Kinetics of carnitine uptake by rat epididymal cells. Androgen-dependence and lack of stereospecificity

Exposure to bleomycin nanoaerosol does not induce fibrosis in mice

Bleomycin (BLM), a cytostatic drug widely used in the treatment of cancer, often induces lung fibrosis as a side effect. This phenomenon has been exploited in biomedical studies to create a fibrosis model. Inhalation of BLM nanoaerosol particles (NAPs) was expected to induce severe fibrosis in mice. Instead, inhalation of BLM NAPs for 13 days with a total inhaled dose of ~1 mg/kg did not cause pneumofibrosis in CD-1 mice; only marked pneumonitis with infiltration of macrophages in all parts of the lungs was observed in all eight mice exposed. These changes were notably reversible within 2 weeks of cessation of exposure. Despite the limited number of animals it was confidently concluded that slow administration of BLM in the form of NAPs did not cause fibrosis as the bolus instillation of similar doses does.

Novel localization of OCTN1, an organic cation/carnitine transporter, to mammalian mitochondria

Carnitine is a zwitterion essential for the beta-oxidation of fatty acids. We report novel localization of the organic cation/carnitine transporter, OCTN1, to mitochondria. We made GFP- and RFP-human OCTN1 cDNA constructs and showed expression of hOCTN1 in several transfected mammalian cell lines. Immunostaining of GFP-hOCTN1 transfected cells with different intracellular markers and confocal fluorescent microscopy demonstrated mitochondrial expression of OCTN1. There was striking co-localization of an RFP-hOCTN1 fusion protein and a mitochondrial-GFP marker construct in transfected MEF-3T3 and no co-localization of GFP-hOCTN1 in transfected human skin fibroblasts with other intracellular markers. L-[(3)H]Carnitine uptake in freshly isolated mitochondria of GFP-hOCTN1 transfected HepG2 demonstrated a K(m) of 422 microM and Western blot with an anti-GFP antibody identified the expected GFP-hOCTN1 fusion protein (90 kDa). We showed endogenous expression of native OCTN1 in HepG2 mitochondria with anti-GST-hOCTN1 antibody. Further, we definitively confirmed intact L-[(3)H]carnitine uptake (K(m) 1324 microM), solely attributable to OCTN1, in isolated mitochondria of mutant human skin fibroblasts having <1% of carnitine acylcarnitine translocase activity (alternate mitochondrial carnitine transporter). This mitochondrial localization was confirmed by TEM of murine heart incubated with highly specific rabbit anti-GST-hOCTN1 antibody and immunogold labeled goat anti-rabbit antibody. This suggests an important yet different role for OCTN1 from other OCTN family members in intracellular carnitine homeostasis.

Epitope shared by functional variant of organic cation/carnitine transporter, OCTN1, Campylobacter jejuni and Mycobacterium paratuberculosis may underlie susceptibility to Crohn’s disease at 5q31

Campylobacter jejuni and Mycobacterium paratuberculosis have been implicated in the pathogenesis of Crohn's disease. The presence of bacterial metabolites in the colonic lumen causing a specific breakdown of fatty acid oxidation in colonic epithelial cells has been suggested as an initiating event in inflammatory bowel disease (IBD). l-Carnitine is a small highly polar zwitterion that plays an essential role in fatty acid oxidation and ATP generation in intestinal bioenergetic metabolism. The organic cation/carnitine transporters, OCTN1 and OCTN2, function primarily in the transport of l-carnitine and elimination of cationic drugs in the intestine. High-resolution linkage disequilibrium mapping has identified a region of about 250kb in size at 5q31 (IBD5) encompassing the OCTN1 and -2 genes, to confer susceptibility to Crohn's disease. Recently, two variants in the OCTN1 and OCTN2 genes have been shown to form a haplotype which is associated with susceptibility to Crohn's. We show that OCTN1 and OCTN2 are strongly expressed in target areas for IBD such as ileum and colon. Further, we have now identified a nine amino acid epitope shared by this functional variant of OCTN1 (Leu503Phe) (which decreases the efficiency of carnitine transport), and by C. jejuni (9 aa) and M. paratuberculosis (6 aa). The prevalence of this variant of OCTN1 (Phe503:Leu503) is 3-fold lower in unaffected individuals of Jewish origin (1:3.44) compared to unaffected individuals of non-Jewish origin (1:1). We hypothesize that a specific antibody raised to this epitope during C. jejuni or M. paratuberculosis enterocolitis would cross-react with the intestinal epithelial cell functional variant of OCTN1, an already less efficient carnitine transporter, leading to an impairment of mitochondrial beta-oxidation which may then serve as an initiating event in IBD. This impairment of l-carnitine transport by OCTN1 may respond to high-dose l-carnitine therapy.

