PDZK1-interacting protein 1 (PDZK1IP1) traps Smad4 protein and suppresses transforming growth factor-β (TGF-β) signaling

Transforming growth factor (TGF)-beta signaling in humans is stringently regulated to prevent excessive TGF-beta signaling. In tumors, TGF-beta signaling can both negatively and positively regulate tumorigenesis dependent on tumor type, but the reason for these opposite effects is unclear. TGF-beta signaling is mainly mediated via the Smad-dependent pathway, and herein we found that PDZK1-interacting protein 1 (PDZK1IP1) interacts with Smad4. PDZK1IP1 inhibited both the TGF-beta and the bone morphogenetic protein (BMP) pathways without affecting receptor-regulated Smad (R-Smad) phosphorylation. Rather than targeting R-Smad phosphorylation, PDZK1IP1 could interfere with TGF-beta- and BMP-induced R-Smad/Smad4 complex formation. Of note, PDZK1IP1 retained Smad4 in the cytoplasm of TGF-beta-stimulated cells. To pinpoint PDZK1IP1's functional domain, we created several PDZK1IP1 variants and found that its middle region, from Phe(40) to Ala(49), plays a key role in its Smad4-regulating activity. PDZK1IP1 knockdown enhanced the expression of the TGF-beta target genes Smad7 and prostate transmembrane protein androgen-induced (TMEPAI) upon TGF-beta stimulation. In contrast, PDZK1IP1 overexpression suppressed TGF-beta-induced reporter activities, cell migration, and cell growth inhibition. In a xenograft tumor model in which TGF-beta was previously shown to elicit tumor-promoting effects, PDZK1IP1 gain of function decreased tumor size and increased survival rates. Taken together, these findings indicate that PDZK1IP1 interacts with Smad4 and thereby suppresses the TGF-beta signaling pathway.