Low-Density Lipoprotein Receptor-Related Proteins in Skeletal Development and Disease

LRP5, Bone Mass Polymorphisms and Skeletal Disorders

The formation and maintenance of the gross structure and microarchitecture of the human skeleton require the concerted functioning of a plethora of morphogenic signaling processes. Through recent discoveries in the field of genetics, numerous genotypic variants have been implicated in pathologic skeletal phenotypes and disorders arising from the disturbance of one or more of these processes. For example, total loss-of-function variants of LRP5 were found to be the cause of osteoporosis-pseudoglioma syndrome (OPPG). LRP5 encodes for the low-density lipoprotein receptor-related protein 5, a co-receptor in the canonical WNT-β-catenin signaling pathway and a crucial protein involved in the formation and maintenance of homeostasis of the human skeleton. Beyond OPPG, other partial loss-of-function variants of LRP5 have been found to be associated with other low bone mass phenotypes and disorders, while LRP5 gain-of-function variants have been implicated in high bone mass phenotypes. This review introduces the roles that LRP5 plays in skeletal morphogenesis and discusses some of the structural consequences that result from abnormalities in LRP5. A greater understanding of how the LRP5 receptor functions in bone and other body tissues could provide insights into a variety of pathologies and their potential treatments, from osteoporosis and a variety of skeletal abnormalities to congenital disorders that can lead to lifelong disabilities.

Proteolytic regulation of a galectin-3/Lrp1 axis controls osteoclast-mediated bone resorption

Bone-resorbing osteoclasts mobilize proteolytic enzymes belonging to the matrix metalloproteinase (MMP) family to directly degrade type I collagen, the dominant extracellular matrix component of skeletal tissues. While searching for additional MMP substrates critical to bone resorption, Mmp9/Mmp14 double-knockout (DKO) osteoclasts-as well as MMP-inhibited human osteoclasts-unexpectedly display major changes in transcriptional programs in tandem with compromised RhoA activation, sealing zone formation and bone resorption. Further study revealed that osteoclast function is dependent on the ability of Mmp9 and Mmp14 to cooperatively proteolyze the β-galactoside-binding lectin, galectin-3, on the cell surface. Mass spectrometry identified the galectin-3 receptor as low-density lipoprotein-related protein-1 (Lrp1), whose targeting in DKO osteoclasts fully rescues RhoA activation, sealing zone formation and bone resorption. Together, these findings identify a previously unrecognized galectin-3/Lrp1 axis whose proteolytic regulation controls both the transcriptional programs and the intracellular signaling cascades critical to mouse as well as human osteoclast function.

Egg Yolk Protein Homologs Identified in Live-Bearing Sharks: Co-Opted in the Lecithotrophy-to-Matrotrophy Shift?

Reproductive modes of vertebrates are classified into two major embryonic nutritional types: yolk deposits (i.e., lecithotrophy) and maternal investment (i.e., matrotrophy). Vitellogenin (VTG), a major egg yolk protein synthesized in the female liver, is one of the molecules relevant to the lecithotrophy-to-matrotrophy shift in bony vertebrates. In mammals, all VTG genes are lost following the lecithotrophy-to-matrotrophy shift, and it remains to be elucidated whether the lecithotrophy-to-matrotrophy shift in nonmammalians is also associated with VTG repertoire modification. In this study, we focused on chondrichthyans (cartilaginous fishes)-a vertebrate clade that underwent multiple lecithotrophy-to-matrotrophy shifts. For an exhaustive search of homologs, we performed tissue-by-tissue transcriptome sequencing for two viviparous chondrichthyans, the frilled shark Chlamydoselachus anguineus and the spotless smooth-hound Mustelus griseus, and inferred the molecular phylogeny of VTG and its receptor very low-density lipoprotein receptor (VLDLR), across diverse vertebrates. As a result, we identified either three or four VTG orthologs in chondrichthyans including viviparous species. We also showed that chondrichthyans had two additional VLDLR orthologs previously unrecognized in their unique lineage (designated as VLDLRc2 and VLDLRc3). Notably, VTG gene expression patterns differed in the species studied depending on their reproductive mode; VTGs are broadly expressed in multiple tissues, including the uterus, in the two viviparous sharks, and in addition to the liver. This finding suggests that the chondrichthyans VTGs do not only function as the yolk nutrient but also as the matrotrophic factor. Altogether, our study indicates that the lecithotrophy-to-matrotrophy shift in chondrichthyans was achieved through a distinct evolutionary process from mammals.