Carnitine/organic cation transporter OCTN2-mediated transport of carnitine in primary-cultured epididymal epithelial cells

Carnitine is essential for the acquisition of motility and maturation of spermatozoa in the epididymis, and is accumulated in epididymal fluid. In this study, carnitine transport into primary-cultured rat epididymal epithelial cells was characterized to clarify the nature of the transporter molecules involved. Uptake of carnitine by primary-cultured epididymal epithelial cells was time, Na+and concentration dependent. Kinetic analysis of carnitine uptake by the cells revealed the involvement of high- and low-affinity transport systems withKm values of 21 μM and 2.2 mM respectively. The uptake of carnitine by the cells was significantly reduced by inhibitors of carnitine/organic cation transporter (OCTN2), such as carnitine analogues and cationic compounds. In RT-PCR analysis, OCTN2 expression was detected. These results demonstrated that the high-affinity carnitine transporter OCTN2, which is localized at the basolateral membrane of epididymal epithelial cells, mediates carnitine supply into those cells from the systemic circulation as the first step of permeation from blood to spermatozoa.

OCTN3 is a mammalian peroxisomal membrane carnitine transporter

Carnitine is a zwitterion essential for the beta-oxidation of fatty acids. The role of the carnitine system is to maintain homeostasis in the acyl-CoA pools of the cell, keeping the acyl-CoA/CoA pool constant even under conditions of very high acyl-CoA turnover, thereby providing cells with a critical source of free CoA. Carnitine derivatives can be moved across intracellular barriers providing a shuttle mechanism between mitochondria, peroxisomes, and microsomes. We now demonstrate expression and colocalization of mOctn3, the intermediate-affinity carnitine transporter (Km 20 microM), and catalase in murine liver peroxisomes by TEM using immunogold labelled anti-mOctn3 and anti-catalase antibodies. We further demonstrate expression of hOCTN3 in control human cultured skin fibroblasts both by Western blotting and immunostaining analysis using our specific anti-mOctn3 antibody. In contrast with two peroxisomal biogenesis disorders, we show reduced expression of hOCTN3 in human PEX 1 deficient Zellweger fibroblasts in which the uptake of peroxisomal matrix enzymes is impaired but the biosynthesis of peroxisomal membrane proteins is normal, versus a complete absence of hOCTN3 in human PEX 19 deficient Zellweger fibroblasts in which both the uptake of peroxisomal matrix enzymes as well as peroxisomal membranes are deficient. This supports the localization of hOCTN3 to the peroxisomal membrane. Given the impermeability of the peroxisomal membrane and the key role of carnitine in the transport of different chain-shortened products out of peroxisomes, there appears to be a critical need for the intermediate-affinity carnitine/organic cation transporter, OCTN3, on peroxisomal membranes now shown to be expressed in both human and murine peroxisomes. This Octn3 localization is in keeping with the essential role of carnitine in peroxisomal lipid metabolism.

Characterization of organic cation/carnitine transporter family in human sperm

Spermatozoan maturation, motility, and fertility are, in part, dependent upon the progressive increase in epididymal and spermatozoal carnitine, critical for mitochondrial fatty acid oxidation, as sperm pass from the caput to the cauda of the epididymis. We demonstrate that the organic cation/carnitine transporters, OCTN1, OCTN2, and OCTN3, are expressed in sperm as three distinct proteins with an expected molecular mass of 63 kDa, using Western blot analysis and our transporter-specific antibodies. Carnitine uptake studies in normal control human sperm samples further support the presence of high-affinity (OCTN2) carnitine uptake (K(m) of 3.39+/-1.16 microM; V(max) of 0.23+/-0.14 pmol/min/mg sperm protein; and mean+/-SD; n=12), intermediate-affinity (OCTN3) carnitine uptake (K(m) of 25.9+/-14.7 microM; V(max) of 1.49+/-1.03 pmol/min/mg protein; n=26), and low-affinity (OCTN1) carnitine uptake (K(m) of 412.6+/-191 microM; V(max) of 32.7+/-20.5 pmol/min/mg protein; n=18). Identification of individuals with defective sperm carnitine transport may provide potentially treatable etiologies of male infertility, responsive to L-carnitine supplementation.

