APPL proteins promote TGFβ-induced nuclear transport of the TGFβ type I receptor intracellular domain

The multifunctional cytokine transforming growth factor-beta (TGF beta) is produced by several types of cancers, including prostate cancer, and promote tumour progression in autocrine and paracrine manners. In response to ligand binding, the TGF beta type I receptor (T beta RI) activates Smad and non-Smad signalling pathways. The ubiquitin-ligase tumour necrosis factor receptor-associated factor 6 (TRAF6) was recently linked to regulate intramembrane proteolytic cleavage of the T beta RI in cancer cells. Subsequently, the intracellular domain (ICD) of T beta RI enters in an unknown manner into the nucleus, where it promotes the transcription of pro-invasive genes, such as MMP2 and MMP9. Here we show that the endocytic adaptor molecules APPL1 and APPL2 are required for TGF beta-induced nuclear translocation of T beta RI-ICD and for cancer cell invasiveness of human prostate and breast cancer cell lines. Moreover, APPL proteins were found to be expressed at high levels in aggressive prostate cancer tissues, and to be associated with T beta RI in a TRAF6-dependent manner. Our results suggest that the APPL-T beta RI complex promotes prostate tumour progression, and may serve as a prognostic marker.