LRP6/filamentous-actin signaling facilitates osteogenic commitment in mechanically induced periodontal ligament stem cells

BACKGROUND

Mechanotransduction mechanisms whereby periodontal ligament stem cells (PDLSCs) translate mechanical stress into biochemical signals and thereby trigger osteogenic programs necessary for alveolar bone remodeling are being deciphered. Low-density lipoprotein receptor-related protein 6 (LRP6), a Wnt transmembrane receptor, has been qualified as a key monitor for mechanical cues. However, the role of LRP6 in the mechanotransduction of mechanically induced PDLSCs remains obscure.

METHODS

The Tension System and tooth movement model were established to determine the expression profile of LRP6. The loss-of-function assay was used to investigate the role of LRP6 on force-regulated osteogenic commitment in PDLSCs. The ability of osteogenic differentiation and proliferation was estimated by alkaline phosphatase (ALP) staining, ALP activity assay, western blotting, quantitative real-time PCR (qRT-PCR), and immunofluorescence. Crystalline violet staining was used to visualize cell morphological change. Western blotting, qRT-PCR, and phalloidin staining were adopted to affirm filamentous actin (F-actin) alteration. YAP nucleoplasmic localization was assessed by immunofluorescence and western blotting. YAP transcriptional response was evaluated by qRT-PCR. Cytochalasin D was used to determine the effects of F-actin on osteogenic commitment and YAP switch behavior in mechanically induced PDLSCs.

RESULTS

LRP6 was robustly activated in mechanically induced PDLSCs and PDL tissues. LRP6 deficiency impeded force-dependent osteogenic differentiation and proliferation in PDLSCs. Intriguingly, LRP6 loss caused cell morphological aberration, F-actin dynamics disruption, YAP nucleoplasmic relocation, and subsequent YAP inactivation. Moreover, disrupted F-actin dynamics inhibited osteogenic differentiation, proliferation, YAP nuclear translocation, and YAP activation in mechanically induced PDLSCs.

CONCLUSIONS

We identified that LRP6 in PDLSCs acted as the mechanosensor regulating mechanical stress-inducible osteogenic commitment via the F-actin/YAP cascade. Targeting LRP6 for controlling alveolar bone remodeling may be a prospective therapy to attenuate relapse of orthodontic treatment.

Immune Checkpoint and Other Receptor-Ligand Pairs Modulating Macrophages in Cancer: Present and Prospects

Immunotherapy, especially immune checkpoint blocking, has become the primary anti-tumor treatment in recent years. However, the current immune checkpoint inhibitor (ICI) therapy is far from satisfactory. Macrophages are a key component of anti-tumor immunity as they are a common immune cell subset in tumor tissues and act as a link between innate and adaptive immunity. Hence, understanding the regulation of macrophage activation in tumor tissues by receptor-ligand interaction will provide promising macrophage-targeting strategies to complement current adaptive immunity-based immunotherapy and traditional anti-tumor treatment. This review aims to offer a systematic summary of the current advances in number, structure, expression, biological function, and interplay of immune checkpoint and other receptor-ligand between macrophages and tumor cells.

Heterozygous LRP1 deficiency causes developmental dysplasia of the hip by impairing triradiate chondrocytes differentiation due to inhibition of autophagy

Developmental dysplasia of the hip (DDH) is one of the most common congenital skeletal malformations; however, its etiology remains unclear. Here, we conducted whole-exome sequencing and identified likely pathogenic variants in the LRP1 (low-density lipoprotein receptor-related protein 1) gene in two families and seven unrelated patients. We found that the timing of triradiate cartilage development was brought forward 1 or 2 wk earlier in the LRP-deficient mice, which leads to malformation of the acetabulum and femoral head. Furthermore, Lrp1 deficiency caused a significant decrease of chondrogenic ability in vitro. Our study reveals a critical role of LRP1 in the etiology and pathogenesis of DDH, opening an avenue for its treatment.