A third human carnitine/organic cation transporter (OCTN3) as a candidate for the 5q31 Crohn's disease locus (IBD5)

Organic cation transporters function primarily in the elimination of cationic drugs in kidney, intestine, and liver. The murine organic cation/carnitine (Octn) transporter family, Octn1, Octn2, and Octn3 is clustered on mouse chromosome 11 (NCBI Accession No. NW_000039). The human OCTN1 and OCTN2 orthologs map to the syntenic IBD5 locus at 5q31, which has been shown to confer susceptibility to Crohn's disease. We show that the human OCTN3 protein, whose corresponding gene is not yet cloned or annotated in the human reference DNA sequence, does indeed exist and is uniquely involved in carnitine-dependent transport in peroxisomes. Its functional properties and inferred chromosomal location implicate it for involvement in Crohn's disease.

Gsalpha-mediated regulation of the carnitine carrier in S49 lymphoma cells

Carnitine is essential for mitochondrial oxidation of long-chain fatty acids. Peripheral cells rely on plasma transport of carnitine which is taken up by an active mechanism in the plasma membrane. This project investigated the plasma membrane bound carnitine carrier in cultured S49 lymphoma cells. We investigated wild-type cells and two mutant cells lines showing deficient activity of adenylyl cyclase, cyc- lacking and H21a containing a deficient Gsalpha. Plasma membranes derived from cyc- cells displayed six times more carnitine binding sites and a 1.35 times faster uptake rate than plasma membranes from wild-type cells. In vitro mixing of plasma membranes from cyc- and wild-type cells transferred a factor reducing the number of expected carnitine binding sites by about 30%. Cyclic AMP could not substitute for wild-type membranes as the inhibitor of carnitine binding to plasma membranes derived from cyc- cells. Cholera toxin induced ADP-ribosylation of Gsalpha causing activation of Gsalpha present in wild-type but not in cyc- cells, further reducing carnitine uptake and carnitine binding to plasma membranes. Our findings thus supported the notion that Gsalpha by a mechanism not involving cyclic AMP inhibited cellular uptake of carnitine by reducing the number of available carnitine binding sites in plasma membranes.

Epididymal epithelium: its contribution to the formation of a luminal fluid microenvironment

To understand the process of sperm maturation, an understanding of interactions between the spermatozoa with the luminal fluid microenvironment and with the epididymal epithelium is necessary. The composition of epididymal luminal fluid of several species is well documented but the manner by which the epididymis contributes to the formation of this specialized milieu is not so well understood. A major role played by the epididymis is to finely regulate the movement of molecules into and out of the lumen. This ensures that as spermatozoa progress along the duct they are exposed to a continually changing, but optimal environment necessary for their maturation and survival. This review focusses on our current understanding of the contributions of the epididymal epithelium to the formation of a specialized luminal fluid microenvironment. The role of the blood-epididymis barrier, the composition of the epididymal luminal fluid, the permeability properties of the epididymal epithelium, and recent studies on a number of luminal fluid proteins and expression of the genes which encode these proteins are discussed.

Stimulation of gonadotropin-releasing hormone secretion by acetyl-L-carnitine in hypothalamic neurons and GT1 neuronal cells

Pulsatile gonadotropin-releasing hormone (GnRH) secretion from perifused hypothalamic cells and GT1-1 neuronal cells was significantly increased after culture in medium containing 100 microM acetyl-L-carnitine (ALC). This action of ALC was largely due to an increase in the spike amplitude of GnRH release. In addition, the receptor-mediated release of GnRH by N-methyl-D-aspartic acid and endothelin was significantly increased in perifused cells cultured in ALC-enriched medium. Stimulatory effects of ALC on basal, high K(+)- and agonist-induced GnRH release were also observed during long-term culture of primary hypothalamic neurons. Similar effects of ALC were evident in cultured GT1-1 cells and were accompanied by a significant increase in cell number. These observations in normal and transformed GnRH neurons demonstrate that ALC promotes the growth and secretory activity of neuropeptide-producing cells of the hypothalamus.

