Projects

The LDL Receptor Family: Biogenesis, Endocytic Trafficking, and Signaling

The low-density lipoprotein receptor (LDLR) family composed of over ten cell surface receptors with the LDLR being the prototype receptor. These receptors function in cellular uptake of various ligands and mediate signal transductions of diverse pathways. The LDLR itself was shown to be an essential receptor for cholesterol homeostasis. Mutations of the LDLR cause familial hypercholesterolemia (FH). Studies in our laboratory have focused on understanding how LDLR family members traffic within cells and how their biological functions are regulated. We have shown that a receptor-associated protein (RAP) promotes the folding of ligand-binding repeats and prevents premature ligand-binding during receptor trafficking along the secretory pathway. Once receptors mature at the cell surface, they enter endocytic trafficking, which including clathrin-mediated endocytosis, intracellular sorting, and recycling/degradation. We have utilized a large member of the family, the LDLR-related protein (LRP), as a model receptor to dissect these processes. A unique feature of LRP endocytic trafficking is its rapid endocytosis, which is mediated by several endocytosis signals including an YXXL motif, a di-leucine motif, and a PKA-mediated phosphorylation of a serine residue. We also found that a cytosolic adaptor protein, sorting nexin 17, interacts with the first NPXY motif of LRP tail and regulates its recycling.

Current work in these areas includes:

1. Define how RAP and a novel chaperone, Mesd, participate in the complex folding process of the LDLR family members and examine how they coordinate with general ER chaperones.
2. Define roles of SNX17 and the ubiquitin-proteasome system in LRP sorting to the recycling and degradation pathways.
3. Analyze the relationship between LRP endocytosis and LRP-mediated signal transduction.

Molecular Pathobiology of LRP1B Tumor Suppressor

LRP1B is a novel member of the LDLR family. It was discovered as a putative tumor suppressor and is frequently deleted in lung cancer cells. It shares high sequence and structural homology with LRP. LRP1B and LRP have overlapping ligand-binding specificities; however, our recent studies demonstrate that LRP1B exhibits a significantly slower rate of endocytosis when compared to LRP. As a result, expression of LRP1B prevents the regeneration of urokinase plasminogen activator receptor (uPAR) to the cell surface and inhibits cell migration. Our working hypothesis is that the tumor suppressive function of LRP1B is related to its ability to retain uPAR in a non-functional state on the cell surface, and that alterations in the expression and endocytic efficiency of LRP1B may impact the level of functional cell surface uPAR, tumor cell migration, invasion, growth, and metastasis.

Current work in this area includes:

1. Define interactions between LRP1B and the components of the uPAR system, and explore the roles of LRP1B in uPAR/integrin-mediated signaling.
2. Dissect the mechanism underlying the slow endocytosis rate of LRP1B.
3. Examine the roles of LRP1B in uPAR-mediated human cancer cell proliferation, migration, and invasion in vitro, and tumorigenesis and metastasis in vivo.
4. Determine whether the LRP1B gene, transcript, and protein levels are altered in human tumor tissues.

LRP and Alzheimer’s Disease

Amyloid beta-peptide (Abeta) deposition in the brain is considered a critical event in the pathogenesis of Alzheimer’s disease (AD). In vitro studies suggest that LRP regulates Abeta production and clearance. However, in vivo evidence of LRP’s role in Abeta metabolism is lacking. We generated transgenic mice that overexpress a minireceptor form of LRP in the brain. These mice were bred with AD model mice, PDAPP, which overexpress a mutant form of human APP and develop AD-like amyloid plaques. We found that PDAPP/LRP double transgenic mice exhibit a significant age-dependent increase of soluble Abeta. Importantly, increased soluble Abeta in aged PDAPP/LRP double transgenic mice was highly correlated with enhanced deficits in spatial learning and memory, suggesting that soluble forms of Abeta can independently impair neuronal function in vivo. Using a conditional gene knockout mouse model to delete LRP in the forebrain, our recent work also showed that LRP regulates brain apoE and cholesterol metabolism. Further, APP processing product, AICD, regulates LRP expression by binding directly to LRP promoter. Our long-term goals are to define the mechanisms by which LRP modulates brain Abeta and apoE/cholesterol metabolism, Abeta oligomerization and accumulation, and to examine how modulation of LRP expression in vivo impacts Abeta and apoE/cholesterol during aging and AD. Our working hypothesis is that LRP modulates brain Abeta levels by influencing its production and clearance in an age-dependent manner, and that certain forms of Abeta can independently cause neuronal toxicity leading to memory deficits seen in AD. We also hypothesize that a decreased LRP level in AD brains results in compromised apoE/cholesterol metabolism, which contributes to synaptic dysfunction and neurodegeneration. Several animal models are used in this research area.

LRP6 in Wnt Signaling and Tumorigenesis

The Wnt signaling pathway is involved in various differentiation events during embryonic development and when aberrantly activated, can lead to tumor formation. LRP6 is a novel member of the expanding LDLR family. Recent studies have demonstrated that LRP6 is an indispensable element of the canonical Wnt pathway by interacting with several components of the Wnt signaling pathway. Upon binding to ligand Wnt and cytosolic protein axin, LRP6 promotes Wnt signaling. On the contrary, when binding to a secreted factor Dickkopf (Dkk), LRP6 antagonizes Wnt signaling. Although the role of LRP6 in embryonic development is well established, the role of LRP6 in tumorigenesis is unclear. We have recently found that stable expression of LRP6 in human fibrosarcoma HT1080 cells results in accumulation of cytosolic beta-catenin, activation of Wnt/beta-catenin signaling, and promotes cell proliferation in vitro, and tumorigenesis in vivo. Furthermore, we found that LRP6 is frequently up-regulated in human malignant tissues. Our working hypothesis is that LRP6 is an oncogenic protein which is related to its ability to modulate Wnt/beta-catenin signaling, and that alteration in the LRP6 expression may impact tumor cell proliferation and migration in vitro, and tumorigenesis and metastasis in vivo.

Our current work in this area includes:

1. Dissect how LRP6 and its endocytic trafficking is regulated by agonists and antagonists.
2. Examine the roles of LRP6 expression in human cancer cell proliferation and migration in vitro, and tumorigenesis and metastasis in vivo.
3. Determine how LRP6 transcript and protein levels are altered in human malignant tissues.