Developmental dysplasia of the hip (DDH) is one of the most common congenital skeletal malformations; however, its etiology remains unclear. Here, we conducted whole-exome sequencing in eight DDH families followed by targeted sequencing of 68 sporadic DDH patients. We identified likely pathogenic variants in the LRP1 (low-density lipoprotein receptor-related protein 1) gene in two families and seven unrelated patients. All patients harboring the LRP1 variants presented a typical DDH phenotype. The heterozygous Lrp1 knockout (KO) mouse (Lrp1^+/−) showed phenotypes recapitulating the human DDH phenotypes, indicating Lrp1 loss of function causes DDH. Lrp1 knockin mice with a missense variant corresponding to a human variant identified in DDH (Lrp1^R1783W) also presented DDH phenotypes, which were milder in heterozygotes and severer in homozygotes than those of the Lrp1 KO mouse. The timing of triradiate cartilage development was brought forward 1 or 2 wk earlier in the LRP-deficient mice, which leads to malformation of the acetabulum and femoral head. Furthermore, Lrp1 deficiency caused a significant decrease of chondrogenic ability in vitro. During the chondrogenic induction of mice bone marrow stem cells and ATDC5 (an inducible chondrogenic cell line), Lrp1 deficiency caused decreased autophagy levels with significant β-catenin up-regulation and suppression of chondrocyte marker genes. The expression of chondrocyte markers was rescued by PNU-74654 (a β-catenin antagonist) in an shRNA-Lrp1–expressed ATDC5 cell. Our study reveals a critical role of LRP1 in the etiology and pathogenesis of DDH, opening an avenue for its treatment.

LRP8, modulated by miR-1262, promotes tumour progression and forecasts the prognosis of patients in breast cancer

This research was designed to detect the function of low-density lipoprotein receptor (LDLR)-related protein 8 (LRP8) in breast cancer (BC). Our results revealed that LRP8 was highly expressed in BC tissues and cell lines compared with human normal breast tissues. The poor prognosis of patients with BC was associated with the up-regulation of LRP8 while inversely connected with overexpression of miR-1262. Functionally, LRP8 depletion in BC cells impaired the proliferative, clonogenic, invasive, and migratory capabilities, which was consistent with the effects of upregulated miR-1262. Bioinformatics prediction and luciferase reporter assay confirmed that miR-1262 was an upstream factor for LRP8 and negatively regulated the expression of LRP8. Further experiments illustrated that the co-transfection of miR-1262 antamir and si-LRP8 could significantly suppress the promoting impacts caused by the transfection of miR-1262 antamir alone. These findings highlighted that LRP8 accelerated the BC development by contributing cellular aggressiveness, which was modulated by miR-1262.

Low-density lipoprotein receptor-related protein 8 facilitates the proliferation and invasion of non-small cell lung cancer cells by regulating the Wnt/β-catenin signaling pathway

Low-density lipoprotein receptor-related protein 8 (LRP8) is involved in the development of multiple tumors, including lung cancer. However, the exact mechanism by which LRP8 exerts its oncogenic role in non-small cell lung cancer (NSCLC) remains elusive. Hence, in this study, we aimed to unravel the expression and role of LRP8 in the progression of NSCLC. We used online bioinformatics databases to identify the expression of LRP8 in multiple types of lung cancer. We validated LRP8 expression in NSCLC cell lines and tissues by Western blotting and immunohistochemistry. The functions of LRP8 in NSCLC carcinogenesis and progression were determined using in vitro and in vivo systems. The Wnt pathway activator LiCl was further used to validate the regulatory role of LRP8 in Wnt/β-catenin signaling. We demonstrated that LRP8 was markedly overexpressed in NSCLC tissues and cell lines, and its overexpression significantly correlated with poor clinicopathological characteristics and prognosis. Moreover, LRP8 depletion suppressed cell proliferation, migration, invasion, and epithelial-mesenchymal transition in vitro and impeded tumor growth in vivo. Mechanistically, LPR8 knockdown elicited tumor-suppressive functions by suppressing the Wnt/β-catenin pathway, which was partially reversed by LiCl. Hence, our study revealed that LRP8 facilitates NSCLC cell proliferation and invasion via the Wnt/β-catenin pathway, and thus LRP8 could be a novel therapeutic target for NSCLC.