Enhanced uptake of carnitine by perfused rat liver following starvation

Previously, the release of carnitine from the perfused rat liver was found to be protein-mediated, dependent on the nutritional state but not on metabolic energy. Further, it was shown to exceed the physiological demand by about 10-fold (Sandor et al. (1985) Biochim. Biophys. Acta 835, 83-91). In the present study the uptake of carnitine by perfused rat liver has been investigated. The liver tissue and the perfusate were in equilibrium when the carnitine concentration in the perfusate was close to 45 microM, physiological in the rat plasma. Under this condition, when no net carnitine transport occurred, an unidirectional uptake of L-[3H]carnitine was observed. Quantitatively, the uptake rate was 355 +/- 60 (S.D.) nmol/h per 100 g body weight at 45-50 microM perfusate concentration. This uptake capacity balances the previously reported excessive release (Sandor et al., op. cit.). On this basis we propose that a futile release/uptake cycle operates in carnitine transport across the liver cell membrane. Liverse of 24-h starved rats took up L-[3H]carnitine at 56% higher rate from the perfusate (75 microM) than livers of fed rats. Kinetic analysis revealed that fasting caused a decrease in Km value from 4.22 mM to 2.59 mM, whereas Vmax remained practically unchanged, average 0.95 mumol/min per 100 g body weight. D-[3H]Carnitine was transported at the same rate as L-carnitine and underwent the effect of fasting as well. The uptake was partially inhibited by 1 mM 2,4-dinitrophenol and 5 mM KCN, showing its dependency on metabolic energy. If Li+ replaced Na+ a strong inhibitory effect (to 20% of control) was observed, which suggests a co-transport of carnitine with Na+. Mersalyl, an SH reagent, had no effect on the uptake, whereas it practically abolished the release of carnitine from the perfused livers. This observation suggests that the inward and outward transport of carnitine are mediated by two different proteins.

Characteristics of the transport of carnitine into the cauda epididymidis of the rat as ascertained by luminal perfusion in vitro

An in vitro luminal perfusion technique was used to monitor the transepithelial movement of [3H]L-carnitine into the cauda epididymal lumen of rats. Movement of the volume marker [14C]inulin into the lumen was limited while the transport of [3H]L-carnitine was saturable and was dependent on respiratory energy. The transport of L-carnitine (50 microM) was significantly inhibited by 500 microM D-carnitine, palmitoyl L-carnitine and deoxycarnitine but not by trimethylammonium iodide, trimethyl phosphate, acetyl-L-carnitine, choline, betaine, gamma-aminobutyric acid or DL-carnitinamide. The Kt (116 microM) and Vmax (0.1 nmol/30 min/cm) for the carriers were calculated from the present data and as the Kt is about twice the normal plasma concentration of carnitine, the rate of transport by the pump is presumed to be limited by the plasma concentration.

Selective luminal absorption of L-carnitine from the proximal regions of the rat epididymis. Possible relationships to development of sperm motility

The absorption of L-carnitine from the duct of the proximal regions of the rat epididymis was investigated using a stopped-flow, split-droplet microperfusion technique. L-carnitine was absorbed from the duct of the proximal caput epididymidis by a time-dependent and saturable transport system (Km = 25 micron; Vmax = 0.65 pmoles absorbed/min/mm3 tubular volume). Furthermore, absorption appeared to be primarily sodium-independent, although the existence of a minor sodium-dependent pathway cannot be ruled out. A similar transport system was not evident along the distal caput epididymidis, where absorption of L-carnitine was attributable to passive diffusion only. The inward and outward movement of L-carnitine across the epithelium of the proximal and distal caput epididymidis appears to be regulated so that the spermatozoa come into contact with high levels of L-carnitine in the distal caput region.

