

Abstract

Cadmium (Cd) is a pervasive toxic metal, present in most food types, cigarette smoke, and contaminated air. It has no nutritional or physiological value, but most cells in the body will assimilate Cd, as its charge and ionic radius are similar to the essential metals, iron, zinc, and calcium (Fe, Zn, Ca), which are required for cellular metabolism and functional integrity. The kidney is regarded as the principal site of Cd toxicity as it preferentially accumulates in the proximal tubular epithelium here. As these cells die due to toxic accumulation, Cd complexed with metallothionein are released into tubular lumen and excreted. Excretion of Cd, signifying current renal cell toxicity, is used as a measure of long-term Cd exposure. Approximately 10% of the global population are now living with diabetes and over 80% of these are overweight or obese. This association has fueled an intense search for any exogenous chemicals and life-style factors that could induce excessive weight gain. Whilst epidemiological studies have clearly linked diabetes to Cd exposure, this appears to be independent of obesity. Indeed, obesity may enhance diabetogenic effects of Cd. This review highlights Cd exposure sources and levels associated with diabetes type 2 along with the pathophysiologic mechanisms by which Cd disrupt glucose metabolism. Special emphasis is on roles of the liver and kidney in glucose homeostasis, heme-glucose cross-talk, and the involvement of heme oxygenase-1 and -2 (HO-1 and HO-2). From heme degradation, both HO-1 and HO-2 release Fe, carbon monoxide and a precursor substrate for producing a cytoprotective biochemical, bilirubin, whilst HO-2 appears to have also anti-diabetic and anti-obese actions.