Lipid based nanoparticles as a novel treatment modality for hepatocellular carcinoma: a comprehensive review on targeting and recent advances

Liver cancer is considered one of the deadliest diseases with one of the highest disease burdens worldwide. Among the different types of liver cancer, hepatocellular carcinoma is considered to be the most common type. Multiple conventional approaches are being used in treating hepatocellular carcinoma. Focusing on drug treatment, regular agents in conventional forms fail to achieve the intended clinical outcomes. In order to improve the treatment outcomes, utilizing nanoparticles-specifically lipid based nanoparticles-are considered to be one of the most promising approaches being set in motion. Multiple forms of lipid based nanoparticles exist including liposomes, solid lipid nanoparticles, nanostructured lipid carriers, microemulsion, nanoemulsion, phytosomes, lipid coated nanoparticles, and nanoassemblies. Multiple approaches are used to enhance the tumor uptake as well tumor specificity such as intratumoral injection, passive targeting, active targeting, and stimuli responsive nanoparticles. In this review, the effect of utilizing lipidic nanoparticles is being discussed as well as the different tumor uptake enhancement techniques used.

LXR-Mediated Regulation of Marine-Derived Piericidins Aggravates High-Cholesterol Diet-Induced Cholesterol Metabolism Disorder in Mice

Reported as two antirenal cell carcinoma (RCC) drug candidates, marine-derived compounds piericidin A (PA) and glucopiericidin A (GPA) exhibit hepatotoxicity in renal carcinoma xenograft mice. Proteomics and transcriptomics reveal the hepatotoxicity related with cholesterol disposition since RCC is characterized by cholesterol accumulation. PA/GPA aggravate hepatotoxicity in high-cholesterol diet (HCD)-fed mice while exhibiting no toxicity in chow diet-fed mice. High cholesterol accumulation in liver is liver X receptor (LXR)-mediated cytochrome P450 family 7 subfamily a member 1 (CYP7A1) depression and low-density lipoprotein receptor (LDLR) activation. The farnesoid X nuclear receptor (FXR) is also depressed with a downregulated target gene OSTα. Different from PA directly combined with LXRα as an inhibitor, GPA exists as a prodrug in the liver and exerts toxic effects due to transformation into PA. Surface plasmon resonance (SPR) and docking results of 17 piericidins illustrate that glycosides exert no LXRα binding activity. A longer survival time of GPA-treated mice indicates that further exploration in anti-RCC drug research should focus on reducing glycosides transformed into PA and concentrating in the kidney tumor rather than the liver for lowering the risk of hepatotoxicity.