Release of carnitine from the perfused rat liver

Perfused rat liver was shown to be the proper model for studies on hepatic cellular transport of carnitine. During recirculating perfusion the livers kept equilibrium with 45 nmol/ml total carnitine in perfusate, exhibited concentrative uptake and there was no sign of artificial leakage. The release side of the carnitine transport was characterized by utilizing outflow perfusions. The livers from fed rats exported daily 9.93 mumol per 100 g body weight total carnitine. This release rate is 4- or 10-fold higher than the estimated daily turnover in vivo or the measured urinary excretion. Therefore, the major part of the released carnitine has to re-enter the liver. The outward carnitine transport does not depend on energy or the Na+-K+ pump, since it did not respond to metabolic poisons and ouabain. However, the release rate was strongly inhibited by mersalyl and showed saturability in function of tissue carnitine levels. The Vmax of the saturable outward transport system was 2.47 nmol . min-1 . g-1 liver, the apparent Km was 0.27 mM tissue level (both as compared to total carnitine). These data showed the outward transport of carnitine from the liver to be protein mediated. The contribution of a diffusion (nonsaturable) component was estimated to be 20-25% in the range of tissue levels occurring in vivo. The rate of carnitine release from the liver decreased as an effect of 24 h starvation from the daily 9.92 mumol release to 6.55 mumol on 100 g body weight basis. This decrease is more pronounced when the release rates are expressed on the basis of tissue carnitine levels. The resulting value can be called rate constant (at the linear part of the saturation curve, Fig. 5) and it decreased to 5.00 min-1 from 8.41 min-1 as an effect of starvation. We have concluded that the altered parameters of carnitine transport across the liver cell is decisive in developing the higher hepatic carnitine concentration in the fasted state.

Absorption of D- and L-carnitine by the intestine and kidney tubule in the rat

The process by which L- and D-carnitine are absorbed was investigated using the live rat and the isolated vascularly perfused intestine. A lumenal dose of 2-6 nmol in the perfused intestine resulted in less than 5% transport of either isomer to the perfusate in 30 min. The L-isomer was taken up by the intestinal tissue about twice as rapidly as the D-isomer by both the perfused intestine (52.8% and 21.6%, respectively) and the live animal (80% and 50%, respectively) in 30 min. After 1 h 90% of the L-carnitine had accumulated in the intestinal tissue and was released to the circulation over the next several hours. Accumulation of D-carnitine reached a maximum of 80% in 2 h and release to the circulations was similar to that of L-carnitine. Uptake of both L-[14C]carnitine and acetyl-L-[14C]carnitine was more rapid in the upper jejunal segment than in other portions of the small intestine. Acetylation occurred in all segments, resulting in nearly 50% conversion to this derivative in 5 min. Increasing the dose of L-carnitine reduced the percent acetylation. The uptake of both isomers was a saturable process and high concentrations of D-carnitine, acetyl-L-carnitine and trimethylaminobutyrate inhibited L-carnitine uptake. In the live animal after 5 h, the distribution of isotope from L-[14C]carnitine and D-[3H]carnitine differed primarily in the muscle where 29.5% of the L-carnitine and 5.3% of the D-carnitine was found and in the urine where 2.9% of the L-carnitine and 7.1% of the D-carnitine was found. The renal threshold for L-carnitine was 80 microM and for D-carnitine 30 microM, in the isolated perfused kidney. Approx. 40% of the L-carnitine but none of the D-carnitine excreted in the urine was acetylated. L-Carnitine and D-carnitine competed for tubular reabsorption.

Purification and properties of carnitine acetyltransferases from bovine spermatozoa and heart

To investigate the physical and kinetic properties of sperm carnitine acetyltransferase, the enzyme was purified from bovine spermatozoa and heart muscle. Carnitine acetyltransferase was purified 580-fold from ejaculated bovine spermatozoa to a specific activity of 85 units/mg protein (95% homogeneity). Sperm carnitine acetyltransferase was characterized as a single polypeptide of Mr 62,000 and pI 8.2. Heart carnitine acetyltransferase was purified 650-fold by the same procedure to a final specific activity of 71 units/mg protein. The kinetic properties of purified bovine sperm carnitine acetyltransferase were consistent with the proposed function of this enzyme in acetylcarnitine pool formation. Product inhibition by either acetyl-L-carnitine or CoASH was not sufficient to predict significant in vivo inhibition of acetyl transfer. At high concentrations of L-carnitine, bovine sperm and heart carnitine acetyltransferases were most active with propionyl- and butyryl-CoA substrates, although octanoyl-, iso-butyryl-, and iso-valeryl-CoA were acceptable substrates. Binding of one substrate was enhanced by the presence of the second substrate. Carnitine analogs that have significance in reproduction, such as phosphorylcholine and taurine, did not inhibit carnitine acetyltransferase. Bovine sperm and heart carnitine acetyltransferases were indistinguishable on the basis of purification behavior, pI, pH optima, kinetic properties, acyl-CoA specificity, and sensitivity to sulfhydryl reagents and divalent cations; thus there was no indication that bovine sperm carnitine acetyltransferase is a sperm-specific isozyme.