A3 adenosine receptors modulate hypoxia-inducible factor-1α expression in human A375 melanoma cells

Hypoxia-inducible factor-1 (HIF-1) is a key regulator of genes crucial to many aspects of cancer biology. The purine nucleoside, adenosine, accumulates within many tissues under hypoxic conditions, including that of tumors. Because the levels of both HIF-1 and adenosine are elevated within the hypoxic environment of solid tumors, we investigated whether adenosine may regulate HIF-1. Here we show that, under hypoxic conditions (< 2% O-2), adenosine upregulates HIF-1 alpha protein expression in a dose-dependent and time-dependent manner, exclusively through the A(3) receptor subtype. The response to adenosine was generated at the cell surface because the inhibition of A(3) receptor expression, by using small interfering RNA, abolished nucleoside effects. A(3) receptor stimulation in hypoxia also increases angiopoietin-2 (Ang-2) protein accumulation through the induction of HIF-1 alpha. In particular, we found that A(3) receptor stimulation activates p44/p42 and p38 mitogen-activated protein kinases, which are required for A(3)-induced increase of HIF-1 alpha and Ang-2. Collectively, these results suggest a cooperation between hypoxic and adenosine signals that ultimately may lead to the increase in HIF-1-mediated effects in cancer cells.