Pcsk9 Knockout Aggravated Experimental Apical Periodontitis via LDLR

Apical periodontitis (AP), an inflammatory lesion around the apex of tooth roots, is mostly caused by dental pulp infection. Proprotein convertase subtilisin/kexin type 9 (PCSK9) plays a vital role in regulating cholesterol homeostasis by targeting low-density lipoprotein receptor (LDLR) and participates in bacterium-induced chronic periodontitis. However, the roles of PCSK9 in AP are unknown. Here, we investigated its role in AP by using Pcsk9^-/- mice. Micro-computed tomography scanning and histological staining revealed that the periapical bone loss of Pcsk9^-/- mice was greater than that of wild-type (WT) mice, and increased expression of inflammation-related factors tumor necrosis factor α (TNF-α) and interleukin (IL)-6 was also observed. Immunofluorescence staining and quantitative real-time polymerase chain reaction showed PCSK9 expression in bone marrow macrophages (BMMs) was increased after treatment with lipopolysaccharide (LPS). This finding was consistent with the in vivo results that the expression level of PCSK9 in exposed WT mice increased compared with that in unexposed WT mice. After LPS challenge, the expression levels of TNF-α, IL-1β, and IL-6 in BMMs were increased, and Pcsk9 knockout aggravated the expression of these inflammatory factors. The number of osteoclasts positive for tartrate-resistant acid phosphatase staining around the apical lesion in Pcsk9^-/- mice was higher than that in WT mice. Then BMMs underwent the osteoclast differentiation. Pcsk9 knockout BMMs induced increased and larger osteoclasts. While this effect of Pcsk9 knockout was abolished by the addition of Ldlr small interfering RNA, revealing that Pcsk9 knockout increased osteoclastogenesis was dependent on the LDLR. Immunohistochemistry staining showed increased expression level of LDLR in exposed Pcsk9^-/- periapical areas. In vitro experiments showed that LPS promoted the expression level of LDLR in Pcsk9^-/- BMMs and increased osteoclast formation ability, indicating that LPS promoted the elevation of osteoclasteogenesis caused by the Pcsk9 knockout. In conclusion, Pcsk9 deficiency aggravated the inflammatory response and promoted the osteoclastogenesis in an LDLR-dependent manner in AP experimental mice.

miR-30b-5p inhibits cancer progression and enhances cisplatin sensitivity in lung cancer through targeting LRP8

Accumulated evidence has demonstrated that miRNAs are closely implicated in lung carcinogenesis. Herein, we explored the expression pattern of miR-30b-5p in lung cancer, and aimed to uncover miR-30b-5p roles in lung cancer progression and drug resistance. miR-30b-5p expression profiles in lung cancer tissues and the matched non-tumor tissues were determined by using qPCR. Cell viability, migration, invasion and in vivo tumorigenesis were determined by using the CCK-8, colony formation, wound healing, transwell chambers experiments and tumor xenograft models. RNA immunoprecipitation (RIP) and dual luciferase reporter experiments were applied to evaluate the relationship between miR-30b-5p and LRP8. The results demonstrated that miR-30b-5p showed a low expression profile in lung cancer tissues and cells, and closely linked to poor prognosis and malignant clinical process. Cell viability, migration, invasiveness and tumorigenesis were significantly weakened following miR-30b-5p overexpression in A549 and NCI-H1299 cells, while cell apoptosis rates were increased. In addition, miR-30b-5p was lowly expressed in A549/DDP (a cisplatin drug resistant cell line) as compared with A549 cells, and miR-30b-5p increased A549/DDP cell sensitivity to DDP. However, these above roles of miR-30b-5p were all significantly impaired following the overexpression of LRP8 which was overexpressed in lung cancer tissues. Collectively, this study demonstrated that miR-30b-5p functions as a tumor suppressor in lung cancer, and re-sensitizes lung cancer cells to DDP by targeting LRP8.

Strategies for delivering therapeutics across the blood-brain barrier

Achieving sufficient delivery across the blood-brain barrier is a key challenge in the development of drugs to treat central nervous system (CNS) disorders. This is particularly the case for biopharmaceuticals such as monoclonal antibodies and enzyme replacement therapies, which are largely excluded from the brain following systemic administration. In recent years, increasing research efforts by pharmaceutical and biotechnology companies, academic institutions and public-private consortia have resulted in the evaluation of various technologies developed to deliver therapeutics to the CNS, some of which have entered clinical testing. Here we review recent developments and challenges related to selected blood-brain barrier-crossing strategies - with a focus on non-invasive approaches such as receptor-mediated transcytosis and the use of neurotropic viruses, nanoparticles and exosomes - and analyse their potential in the treatment of CNS disorders.

