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Background LRP5/6 are co-receptors in Wnt/β-catenin pathway. Recently, we discovered multiple β-catenin independent functions of LRP5/6 in tumor cells and in the diseased heart. Nucleoporin 37 (NUP37) is an important component of the nuclear pore complex (NPC), whose elevated expression is associated with worsened prognosis in liver cancer. Previous studies have shown that NUP37 interacted with YAP and activated YAP/TEAD signaling in liver cancer. Our preliminary findings showed a nuclear location of LRP5. We thus tested the hypothesis that LRP5 may act as a genuine regulator of YAP/TEAD signaling via modulating NUP37 in a β-catenin-independent way. Methods We performed siRNA knockdown of LRP5, LRP6, or β-catenin in liver cancer HepG2 cells to determine the effect on tumor cell proliferation. Protein expressions and interaction between LRP5 and NUP37 were determined using immunoprecipitation and western blot analyses. Results HepG2 cell proliferation was markedly inhibited by knockdown of LRP5 but not LRP6 or β-catenin, suggesting that LRP5 has a specific, β-catenin-independent role in inhibiting HepG2 cell proliferation. Knockdown of NUP37 by siRNA inhibited the proliferation of HepG2 cells, whereas overexpression of NUP37 reversed the decrease in cell proliferation induced by LRP5 knockdown. Immunoprecipitation assays confirmed that LRP5 bound to NUP37. Furthermore, LRP5 overexpression restored NUP37 knockdown-induced downregulation of YAP/TEAD pathway. Conclusions LRP5 deletion attenuates cell proliferation via destabilization of NUP37, in a β-catenin-independent manner. LRP5 therefore acts as a genuine regulator of YAP/TEAD signaling via maintaining the integrity of the NPC, and implicates a therapeutic strategy in targeting LRP5 for inhibiting liver cancer cell proliferation.

It is known that bacterial group II phosphopantetheinyl transferases (PPTases) usually phosphopantetheinylate acyl carrier proteins (ACPs) involved in the secondary metabolism. For example, a bacterial group II PPTase SchPPT has been known to phosphopantetheinylate only ACPs involved in secondary metabolism, such as scn ACP0-2 and scn ACP7. In this study, we found two bacterial group II PPTases, Hppt and Sppt, could phosphopantetheinylate not only scn ACP0-2 and scn ACP7, but also sch FAS ACP, an ACP involved in primary metabolism. Swapping of the N terminus and C terminus of PPTases showed that (i) both the hybrids Hppt-Sppt and Sppt-Hppt could phosphopantetheinylate sch FAS ACP but not scn ACP0-2; (ii) both the hybrids Sppt-SchPPT and SchPPT-Sppt lost abilities to phosphopantetheinylate sch FAS ACP and scn ACP0-2. Hppt and Sppt represent group II PPTases which phosphopantetheinylate both ACPs involved in primary metabolism and ACPs involved in secondary metabolism.

How Wnt signalling including canonical and non-canonical pathways are initiated at the cell surface is not completely understood. Here we report that Wnt receptor Frizzled (Frz) and theco-receptors LRP5 and LRP6 (LRP5/6) directly interact with each other and this interaction is regulated by the LRP6 ectodomain. Importantly, through direct binding to Frz, LRP5/6 are able to prevent Frz-regulated non-canonical pathway activation and further non-canonical pathway-mediated tumour metastasis. Knockdown of endogenous LRP5/6 promotes otherwise-nonaggressive tumour cells to migrate in vitro, whereas a soluble recombinant protein of LRP6 ectodomain suppresses migration and metastasis of otherwise-aggressive tumour cells in vitro and in vivo. Furthermore, the expression level of membrane LRP5/6 correlates inversely with metastasis in mouse and human breast cancer. Our study suggests a previously unrecognized mode of receptor interaction, revealing the mechanism of LRP5/6 in inhibition of non-canonical pathway, and a possible clinical use of the LRP6 ectodomain to impede metastasis.