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Biomedical subjects

F Houghton

Publications and source records attributed to F Houghton.

18 recordsLinked to original sources
PubMed2001

A large family of endosome-localized proteins related to sorting nexin 1.

Sorting nexin 1 (SNX1), a peripheral membrane protein, has previously been shown to regulate the cell-surface expression of the human epidermal growth factor receptor [Kurten, Cadena and Gill (1996) Science 272, 1008-1010]. Searches of human expressed sequence tag databases with SNX1 revealed eleven related human cDNA sequences, termed SNX2 to SNX12, eight of them novel. Analysis of SNX1-related sequences in the Saccharomyces cerevisiae genome clearly shows a greatly expanded SNX family in humans in comparison with yeast. On the basis of the predicted protein sequences, all members of this family of hydrophilic molecules contain a conserved 70-110-residue Phox homology (PX) domain, referred to as the SNX-PX domain. Within the SNX family, subgroups were identified on the basis of the sequence similarities of the SNX-PX domain and the overall domain structure of each protein. The members of one subgroup, which includes human SNX1, SNX2, SNX4, SNX5 and SNX6 and the yeast Vps5p and YJL036W, all contain coiled-coil regions within their large C-terminal domains and are found distributed in both membrane and cytosolic fractions, typical of hydrophilic peripheral membrane proteins. Localization of the human SNX1 subgroup members in HeLa cells transfected with the full-length cDNA species revealed a similar intracellular distribution that in all cases overlapped substantially with the early endosome marker, early endosome autoantigen 1. The intracellular localization of deletion mutants and fusions with green fluorescent protein showed that the C-terminal regions of SNX1 and SNX5 are responsible for their endosomal localization. On the basis of these results, the functions of these SNX molecules are likely to be unique to endosomes, mediated in part by interactions with SNX-specific C-terminal sequences and membrane-associated determinants.

Amino Acid Sequence↗
PubMed2001

Steady-state localization of a medial-Golgi glycosyltransferase involves transit through the trans-Golgi network.

The steady-state localization of medial-Golgi enzymes is likely to involve retrograde transport pathways; however, the trafficking of these resident enzymes through the Golgi stack is unclear. To investigate if the medial-Golgi enzyme beta-1,2-N-acetylglucosaminyltransferase I (GlcNAc-TI) is transported to the late Golgi, a modified GlcNAc-TI bearing an N-glycan site on the C-terminus was constructed. The modified GlcNAc-TI was demonstrated to be functionally active in vivo, and was localized to the Golgi stack of transfected cells. In stable Chinese-hamster ovary (CHO) cell clones, the N-glycosylated GlcNAc-TI carried sialylated complex N-glycan chains. Pulse-chase studies showed that the majority of GlcNAc-TI was sialylated within 60 min of synthesis. Treatment of transfected CHO cells with Brefeldin A resulted in the glycosylated GlcNAc-TI bearing endo-beta-N-acetylglucosaminidase H resistant chains; however, the sialylation of glycosylated GlcNAc-TI was dramatically reduced. These data imply that, in CHO cells, newly synthesized GlcNAc-TI is transported rapidly through the Golgi stack to the trans-Golgi network, suggesting that GlcNAc-TI continuously recycles from the late Golgi. Furthermore, this data suggests that retrograde transport pathways play an important role in establishing the asymmetric distribution of GlcNAc-TI within the Golgi stack.

Amidohydrolases↗
PubMed2000

Medial Golgi but not late Golgi glycosyltransferases exist as high molecular weight complexes. Role of luminal domain in complex formation and localization.

To investigate the organization of Golgi glycosyltransferases and their mechanism of localization, we have compared the properties of a number of medial and late acting Golgi enzymes. The medial Golgi enzymes, N-acetylglucosaminyltransferase I and II (GnTI and GnTII) required high salt for solubilization and migrated as high molecular weight complexes on sucrose density gradients. In contrast, the late acting Golgi enzymes, beta1,4-galactosyltransferase and alpha1, 2-fucosyltransferase, were readily solubilized in low salt and migrated as monomers/dimers by sucrose density gradient centrifugation. Analysis of membrane-bound GnTI chimeras indicates that the formation of high molecular weight complexes does not require the transmembrane domain and cytoplasmic tail sequences of GnTI. Furthermore, a soluble form of GnTI, containing the stem region and catalytic domain, accumulated in the Golgi prior to secretion, in contrast to beta1,4-galactosyltransferase. Soluble GnTI, which also associated with high molecular weight complexes, was comparable with membrane-bound GnTI in its ability to glycosylate newly synthesized glycoproteins in vivo. Mutation of charged residues within the stem region of GnTI, known to be important for "kin recognition", had no effect on the efficiency of Golgi localization, the inclusion into high molecular weight complexes, nor functional activity in vivo. The differences in behavior between the medial and late acting Golgi enzymes may contribute to their differential localization and their ability to glycosylate efficiently in the correct Golgi subcompartment.

Animals↗
PubMed1995

Double-blind, placebo-controlled, dose-response trial of oral clodronate in patients with bone metastases.

PURPOSE: Despite evidence that clodronate inhibits tumor-induced osteolysis, no studies have directly assessed the optimal dose for long-term treatment. The aim of this double-blind, placebo-controlled study was to determine the safety and efficacy of different doses of clodronate in affected patients. PATIENTS AND METHODS: Eighty-four patients with tumor-induced osteolysis were randomized to receive treatment with placebo, or 400 mg, 1,600 mg, or 3,200 mg of clodronate, daily for 4 weeks. Patients were reviewed weekly during treatment. Fasting urinary calcium excretion was the primary variable used to assess response. Visual analog pain scores and adverse events were documented. RESULTS: In the clodronate-treated groups, there was a dose-dependent reduction in fasting calcium excretion with a highly significant difference between placebo and 1,600 mg clodronate (P = .0002) and placebo and 3,200 mg clodronate (P = .0001), but no significant difference between 1,600 mg and 3,200 mg clodronate. There was no discernible change in pain scores or analgesic requirements. Bone-derived isoenzyme alkaline phosphatase values increased in all groups, with a significant difference between baseline and final values in the 1,600-mg and 3,200-mg groups (P < .01 and P = .03, respectively). Adverse events were distributed evenly across the four treatment groups. Compliance was greater than 99% in all treatment groups. CONCLUSION: Oral clodronate at a dose of 1,600 mg or 3,200 mg will inhibit bone resorption. Since there was no significant difference between these two doses in terms of efficacy at 4 weeks, 1,600 mg/d can be recommended for long-term treatment. This dose is well tolerated and may promote bone repair, as judged by increases in bone alkaline phosphatase levels.

Administration, Oral↗