Preclinical techniques to investigate exercise training in vascular pathophysiology

Atherosclerosis is a dynamic process starting with endothelial dysfunction and inflammation and eventually leading to life-threatening arterial plaques. Exercise generally improves endothelial function in a dose-dependent manner by altering hemodynamics, specifically by increased arterial pressure, pulsatility, and shear stress. However, athletes who regularly participate in high-intensity training can develop arterial plaques, suggesting alternative mechanisms through which excessive exercise promotes vascular disease. Understanding the mechanisms that drive atherosclerosis in sedentary versus exercise states may lead to novel rehabilitative methods aimed at improving exercise compliance and physical activity. Preclinical tools, including in vitro cell assays, in vivo animal models, and in silico computational methods, broaden our capabilities to study the mechanisms through which exercise impacts atherogenesis, from molecular maladaptation to vascular remodeling. Here, we describe how preclinical research tools have and can be used to study exercise effects on atherosclerosis. We then propose how advanced bioengineering techniques can be used to address gaps in our current understanding of vascular pathophysiology, including integrating in vitro, in vivo, and in silico studies across multiple tissue systems and size scales. Improving our understanding of the antiatherogenic exercise effects will enable engaging, targeted, and individualized exercise recommendations to promote cardiovascular health rather than treating cardiovascular disease that results from a sedentary lifestyle.

Association of single nucleotide polymorphism in NLRC3, NLRC5, HIP1, and LRP8 genes with fecal egg counts in goats naturally infected with Haemonchus contortus

Haemonchus contortus is a common, intractably pathogenic and economically important gastrointestinal nematode for goat producers worldwide, especially in tropical and subtropical regions. The objective of this study is to identify single nucleotide polymorphisms (SNPs) of 12 candidate goat genes mainly related to the innate immune response associated with fecal egg counts (FECs) of Haemonchus contortus in goat as an indicator of the level of parasite infection. Phenotypic data including FEC and blood traits were recorded in 189 native goats from China and 191 ones from Bangladesh, respectively. Bangladeshi goats had significantly (P < 0.01) lower FEC compared to that of Chinese goats, suggesting higher susceptible and infection rates in Chinese goat populations. FEC was significantly positive correlated with body weight (r = 0.64, P < 0.01) and hemoglobin (r = 0.49, P < 0.01) value, but negative with pack cell volume (r = - 0.63, P < 0.05) in goats. Genotyping of SNPs was performed using a matrix-assisted laser desorption ionization time of flight mass spectrometry assay and a generalized linear model was used to evaluate the association between each SNP and goat FEC trait. Eleven novel SNPs in the NLRC3, NLRC5, HIP1, and LRP8, out of 46 variants from these 12 genes, were significantly associated with FEC of goats with a nominal significance level of P < 0.05. Of these 11 SNPs, linkage disequilibrium were revealed among SNPs in LRP8 (r² = 0.87 to 1), between SNPs in NLRC3, NLRC5, and HIP1 (r² = 0.96 to 0.99), respectively. Further, haplotypes within NLRC3, NLRC5, and HIP1 were significantly associated (P < 0.001) with FEC. In artificial challenge trail, quantitative real-time PCR exposed that the relative expression of mRNA was higher in the resistant group for NLRC3 (P < 0.01), LRP8 and HIP1 (P < 0.001) but lower in the resistant group for NLRC5 (P < 0.0001), compared to the susceptible group. The possible SNP markers and genes identified in this study could be potentially used in marker-assisted selection for breeding local goats breeds resistant to gastrointestinal nematode parasite particularly for Haemonchus contortus, and then for improving health and productivity of goat.

Significant association between RETN genetic polymorphisms and alcohol-induced osteonecrosis of femoral head

BACKGROUND

Alcohol-induced osteonecrosis of femoral head (ONFH) is a complex disease and genetic factors are one of the causes. The purpose of this study is to investigate the effects of RETN (resistin; OMIM: 605565) and LDLR (low density lipoprotein receptor; OMIM: 606945) polymorphisms on the risk of alcohol-induced ONFH in Chinese Han population.

