Fc gamma receptor IIIb enhances Fc gamma receptor IIa function in an oxidant-dependent and allele-sensitive manner.

Two classes of receptors for IgG, Fc gamma RIIa and Fc gamma RIIIb, both of which exist in two allelic forms, are expressed on human neutrophils. Neutrophils from normal donors, homozygous for the different allelic phenotypes of Fc gamma RIIIb, have significantly different levels of Fc gamma receptor-mediated phagocytosis of IgG-opsonized erythrocytes (EA). However, the observation that Fc gamma RIIIb mediates phagocytosis of specific mAb-targeted erythrocytes poorly suggests that this receptor may influence EA internalization by Fc gamma RIIa in an allele-sensitive fashion. Donors homozygous for the NA1 allele of Fc gamma RIIIb showed greater activation of Fc gamma RIIa after Fc gamma RIIIb cross-linking than donors homozygous for the NA2 allele of Fc gamma RIIIb. This increase in receptor-specific internalization reflects both an increase in ligand binding by Fc gamma RIIa and an increase in internalization efficiency of targets bound. Activation of Fc gamma RIIa by Fc gamma RIIIb is transferable by supernatants from activated cells and is blocked by inhibitors of reactive oxygen species and the H2O2-myeloperoxidase-chloride system and by serine protease inhibitors. Thus, cross-linking of Fc gamma RIIIb, which leads to neutrophil degranulation and the generation of reactive oxygen intermediates, in turn alters Fc gamma RIIa avidity and efficiency. These oxidant-mediated changes in Fc gamma RIIa function provide a novel mechanism for receptors to collaborate in both an autocrine and paracrine fashion. The allele sensitivity of these effects suggests that Fc gamma receptor polymorphisms may be inherited disease susceptibility factors in host defense against infection and in the development of autoimmunity.

A U-rich element in the 5' untranslated region is necessary for the translation of p27 mRNA.

Increased translation of p27 mRNA correlates with withdrawal of cells from the cell cycle. This raised the possibility that antimitogenic signals might mediate their effects on p27 expression by altering complexes that formed on p27 mRNA, regulating its translation. In this report, we identify a U-rich sequence in the 5' untranslated region (5'UTR) of p27 mRNA that is necessary for efficient translation in proliferating and nonproliferating cells. We show that a number of factors bind to the 5'UTR in vitro in a manner dependent on the U-rich element, and their availability in the cytosol is controlled in a growth- and cell cycle-dependent fashion. One of these factors is HuR, a protein previously implicated in mRNA stability, transport, and translation. Another is hnRNP C1 and C2, proteins implicated in mRNA processing and the translation of a specific subset of mRNAs expressed in differentiated cells. In lovastatin-treated MDA468 cells, the mobility of the associated hnRNP C1 and C2 proteins changed, and this correlated with increased p27 expression. Together, these data suggest that the U-rich dependent RNP complex on the 5'UTR may regulate the translation of p27 mRNA and may be a target of antimitogenic signals.

Distinct altered patterns of p27KIP1 gene expression in benign prostatic hyperplasia and prostatic carcinoma.

BACKGROUND: The p27KIP1 gene, whose protein product is a negative regulator of the cell cycle, is a potential tumor suppressor gene; however, no tumor-specific mutations of this gene have been found in humans. This study was undertaken to identify and to assess potential alterations of p27KIP1 gene expression in patients with benign prostatic hyperplasia (BPH) and patients with prostate cancer. METHODS: We analyzed 130 prostate carcinomas from primary and metastatic sites, as well as prostate samples from normal subjects and from patients with BPH. Immunohistochemistry and in situ hybridization were used to determine the levels of expression and the microanatomical localization of p27 protein and messenger RNA (mRNA), respectively. Immunoblotting and immunodepletion assays were performed on a subset of the prostate tumors. Associations between alterations in p27KIP1 expression and clinicopathologic variables were evaluated with a nonparametric test. The Kaplan-Meier method and the logrank test were used to compare disease-relapse-free survival. Prostate tissues of p27Kip1 null (i.e., knock-out) and wild-type mice were also evaluated. RESULTS: Normal human prostate tissue exhibited abundant amounts of p27 protein and high levels of p27KIP1 mRNA in both epithelial cells and stromal cells. However, p27 protein and p27KIP1 mRNA were almost undetectable in epithelial cells and stromal cells of BPH lesions. Furthermore, p27Kip1 null mice developed enlarged (hyperplastic) prostate glands. In contrast to BPH, prostate carcinomas were found to contain abundant p27KIP1 mRNA but either high or low to undetectable levels of p27 protein. Primary prostate carcinomas expressing lower levels of p27 protein appeared to be biologically more aggressive (two-sided P = .019 [Cox regression analysis]). CONCLUSIONS/IMPLICATIONS: On the basis of these results, we infer that loss of p27Kip1 expression in the human prostate may be causally linked to BPH and that BPH is not a precursor to prostate cancer.

Enhanced ribosomal association of p27(Kip1) mRNA is a mechanism contributing to accumulation during growth arrest.

p27(Kip1) regulates the decision to enter into S-phase or withdraw from the cell cycle by establishing an inhibitory threshold above which G1 cyclin-dependent kinases accumulate before activation. We have used the HL-60 cell line to study regulation of p27 as cells withdraw from the cell cycle following treatment with 12-O-tetradecanoylphorbol-13-acetate (TPA). We found that the amount of p27 is maximal in G0 cells, lower in G1 cells, and undetectable in S-phase cells. In contrast to the protein, the amount of p27 mRNA was the same in these populations, suggesting that accumulation of p27 during the cell cycle and as cells withdraw from the cell cycle is controlled by post-transcriptional mechanisms. In S-phase cells, the degradation of p27 appears to predominate as a regulatory mechanism. In G0 cells, there was an increase in the synthesis rate of p27. Our data demonstrate that, in G0 cells, accumulation of p27 is due to an increase in the amount of p27 mRNA in polyribosomes.

A naturally occurring mutation in Fc gamma RIIA: a Q to K127 change confers unique IgG binding properties to the R131 allelic form of the receptor.

Fc gamma RIIa is widely expressed on hematopoietic cells. There are two known allelic polymorphic forms of Fc gamma RIIa, Fc gamma RIIa-R131 and Fc gamma RIIa-H131, which differ in the amino acid at position 131 in the second lg-like domain. In contrast to Fc gamma RIIa-R131, Fc gamma RIIa-H131 binds hlgG2 but not mIgG1, and this differential binding has clinical implications for host defense, autoimmune disease, immunohematologic disease, and response to therapeutic monoclonal antibodies. We identified a novel Fc gamma RIIA genotype in a healthy individual homozygous for Fc gamma RIIA R/R131 in whom a C to A substitution at codon 127 changes glutamine (Q) to lysine (K) in one of the two Fc gamma RIIA genes. This individual's homozygosity for Fc gamma RIIA-R/R131 leads to the prediction that the receptors on her cells would not bind hIgG2. Monocyte and neutrophil phagocytosis of hIgG2-opsonized erythrocytes was significantly higher (P < .05) for cells from this K/Q127, R/R131 individual than for Q/Q127, R/R131 donors. Platelet aggregation stimulated by an mIgG1 anti-CD9 antibody in this individual was significantly different (P < .05) from Q/Q127, H/R131 and Q/Q127, H/H131 donors and similar to Q/Q127, R/R131. Our data show that the K127/R131 receptors have a unique phenotype, binding both hIgG2 and mIgG1. Further functionally significant mutations in human Fc gamma receptors and possible novel mechanisms for inherited differences in disease susceptibility should be sought with unbiased screening methods.