Resveratrol induces sumoylated COX-2-dependent anti-proliferation in human prostate cancer LNCaP cells

Cyclooxygenase (COX)-2 has been implicated in cancer development. However, resveratrol-induced nuclear accumulation of COX-2 enhances p53-dependent anti-proliferation in different types of cancers. Treatment with resveratrol leads to phosphorylation and nuclear translocation of mitogen-activated protein kinase (ERK1/2), and accumulation of nuclear COX-2 to complex with pERK1/2 and p53. The consequence is Ser-15 phosphorylation of p53 (pSer15-p53), and induction of anti-proliferation in cancer cells. We investigated the mechanisms by which resveratrol-inducible COX-2 facilitates p53-dependent anti-proliferation in prostate cancer LNCaP cells. Resveratrol treatment caused nuclear accumulation and complexing of ERK1/2, pSer15-p53 and COX-2 which was activated ERK1/2-dependent. Knockdown of SUMO-1 by shRNA also reduced nuclear accumulation of COX-2. Inhibition of nuclear accumulation by the COX-2 specific inhibitor, NS-398, inhibited co-localization of nuclear COX-2 and SUMO-1. Similar results were observed in the PD98059-treated cells. Finally, inhibition of SUMO-1 expression also reduced resveratrol-induced expression of pro-apoptotic genes but increased the expression of proliferative genes. In summary, these results demonstrate that inducible COX-2 associates with phosphorylated ERK1/2 to induce the phosphorylation of Ser-15 in p53 and then complexes with p53 and SUMO-1 which binds to p53-responsive pro-apoptotic genes to enhance their expression. The inhibition of COX-2 expression and activity significantly blocks the pro-apoptotic effect of resveratrol.

Identification of the membrane remnants of transferrin receptor with domain-specific antibodies

Tissue culture studies with K562 and HL60 cells have demonstrated the production of a soluble form of transferrin receptor identical to that identified in human serum. The present study was undertaken to search for membrane remnants of the truncated receptor with peptide antibodies specific for the extracellular and cytoplasmic domain of transferrin receptor. In cell membranes, a 105K remnant was identified that is consistent with truncation of one extracellular domain monomer of the transferrin receptor. In the exosomal fraction of the culture supernatant, a smaller 20K remnant consistent with truncation of both extracellular domains was also demonstrated. These findings provide evidence that soluble receptor is the product of proteolytic cleavage of intact membrane-bound transferrin receptor. Prior studies showing that the concentration of the extracellular domain in exosomes remained stable during incubation in culture supernatant suggest that this cleavage possibly occurs intracellularly.

Serum form of the erythropoietin receptor identified by a sequence-specific peptide antibody

The present investigation was undertaken to search for soluble forms of the erythropoietin receptor in human serum using polyclonal antibody against an amino terminal peptide sequence in the extracellular domain. This sequence was located adjacent to the amino terminus at residues 25-38. When this antibody was used for Western blots of solubilized membranes from nucleated bone marrow cells, a protein consistent with native erythropoietin receptor was seen. Purified soluble ectodomain of the erythropoietin receptor displayed appropriate reactivity with this antibody. When sera from normal subjects and patients with a range of hematologic disorders were examined by Western blotting, a protein with a molecular mass of 34 Kd was detected in sera from patients with enhanced erythropoiesis including sickle cell anemia, thalassemia, and megaloblastic anemia. This protein was rarely detected in normal serum but appeared when normal subjects were treated with recombinant erythropoietin and disappeared after full treatment of patients with megaloblastic anemia due to vitamin B12 deficiency. The protein was not detected after myeloablation for bone marrow transplantation but appeared with marrow engraftment. Reactivity of this protein with the peptide antibody was competitively inhibited by the amino terminal peptide sequence. An additional 48 Kd protein was detected that showed minimal variation in intensity with differing degrees of erythropoietic activity. Detection of this protein could not be inhibited by the addition of synthetic peptide. Our findings indicate the presence of a soluble form of the erythropoietin receptor related to the extracellular domain that is highly correlated with enhanced erythropoiesis.

Production of the serum form of the transferrin receptor by a cell membrane-associated serine protease

Recent investigations have demonstrated that the soluble form of the transferrin receptor in human serum is an 85-kDa fragment of intact receptor containing most of the extracellular domain. The recent demonstration of a remnant of the truncated receptor in cell membranes suggests that the soluble form arises from proteolytic cleavage of intact receptor. In the present investigation, domain-specific antibodies were used to further examine the subcellular location and nature of this proteolysis. HL60 cells were used in the investigation because the cells release an 85-kDa soluble form of the receptor to the culture supernatant that is identical to that found in serum. When intact, purified transferrin receptor from human placenta was incubated with the culture supernatant, no proteolytic activity could be demonstrated. However, when purified membrane fractions from HL60 cells were used in this incubation system, the 85-kDa fragment was produced. This activity was inhibited by serine protease inhibitors indicating that cell membrane fractions contain a serine protease capable of producing the serum form of the transferrin receptor.