METHODS

A case-control study including 201 patients and 201 controls was designed. Seven single nucleotide polymorphisms (SNPs) in RETN gene and four SNPs in LDLR gene were genotyped using Agena MassARRAY platform. In allele model and genetic model, chi-square test and logistic regression were used to study the associations between these SNPs and ONFH susceptibility. In addition, the relationships between these SNPs, clinical phenotypes, and blood lipid level with one-way analysis of variance were analyzed.

RESULTS

In the allele model, rs7408174 and rs3745369 in RETN were associated with increased risk of alcohol-induced ONFH, whereas rs34861192 and rs3219175 in RETN showed reduced risk of alcohol-induced ONFH. In the genetic model, rs7408174 was associated with increased risk of alcohol-induced ONFH in dominant model and log-additive model. Rs3745369 showed an increased risk in codominant model, recessive model, and log-additive model. Rs34861192 showed a decreased risk in codominant model, dominant model, and log-additive model, and rs3219175 showed a decreased risk in dominant model and log-additive model. The rs3745368 in RETN was associated with the clinical stage of the disease.

CONCLUSION

These results suggest that RETN genetic polymorphisms are associated with the susceptibility of alcohol-induced ONFH in Chinese Han population.

The Roles of Low-Density Lipoprotein Receptor-Related Proteins 5, 6, and 8 in Cancer: A Review

Low-density lipoprotein receptor (LDLR) has been an object of research since the 1970s because of its role in various cell functions. The LDLR family members include LRP5, LRP6, and LRP8. Even though LRP5, 6, and 8 are in the same family, intriguingly, these three proteins have various roles in physiological events, as well as in regulating different mechanisms in various kinds of cancers. LRP5, LRP6, and LRP8 have been shown to play important roles in a broad panel of cancers. LRP5 is highly expressed in many tissues and is involved in the modulation of glucose-induced insulin secretion, bone development, and cholesterol metabolism, as well as cancer progression. Recently, LRP5 has also been shown to play a role in chondroblastic subtype of osteosarcoma (OS) and prostate cancer and also in noncancer case such as osteoporosis. LRP6, which has been previously discovered to share the same structures as LRP5, has also been associated with many cancer progressions such as human triple negative breast cancer (TNBC), primary chronic lymphocytic leukemia (CLL), nonsmall cell lung cancer (NSCL), lung squamous cell carcinoma (LSCC), and hepatocellular carcinoma (HCC). In addition to its role in cancer progression, LRP8 (apolipoprotein E receptor 2 [APOER2]) has also been demonstrated to regulate canonical Wnt/β-catenin signaling pathway whereby this pathway plays a role in cell migration and development. Therefore, this review aimed to elucidate the role of LRP 5, 6, and 8 in regulating the cancer progression.

Lrp4 expression by adipocytes and osteoblasts differentially impacts sclerostin's endocrine effects on body composition and glucose metabolism

Sclerostin exerts profound local control over bone acquisition and also mediates endocrine communication between fat and bone. In bone, sclerostin's anti-osteoanabolic activity is enhanced by low-density lipoprotein receptor-related protein 4 (Lrp4), which facilitates its interaction with the Lrp5 and Lrp6 Wnt co-receptors. To determine whether Lrp4 similarly affects sclerostin's endocrine function, we examined body composition as well as glucose and fatty acid metabolism in mice rendered deficient of Lrp4 in the adipocyte (AdΔLrp4) or the osteoblast (ObΔLrp4). AdΔLrp4 mice exhibit a reduction in adipocyte hypertrophy and improved glucose and lipid homeostasis, marked by increased glucose and insulin tolerance and reduced serum fatty acids, and mirror the effect of sclerostin deficiency on whole-body metabolism. Indeed, epistasis studies place adipocyte-expressed Lrp4 and sclerostin in the same genetic cascade that regulates adipocyte function. Intriguingly, ObΔLrp4 mice, which exhibit dramatic increases in serum sclerostin, accumulate body fat and develop impairments in glucose tolerance and insulin sensitivity despite development of a high bone mass phenotype. These data indicate that expression of Lrp4 by both the adipocyte and osteoblast is required for normal sclerostin endocrine function and that the impact of sclerostin deficiency on adipocyte physiology is distinct from the effect on osteoblast function.

