Cytoplasmic PML Function in TGF-β Signaling.

TGF-β is a pluripotent cytokine that controls key tumor suppressive functions such as cell growth inhibition, induction of apoptosis, cellular senescence and differentiation. Cancer cells are often unresponsive to TGF-β. Blasts from several leukemia subtypes including acute promyelocytic leukemia (APL) display resistance to TGF-β-growth inhibitory and differentiating activity. As TGF-β, the PML tumor suppressor of APL is also known to modulate key tumor suppressive functions. PML is found to accumulate in the PML-nuclear body, although cytoplasmic isoforms of unknown function have also been described. The PML-RARα fusion oncoprotein of APL is found to accumulate both in the nucleus and cytoplasm and is thought to act as a dominant negative PML mutant in view of its ability to heterodimerize with PML. Given the functional similarities between PML and TGF-β and their functional loss in APL, we asked whether PML would modulate TGF-β function. Here we show that cytoplasmic Pml is an essential modulator of TGF-β signaling. Pml-null primary cells of diverse histological origins are resistant to TGF-β-dependent growth arrest, induction of cellular senescence and apoptosis. Induction of TGF-β target genes such as p21 and p15, as well as Smad2 and Smad3 phosphorylation and nuclear translocation is also dramatically impaired in the absence of Pml. Cytoplasmic Pml is induced by TGF-β and its add-back to Pml-null cells can fully rescue these biological and biochemical defects. Furthermore, we find that cytoplasmic PML physically interacts with Smad2/3 and SARA (Smad anchor for receptor activation), colocalizes with these proteins in cytosolic regions and is required for association of Smad2/3 with SARA and for the accumulation of SARA and TGF-β receptor in the early endosome. We demonstrate that the PML-RARα oncoprotein of APL can antagonize cytoplasmic PML function and that APL blasts display defects in TGF-β signaling similar to that observed in Pml-null cells, which can be overcome by retinoic acid (RA) and As2O3, drugs that induce PML-RARα degradation and are effective in APL treatment. Our findings identify cytoplasmic PML as a critical TGF-β regulator, and further implicate deregulated TGF-β signaling in cancer pathogenesis.