TGF-β receptor I/II trafficking and signaling at primary cilia are inhibited by ceramide to attenuate cell migration and tumor metastasis

Signaling by the transforming growth factor–β (TGF-β) receptors I and II (TβRI/II) and the primary cilia-localized sonic hedgehog (Shh) pathway promote cell migration and, consequently, tumor metastasis. In contrast, the sphingolipid ceramide inhibits cell proliferation and tumor metastasis. We investigated whether ceramide metabolism inhibited TβRI/II trafficking to primary cilia to attenuate cross-talk between TβRI/II and the Shh pathway. We found that ceramide synthase 4 (CerS4)–generated ceramide stabilized the association between TβRI and the inhibitory factor Smad7, which limited the trafficking of TβRI/II to primary cilia. Expression of a mutant TβRI that signals but does not interact with Smad7 prevented the CerS4-mediated inhibition of migration in various cancer cells. Genetic deletion or knockdown of CerS4 prevented the formation of the Smad7-TβRI inhibitory complex and increased the association between TβRI and the transporter Arl6 through a previously unknown cilia-targeting signal (Ala 31 Thr 32 Ala 33 Leu 34 Gln 35 ) in TβRI. Mutating the cilia-targeting signal abolished the trafficking of TβRI to the primary cilia. Localization of TβRI to primary cilia activated a key mediator of Shh signaling, Smoothened (Smo), which stimulated cellular migration and invasion. TβRI-Smo cross-talk at the cilia in CerS4-deficient 4T1 mammary cancer cells induced liver metastasis from orthotopic allografts in both wild-type and CerS4-deficient mice, which was prevented by overexpression of Smad7 or knockdown of intraflagellar transport protein 88 (IFT88). Overall, these data reveal a ceramide-dependent mechanism that suppresses cell migration and invasion by restricting TβRI/II-Shh signaling selectively at the plasma membrane of the primary cilium.