WNT4 and WNT3A activate cell autonomous Wnt signaling independent of PORCN or secretion

The enzyme PORCN is considered essential for Wnt secretion and signaling, however, we observed PORCN inhibition did not phenocopy the effects of WNT4 knockdown in WNT4-dependent breast cancer cells. This suggests a unique relationship between PORCN and WNT4 signaling. To examine the role of PORCN in WNT4 signaling, WNT4 or WNT3A were over-expressed in breast and ovarian cancer, and fibrosarcoma cell lines. Conditioned medium from these lines, and co-culture systems, were used to assess the dependence of Wnt secretion and activity on critical Wnt secretion proteins PORCN and WLS. We observed that WLS was universally required for Wnt secretion and paracrine signaling. In contrast, the dependence of WNT3A secretion and activity on PORCN varied across cell lines, and WNT4 secretion was PORCN-independent in all models. Surprisingly, WNT4 did not present paracrine activity in any tested context. Absent the expected paracrine activity of secreted WNT4, we identified cell autonomous Wnt signaling activation by WNT4 and WNT3A, independent of PORCN or secretion. The PORCN-independent, cell-autonomous Wnt signaling demonstrated herein may be critical in WNT4-driven cellular contexts, or those that are otherwise considered to have dysfunctional Wnt signaling. Summary Statement Wnt proteins can mediate an atypical mode of cell-autonomous signaling, distinct from paracrine signaling, that is independent of both palmitoylation by PORCN and Wnt secretion.