Identification of a major cyclic AMP-dependent protein kinase A phosphorylation site within the cytoplasmic tail of the low-density lipoprotein receptor-related protein: implication for receptor-mediated endocytosis

Proteasome regulates the delivery of LDL receptor-related protein into the degradation pathway

The low-density lipoprotein receptor (LDLR)-related protein (LRP) is a multiligand endocytic receptor that has broad cellular and physiological functions. Previous studies have shown that both tyrosine-based and di-leucine motifs within the LRP cytoplasmic tail are responsible for mediating its rapid endocytosis. Little is known, however, about the mechanism by which LRP is targeted for degradation. By examining both endogenous full-length and a minireceptor form of LRP, we found that proteasomal inhibitors, MG132 and lactacystin, prolong the cellular half-life of LRP. The presence of proteasomal inhibitors also significantly increased the level of LRP at the cell surface, suggesting that the delivery of LRP to the degradation pathway was blocked at a compartment from which recycling of the receptor to the cell surface still occurred. Immunoelectron microscopy analyses demonstrated a proteasomal inhibitor-dependent reduction in LRP minireceptor within both limiting membrane and internal vesicles of the multivesicular bodies, which are compartments that lead to receptor degradation. In contrast to the growth hormone receptor, we found that the initial endocytosis of LRP minireceptor does not require a functional ubiquitin-proteasome system. Finally, using truncated cytoplasmic mutants of LRP minireceptors, we found that a region of 19 amino acids within the LRP tail is required for proteasomal regulation. Taken together our results provide strong evidence that the cellular turnover of a cargo receptor, i.e., LRP, is regulated by the proteasomal system, suggesting a broader function of the proteasome in regulating the trafficking of receptors into the degradation pathway.

Low-density lipoprotein receptor-related protein levels and endocytic function are reduced by overexpression of the FE65 adaptor protein, FE65L1

The FE65 adaptor protein family was identified in two-hybrid screens as proteins that bind the cytoplasmic domain of the amyloid precursor protein (APP). Studies have shown that FE65 binding to APP modulates APP processing. Increased levels of alpha-secretase derived secreted APP (APPsalpha) and beta-amyloid (Abeta) were recovered from conditioned media upon FE65L1 or FE65 overexpression. These effects were associated with an increase in the ratio of mature/immature APP and increased cell-surface APP. FE65 has also been reported to bind low-density lipoprotein receptor-related protein (LRP). Here we show that FE65L1 overexpression results in decreased LRP steady state levels, LRPs, and LRP endocytic receptor function. These changes in LRP protein levels are not due to decreased transcription of LRP. Furthermore, pulse/chase experiments demonstrate that changes in LRP protein only occurred 12-18 h after translation. We conclude that the decreases in LRP levels likely reflect routing of LRP away from the cell surface into a degradative pathway. Previous studies suggested that LRP plays an important role for Abeta production of Kunitz protease inhibitor forms of APP in the endocytic pathway. These data show that FE65L1 can differentially affect the metabolic fate of APP and LRP. In addition, these data suggest that the LRP decrease observed in FE65L1 overexpressing cells may in part contribute to altered APP processing.

Novel mechanism for regulation of epidermal growth factor receptor endocytosis revealed by protein kinase A inhibition

Current models put forward that the epidermal growth factor receptor (EGFR) is efficiently internalized via clathrin-coated pits only in response to ligand-induced activation of its intrinsic tyrosine kinase and is subsequently directed into a lysosomal-proteasomal degradation pathway by mechanisms that include receptor tyrosine phosphorylation and ubiquitylation. Herein, we report a novel mechanism of EGFR internalization that does not require ligand binding, receptor kinase activity, or ubiquitylation and does not direct the receptor into a degradative pathway. Inhibition of basal protein kinase A (PKA) activity by H89 and the cell-permeable substrate peptide Myr-PKI induced internalization of 40-60% unoccupied, inactive EGFR, and its accumulation into early endosomes without affecting endocytosis of transferrin and mu-opioid receptors. This effect was abrogated by interfering with clathrin function. Thus, the predominant distribution of inactive EGFR at the plasma membrane is not simply by default but involves a PKA-dependent restrictive condition resulting in receptor avoidance of endocytosis until it is stimulated by ligand. Furthermore, PKA inhibition may contribute to ligand-induced EGFR endocytosis because epidermal growth factor inhibited 26% of PKA basal activity. On the other hand, H89 did not alter ligand-induced internalization of EGFR but doubled its half-time of down-regulation by retarding its segregation into degradative compartments, seemingly due to a delay in the receptor tyrosine phosphorylation and ubiquitylation. Our results reveal that PKA basal activity controls EGFR function at two levels: 1) residence time of inactive EGFR at the cell surface by a process of "endocytic evasion," modulating the accessibility of receptors to stimuli; and 2) sorting events leading to the down-regulation pathway of ligand-activated EGFR, determining the length of its intracellular signaling. They add a new dimension to the fine-tuning of EGFR function in response to cellular demands and cross talk with other signaling receptors.

