Sprouty4 levels are increased under hypoxic conditions by enhanced mRNA stability and transcription.

Sprouty (Spry) proteins are well-known negative regulators of receptor tyrosine kinase-mediated signalling. Their expression is controlled by mitogens, implying a negative feedback loop. Correspondingly, the different members of the family fulfil important roles during organogenesis by adjustment of growth factor-induced processes. In addition, Spry4, one member of this protein family, has been shown to regulate angiogenesis by inhibiting vascular endothelial cell growth factor-induced extracellular signalling-regulated kinase (ERK) activation. Because oxygen is an important regulator of angiogenesis, we investigated Spry4 expression patterns under hypoxic conditions. Our data demonstrate that both hypoxia and desferrioxamine (DFO) treatment increased Spry4 expression. Following iron depletion, elevated Spry4 levels were detected in several cell types independent of tissue origin, presence of mitogens, cell differentiation and malignancy. Evaluation of the underlying regulative mechanisms revealed that augmented transcription and increased mRNA stability enhance mRNA levels of Spry4 in response to DFO. This study unveils a growth factor-independent regulation mechanism of Spry4 expression. Because increased Spry4 levels are accompanied by disappearing ERK phosphorylation, Spry4 might be involved in the timely restriction of MAPK signals under hypoxic conditions, similar to its role in mitogen-regulated processes. However, the functional significance of the observed upregulation of Spry4 during iron depletion remains to be clarified.