Abstract
The very-low-density lipoprotein receptor (VLDLR) negatively regulates Wnt signaling. VLDLR has two major alternative splice variants, VLDLRI and VLDLRII, but their biological significance and distinction are unknown. Here we found that most tissues expressed both VLDLRI and VLDLRII, while the retina expressed only VLDLRII. The shed soluble VLDLR extracellular domain (sVLDLR-N) was detected in the conditioned medium of retinal pigment epithelial cells, interphotoreceptor matrix, and mouse plasma, indicating that ectodomain shedding of VLDLR occurs endogenously. VLDLRII displayed a higher ectodomain shedding rate and a more potent inhibitory effect on Wnt signaling than VLDLRI in vitro and in vivo. O-glycosylation, which is present in VLDLRI but not VLDLRII, determined the differential ectodomain shedding rates. Moreover, the release of sVLDLR-N was inhibited by a metalloproteinase inhibitor, TAPI-1, while it was promoted by phorbol 12-myristate 13-acetate (PMA). In addition, sVLDLR-N shedding was suppressed under hypoxia. Further, plasma levels of sVLDLR-N were reduced in both type 1 and type 2 diabetic mouse models. We concluded that VLDLRI and VLDLRII had differential roles in regulating Wnt signaling and that decreased plasma levels of sVLDLR-N may contribute to Wnt signaling activation in diabetic complications. Our study reveals a novel mechanism for intercellular regulation of Wnt signaling through VLDLR ectodomain shedding.
Highlights
The very-low-density lipoprotein receptor (VLDLR) negatively regulates Wnt signaling
It is known that the structural difference between VLDLRI and VLDLRII is a lack of the O-linked sugar domain in VLDLRII (Fig. 1A)
As shown by RT-PCR using primers spanning the sequence encoding the O-linked sugar domain, the VLDLRI mRNA was abundantly expressed in the murine heart, adipose tissue, and skeletal muscle, while the VLDLRII mRNA was highly expressed in the murine brain, retina, spleen, kidney, ovary, and uterus (Fig. 1B)
Summary
The very-low-density lipoprotein receptor (VLDLR) negatively regulates Wnt signaling. It has been reported that VLDLRI and VLDLRII are differentially expressed in cancer cell lines with various differentiations [19, 20] These studies suggest that the VLDLR splice variants may have different functions in tissues. We have demonstrated that the shed soluble extracellular domain of recombinant VLDLR (sVLDLR-N) is necessary and sufficient to suppress Wnt signaling [16]. It was still unclear whether the release of sVLDLR-N naturally occurs in vivo and how sVLDLR-N is generated and regulated.
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