Apolipoprotein E activates Akt pathway in neuro-2a in an isoform-specific manner

Apolipoprotein E (apoE) is a ligand for members of the low density lipoprotein (LDL) receptor family, receptors highly expressed in neurons. A study of one of the mechanisms by which apoE might affect neuronal cell metabolism is reported herein. ApoE can induce Akt/protein kinase B phosphorylation in Neuro-2a via two different pathways. Both pathways are mediated by phosphatidylinositol 3-kinase and cAMP-dependent protein kinase. The first pathway is stimulated by apoE3 and E4, but not by E2, after a 1-h incubation. The process requires the binding of apoE to the heparan sulfate proteoglycan/LDL receptor-related protein complex. The second pathway is activated after a 2-h incubation of the cells, in another isoform-dependent manner (E2 = E3 dbl greater-than sign E4) and is mediated by calcium. Our results suggest that apoE might affect cell metabolism and survival in neurons in an isoform-specific manner by inducing novel signaling pathways.

The roles of receptor-associated protein (RAP) as a molecular chaperone for members of the LDL receptor family

Members of the LDL receptor family mediate endocytosis and signal transduction of many extracellular ligands which participate in lipoprotein metabolism, protease regulation, embryonic development, and the pathogenesis of disease (e.g., Alzheimer's disease). Structurally, these receptors share common motifs and modules that are highlighted with clusters of cysteine-rich ligand-binding repeats. Perhaps, the most significant feature that is shared by members of the LDL receptor family is the ability of a 39-kDa receptor-associated protein (RAP) to universally inhibit ligand interaction with these receptors. Under physiological conditions, RAP serves as a molecular chaperone/escort protein for these receptors to prevent premature interaction of ligands with the receptors and thereby ensures their safe passage through the secretory pathway. In addition, RAP promotes the proper folding of these receptors, a function that is likely independent from its ability to inhibit ligand binding. The molecular mechanisms underlying these functions of RAP, as well as the molecular determinants that contribute to RAP-receptor interaction will be discussed in this review. Elucidation of these mechanisms should help to clarify how a specialized chaperone promotes the biogenesis of LDL receptor family members, and may provide insights into how the expression and function of these receptors can be regulated via the expression of RAP under pathological states.

The putative tumor suppressor LRP1B, a novel member of the low density lipoprotein (LDL) receptor family, exhibits both overlapping and distinct properties with the LDL receptor-related protein

The low density lipoprotein receptor-related protein-deleted in tumor (LRP1B, initially referred to as LRP-DIT) was cloned and characterized as a candidate tumor suppressor. It is a new member of the low density lipoprotein receptor gene family. Its overall domain structure and large size (approximately 600 kDa) are similar to LRP and suggest that it is a multifunctional cell surface receptor. Herein, we characterize a series of ligands for the receptor using cell lines that stably express it as a domain IV minireceptor (mLRP1B4). Ligands of LRP including receptor-associated protein, urokinase plasminogen activator, tissue-type plasminogen activator, and plasminogen activator inhibitor type-1 each demonstrate binding, internalization, and degradation via mLRP1B4. Interestingly, the kinetics of ligand endocytosis is distinctly different from that of LRP, with LRP1B exhibiting a markedly diminished internalization rate. In addition, tissue expression analysis reveals that the LRP1B gene is expressed in brain, thyroid, and salivary gland. These studies thus extend the physiological roles of members of the LDL receptor family.

Differential functions of members of the low density lipoprotein receptor family suggested by their distinct endocytosis rates

The low density lipoprotein receptor (LDLR) family is composed of a class of cell surface endocytic receptors that recognize extracellular ligands and internalize them for degradation by lysosomes. In addition to LDLR, mammalian members of this family include the LDLR-related protein (LRP), the very low density lipoprotein receptor (VLDLR), the apolipoprotein E receptor-2 (apoER2), and megalin. Herein we have analyzed the endocytic functions of the cytoplasmic tails of these receptors using LRP minireceptors, its chimeric receptor constructs, and full-length VLDLR and apoER2 stably expressed in LRP-null Chinese hamster ovary cells. We find that the initial endocytosis rates mediated by different cytoplasmic tails are significantly different, with half-times of ligand internalization ranging from less than 30 s to more than 8 min. The tail of LRP mediates the highest rate of endocytosis, whereas those of the VLDLR and apoER2 exhibit least endocytosis function. Compared with the tail of LRP, the tails of the LDLR and megalin display significantly lower levels of endocytosis rates. Ligand degradation analyses strongly support differential endocytosis rates initiated by these receptors. Interestingly apoER2, which has recently been shown to mediate intracellular signal transduction, exhibited the lowest level of ligand degradation efficiency. These results thus suggest that the endocytic functions of members of the LDLR family are distinct and that certain receptors in this family may play their main roles in areas other than receptor-mediated endocytosis.

Low-density lipoprotein receptor family: endocytosis and signal transduction

The low-density lipoprotein receptor (LDLR) family is composed of a class of single transmembrane glycoproteins, generally recognized as cell surface endocytic receptors, which bind and internalize extracellular ligands for degradation by lysosomes. Structurally, members of the LDLR family share homology within their extracellular domains, which are highlighted by the presence of clusters of ligand-binding repeats. Recently, information regarding the structural and functional elements within their cytoplasmic tails has begun to emerge, which suggests that members of the LDLR family function not only in receptor-mediated endocytosis, but also in transducing signals that are important during embryonic development and the pathogenesis of Alzheimer's disease. This review focuses on recent knowledge of the structural and functional aspects of LDLR family members in endocytosis and signal transduction. The relationship of these functions to the development of the neuronal system and in the pathogenesis of Alzheimer's disease is specifically discussed.