Cd stabilizes HIF-1α under normoxic conditions via lysine-63-linked ubiquitination and induces ER stress and cell proliferation

Cadmium, a carcinogenic and toxic substance released into the environment, has emerged as a potent activator of lysine-63 ubiquitination, and lysine-63 is a crucial regulator of signal transduction pathways. Although critical, very little information is currently available about how the activation of lysine 63 ubiquitination by Cd might contribute to cancers and inflammatory diseases. The present study provides the first evidence that Cd stabilizes hypoxia-inducible factor-1-alpha, a transcription factor, under normoxic conditions via lysine 63 ubiquitination. Cd induces the accumulation of lysine 63 polyubiquitinated proteins. Importantly, Cd-induced ubiquitination does not prevent oxidative damage or proteasome impairment. Instead, we demonstrated that Cd activates lysine 63 ubiquitination and amplifies its accumulation by overloading the capacity of the autophagy pathway, thus promoting endoplasmic reticulum stress and cell death. At the molecular level, Cd-induced lysine 63 polyubiquitination is correlated with the stabilization of hypoxia-inducible factor-1-alpha, which translocates into the nucleus and promotes the expression of oncogenes such as interleukin 8 and vascular endothelial growth factor. Strikingly, prolonged cell exposure to high Cd concentrations induces increased lysine-63 polyubiquitination, which promotes aggresome formation, thus preventing this protein from interacting with its downstream nuclear targets. Our results showed that Cd is an activator of K63-linked ubiquitination that stabilizes and promotes the accumulation of HIF-1α, which blocks autophagy, thus resulting in endoplasmic reticulum stress. In addition, a small amount of HIF-1α was observed in the nucleus. We therefore propose that the aberrant activation of lysine 63 polyubiquitination by the carcinogen Cd could promote cell proliferation and inflammation at low levels, while high levels lead to cell death.