EXPRESSION STUDIES AND FUNCTIONAL CHARACTERIZATION OF RENAL HUMAN ORGANIC ANION TRANSPORTER 1 ISOFORMS

The human organic anion transporter 1 (hOAT1) facilitates the basolateral entry of organic anions such as endogenous metabolites, xenobiotics, and drugs into the proximal tubule cells. In the present study we investigated the general occurrence of hOAT1 isoforms in the kidneys and performed functional characterizations. Kidney specimens of 10 patients were analyzed by reverse transcription-polymerase chain reaction. We detected hOAT1-2 as the main transcript in almost all patients, and weak transcripts of hOAT1-1, hOAT1-3, and hOAT1-4 in many of them. An evaluation of the renal distribution showed all four mRNAs mostly restricted to the cortex. Western blot analysis of membrane fractions from two kidney specimens yielded two bands corresponding to the observed mRNA expression, suggesting hOAT1-3 and hOAT1-4 to be expressed on the protein level in vivo. This observation is further supported by immunofluorescence analyses of all four cloned hOAT1 isoforms transiently transfected in COS 7 cells. Functional characterizations did not show any transport activity of hOAT1-3 and hOAT1-4 for the tested substrates. Cotransfection studies of each of them with hOAT1-1 did not alter fluorescein uptake indicating no regulatory impact of these isoforms. Further functional comparisons of hOAT1-1 and hOAT1-2 in fluorescein uptake studies exhibited almost identical affinities for fluorescein with Michaelis constants of 11.6 +/- 3.7 microM (hOAT1-1) and 11.9 +/- 6.4 microM (hOAT1-2), and similar sensitivities to inhibition by p-aminohippurate [IC(50): 16 microM (hOAT1-1), 10 microM (hOAT1-2)], urate [IC(50): 440 microM (hOAT1-1), 385 microM (hOAT1-2)], and furosemide (IC(50): 14 microM (hOAT1-1), 20 microM (hOAT1-2)], implying functional equivalence.

Functional and selective targeting of adenovirus to high-affinity Fcgamma receptor I-positive cells by using a bispecific hybrid adapter

Adenovirus (Ad) efficiently delivers its DNA genome into a variety of cells and tissues, provided that these cells express appropriate receptors, including the coxsackie-adenovirus receptor (CAR), which binds to the terminal knob domain of the viral capsid protein fiber. To render CAR-negative cells susceptible to Ad infection, we have produced a bispecific hybrid adapter protein consisting of the amino-terminal extracellular domain of the human CAR protein (CARex) and the Fc region of the human immunoglobulin G1 protein, comprising the hinge and the CH2 and CH3 regions. CARex-Fc was purified from COS7 cell supernatants and mixed with Ad particles, thus blocking Ad infection of CAR-positive but Fc receptor-negative cells. The functionality of the CARex domain was further confirmed by successful immunization of mice with CARex-Fc followed by selection of a monoclonal anti-human CAR antibody (E1-1), which blocked Ad infection of CAR-positive cells. When mixed with Ad expressing eGFP, CARex-Fc mediated an up to 250-fold increase of transgene expression in CAR-negative human monocytic cell lines expressing the high-affinity Fcgamma receptor I (CD64) but not in cells expressing the low-affinity Fcgamma receptor II (CD32) or III (CD16). These results open new perspectives for Ad-mediated cancer cell vaccination, including the treatment of acute myeloid leukemia.

Genomic structure and in vivo expression of the human organic anion transporter 1 (hOAT1) gene

The human organic anion transporter 1 (hOAT1) plays a key role in the secretion of an array of potentially toxic organic anions including many clinically important drugs. Here we report on the genomic cloning of hOAT1. A human genomic library was used for screening of a PAC (P1 artificial chromosome) clone applying PCR techniques. Sequencing of several restriction subclones and of a PCR-generated clone revealed that the hOAT1 gene spans 8.2 kb and is composed of 10 exons divided by 9 introns. RT-PCR studies in a human kidney specimen led to the detection of two new splice variants, hOAT1-3 and hOAT1-4, showing a 132-bp in-frame deletion. Using fluorescence in situ hybridization (FISH) we mapped the hOAT1 gene as a single signal to chromosome 11q13.1-q13.2. Additionally, 600 bp of the 5' flanking region was analyzed, illustrating the probable transcription start site at nt -280, a NF-kappaB-site at nt -397 and several putative transcription factor binding sites.

Multiple regulatory steps are involved in the control of lipoprotein lipase activity in brown adipose tissue

Lipoprotein lipase (LPL) supplies brown adipose tissue with fatty acids for nonshivering thermogenesis. In brown adipose tissue of the Djungarian hamster we studied i) the molecular mechanisms involved in cold-induced stimulation of LPL activity, ii) the adrenergic control of LPL expression, and iii) compared LPL expression in brown and white adipose tissue. i) After 8 h cold exposure we detected a 2-fold increase in LPL activity and protein level in brown adipose tissue, whereas LPL mRNA level remained unchanged. A cold-induced increase (1.5-fold) in LPL activity was observed in brown adipose tissue of hamsters treated with actinomycin D prior to 4 h cold exposure, whereas cycloheximide treatment completely abolished LPL stimulation. Thus, these data suggest that during the initial phase (< 24 h) of cold exposure the stimulation of LPL activity in brown adipose tissue is most likely due to increased translation. In contrast, during prolonged cold exposure, we detected a maximal 7-fold increase in LPL activity and a 2- to 3-fold increase in LPL mRNA level in brown adipose tissue indicating LPL regulation at the pretranslational level. Furthermore, comparison of LPL protein and activity in brown adipose tissue during prolonged (> 24 h) cold exposure provides some evidence that the active fraction of the enzyme pool in brown adipose tissue is increased in response to cold. ii) Surgical denervation and noradrenaline treatment revealed a complex role of the sympathetic innervation in the control of LPL expression in brown adipose tissue. Denervation decreased LPL mRNA level, but increased LPL activity. Noradrenaline treatment stimulated LPL activity to a similar extent as cold exposure. However, cold-induced stimulation of LPL activity was not impaired by denervation. iii) Cold exposure significantly elevated LPL mRNA content of inguinal white adipose tissue, although LPL activity was not affected. Posttranscriptional mechanisms appear to be involved in the tissue specific control of LPL expression.

Inhibition of 5'-deiodination of thyroxine suppresses the cold-induced increase in brown adipose tissue messenger ribonucleic acid for mitochondrial uncoupling protein without influencing lipoprotein lipase activity

Young adult male and female Djungarian hamsters were exposed to ambient temperatures of 23 or 0 C for 12 h; half of the animals in each group were treated with iopanoic acid to suppress the peripheral conversion of T4 to the thermotropically active thyroid hormone T3 by the enzyme 5'-deiodinase (5'D). Brown adipose tissue (BAT) mRNA for uncoupling protein (UCP), BAT lipoprotein lipase (LPL) activity, and 5'D activity were measured at the conclusion of the study. A temperature of 0 C produced large rises in 5'D and LPL activities and a similar large increase in UCP mRNA within the 12-h exposure period. When 5'D activity was inhibited with iopanoic acid, mRNA for UCP was reduced, while LPL activity was unaffected. The results show that the optimal production of mRNA for BAT UCP depends on the availability of T3; however, T3 is not required for the cold-induced activation of LPL activity in BAT.