Cytoplasmic PML function in TGF-β signalling

Transforming growth factor β (TGF-β) is a pluripotent cytokine that controls key tumour suppressive functions, but cancer cells are often unresponsive to it. The promyelocytic leukaemia (PML) tumour suppressor of acute promyelocytic leukaemia (APL) accumulates in the PML nuclear body, but cytoplasmic PML isoforms of unknown function have also been described. Here we show that cytoplasmic Pml is an essential modulator of TGF-β signalling. Pml-null primary cells are resistant to TGF-β-dependent growth arrest, induction of cellular senescence and apoptosis. These cells also have impaired phosphorylation and nuclear translocation of the TGF-β signalling proteins Smad2 and Smad3, as well as impaired induction of TGF-β target genes. Expression of cytoplasmic Pml is induced by TGF-β. Furthermore, cytoplasmic PML physically interacts with Smad2/3 and SARA (Smad anchor for receptor activation) and is required for association of Smad2/3 with SARA and for the accumulation of SARA and TGF-β receptor in the early endosome. The PML-RARα oncoprotein of APL can antagonize cytoplasmic PML function and APL cells have defects in TGF-β signalling similar to those observed in Pml-null cells. Our findings identify cytoplasmic PML as a critical TGF-β regulator, and further implicate deregulated TGF-β signalling in cancer pathogenesis.