p73 engages A2B receptor signalling to prime cancer cells to chemotherapy-induced death

Tumour cells often acquire the ability to escape cell death, a key event leading to the development of cancer. In almost half of all human cancers, the capability to induce cell death is reduced by the mutation and inactivation of p53, a tumour suppressor protein that is a central regulator of apoptosis. As a result, there is a crucial need to identify different cell death pathways that could be targeted in malignancies lacking p53. p73, the closely related p53 family member, can regulate many p53 target genes and therefore some of the same cellular responses as p53. Unlike p53, however, p73 is seldom mutated in cancer, making it an attractive, alternative death effector to target. We report here the ability of p73 to upregulate the expression of the A2B receptor, a recently characterized p53 target that effectively promotes cell death in response to extracellular adenosine—a metabolite that accumulates during various forms of cellular stress. Importantly, we show that p73-dependent stimulation of A2B signalling markedly enhances apoptosis in cancer cells that are devoid of p53. This mode of death is caspase- and puma-dependent, and can be prevented by the overexpression of anti-apoptotic Bcl-X(L). Moreover, treatment of p53-null cancer cells with the chemotherapeutic drug adriamycin (doxorubicin) induces A2B in a p73-dependent manner and, in combination with an A2B agonist, substantially enhances apoptotic death. We therefore propose an alternate and distinct p53-independent pathway to stimulate programmed cell death involving p73-mediated engagement of adenosine signalling.