LRP8 is overexpressed in estrogen-negative breast cancers and a potential target for these tumors

Triple‐negative breast cancer (TNBC) is the breast cancer subtype with the worst prognosis. New treatments improving the survival of TNBC patients are, therefore, urgently required. We performed a transcriptome microarray analysis to identify new treatment targets for TNBC. We found that low‐density lipoprotein receptor‐related protein 8 (LRP8) was more strongly expressed in estrogen receptor‐negative breast tumors, including TNBCs and those overexpressing HER2, than in luminal breast tumors and normal breast tissues. LRP8 depletion decreased cell proliferation more efficiently in estrogen receptor‐negative breast cancer cell lines: TNBC and HER2 overexpressing cell lines. We next focused on TNBC cells for which targeted therapies are not available. LRP8 depletion induced an arrest of the cell cycle progression in G1 phase and programmed cell death. We also found that LRP8 is required for anchorage‐independent growth in vitro, and that its depletion in vivo slowed tumor growth in a xenograft model. Our findings suggest that new approaches targeting LRP8 may constitute promising treatments for hormone‐negative breast cancers, those overexpressing HER2 and TNBCs.

Lrp4/Wise regulates palatal rugae development through Turing-type reaction-diffusion mechanisms

Periodic patterning of iterative structures is diverse across the animal kingdom. Clarifying the molecular mechanisms involved in the formation of these structure helps to elucidate the process of organogenesis. Turing-type reaction-diffusion mechanisms have been shown to play a critical role in regulating periodic patterning in organogenesis. Palatal rugae are periodically patterned ridges situated on the hard palate of mammals. We have previously shown that the palatal rugae develop by a Turing-type reaction-diffusion mechanism, which is reliant upon Shh (as an inhibitor) and Fgf (as an activator) signaling for appropriate organization of these structures. The disturbance of Shh and Fgf signaling lead to disorganized palatal rugae. However, the mechanism itself is not fully understood. Here we found that Lrp4 (transmembrane protein) was expressed in a complementary pattern to Wise (a secreted BMP antagonist and Wnt modulator) expression in palatal rugae development, representing Lrp4 expression in developing rugae and Wise in the inter-rugal epithelium. Highly disorganized palatal rugae was observed in both Wise and Lrp4 mutant mice, and these mutants also showed the downregulation of Shh signaling, which was accompanied with upregulation of Fgf signaling. Wise and Lrp4 are thus likely to control palatal rugae development by regulating reaction-diffusion mechanisms through Shh and Fgf signaling. We also found that Bmp and Wnt signaling were partially involved in this mechanism.

Endocytosis of lipoproteins

During their metabolism, all lipoproteins undergo endocytosis, either to be degraded intracellularly, for example in hepatocytes or macrophages, or to be re-secreted, for example in the course of transcytosis by endothelial cells. Moreover, there are several examples of internalized lipoproteins sequestered intracellularly, possibly to exert intracellular functions, for example the cytolysis of trypanosoma. Endocytosis and the subsequent intracellular itinerary of lipoproteins hence are key areas for understanding the regulation of plasma lipid levels as well as the biological functions of lipoproteins. Indeed, the identification of the low-density lipoprotein (LDL)-receptor and the unraveling of its transcriptional regulation led to the elucidation of familial hypercholesterolemia as well as to the development of statins, the most successful therapeutics for lowering of cholesterol levels and risk of atherosclerotic cardiovascular diseases. Novel limiting factors of intracellular trafficking of LDL and the LDL receptor continue to be discovered and to provide drug targets such as PCSK9. Surprisingly, the receptors mediating endocytosis of high-density lipoproteins or lipoprotein(a) are still a matter of controversy or even new discovery. Finally, the receptors and mechanisms, which mediate the uptake of lipoproteins into non-degrading intracellular itineraries for re-secretion (transcytosis, retroendocytosis), storage, or execution of intracellular functions, are largely unknown.