Characterization and quantitation of the circulating forms of serum transferrin receptor using domain-specific antibodies

To characterize the nature of the immunoreactive transferrin receptor in human serum, antisera were developed to peptide sequences of the extracellular domain of human transferrin receptor between amino acids 107 and 120 and the intracellular domain between amino acids 40 and 54. Antisera against the extracellular domain exhibited reactivity against both purified intact receptor and immunopurified circulating receptor, whereas antisera against the intracellular domain reacted only with intact receptor. Using competitive binding enzyme-linked immunosorbent assays, transferrin receptor in ultracentrifuged sera from normal subjects and patients with sickle cell anemia could be detected with antisera against the extracellular but not the intracellular domain. When the pellet obtained by ultracentrifugation of these sera was assayed after solubilization in 1% teric (polyoxyethylene-9-lauryl ether), only 0.6% of total serum receptor was detected in normal subjects and 3.8% in subjects with sickle cell disease. Roughly equal amounts of this pelleted immunoactivity were detected with antibodies against the extracellular and intracellular domains. These results indicate that less than 1% of transferrin receptor in normal human sera is intact receptor consistent with an exosomal origin and that virtually all circulating transferrin receptor is in the form of a truncated extracellular domain.

Characterization of transferrin receptor released by K562 erythroleukemia cells

A soluble form of transferrin receptor has been detected in human serum and has been shown recently to be a truncated form of the intact membrane bound receptor. Mechanisms governing the release of transferrin receptor by cells are poorly understood and could be better defined by tissue culture. The present investigation was undertaken to characterize the transferrin receptor released by K562 erythroleukemic cells. In contrast with maturing sheep reticulocytes, which have been shown to release transferrin receptor in small vesicles termed exosomes, we demonstrated, with a monoclonal enzyme-linked immunoassay, that less than 30% of the transferrin receptor released by K562 cells in log phase growth was in a particulate form. The relative amounts of soluble and particulate receptor released to the supernatant did not change significantly during 48 hr of incubation. Soluble receptor was purified by immunoaffinity chromatography. On polyacrylamide gel electrophoresis, its mobility was the same (85 kDa) as that of the truncated monomeric form recently identified in human serum. Further evidence that serum and soluble receptors released by K562 cells are identical was provided by amino acid sequence analysis, which demonstrated that 16 of the first 19 residues of the N-terminal sequence of soluble K562 receptor are homologous with the serum receptor. The remaining three were not identifiable. K562 cells provide a useful in vitro model for studying the production of membrane-bound and soluble forms of released transferrin receptor.

Production of soluble transferrin receptor by K562 erythroleukaemia cells

The present study was undertaken to examine the production of soluble transferrin receptor by K562 erythroleukaemia cells under controlled experimental conditions. The concentrations of soluble and cellular transferrin receptor were measured by immunoassay employing monoclonal antibodies. Cellular ferritin was also measured as an index of iron supply. With incubation up to 48 h there was a progressive increase in the concentration of soluble transferrin receptor. Manipulating iron supply by adding iron chelators or diferric transferrin to the incubation medium produced marked alterations in cellular receptor and ferritin content. Under all such conditions examined, the relationship between soluble and cellular receptor remained highly constant. These findings support clinical studies of serum receptor suggesting that over a broad spectrum of haematological disorders there is a fixed relationship between serum receptor and tissue receptor mass.

Serum transferrin receptor is a truncated form of tissue receptor

Recent studies have provided immunological evidence for the existence of transferrin receptor in human serum and have revealed that its concentration is a sensitive measure of erythropoiesis and iron deficiency. The present study was undertaken to establish the molecular identity of this immunoreactive component. Purification from human serum was accomplished by immunoaffinity chromatography using, as the ligand, monoclonal antitransferrin receptor antibody. The receptor preparation contained two major components with Mr of 75,000 and 85,000, which were identified as transferrin and transferrin receptor, respectively. The physicochemical and immunochemical properties of the 85,000 serum receptor were compared with those established for intact placental transferrin receptor. The serum receptor exhibited an apparent Mr = 85,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis under non-reducing conditions, as compared with 190,000 for placental transferrin receptor. Upon reduction, the Mr of serum receptor was unaltered, whereas, the 190,000 placental receptor dimer decreased to the expected monomer value of 95,000. Amino-terminal amino acid sequence analysis revealed that residues 1-19 of serum receptor were identical to residues 101-119 of intact receptor. These findings provide physicochemical evidence for the existence of transferrin receptor in human serum, establish its molecular identity as a truncated form lacking the cytoplasmic and transmembrane domains (residues 1-100) of intact receptor, and demonstrate that it exists as a transferrin-receptor complex in serum.