Phorbol esters augment spermidine transport without protein kinase C activation

Polyunsaturated fatty acids in the modulation of T-cell signalling

n-3 polyunsaturated fatty acids (PUFA) have been shown to modulate immune responses. These agents, being considered as adjuvant immunosuppressants, have been used in the treatment of various inflammatory and autoimmune diseases. However, the molecular mechanisms of action of n-3 PUFA-induced immunosuppressive effects are not well-understood. Since exogenous n-3 PUFA, under in vitro and in vivo conditions, are efficiently incorporated into T-cell plasma membranes, a number of recent studies have demonstrated that these agents may modulate T-cell signalling. In this review, the interactions of n-3 PUFA with the second messenger cascade initiated during early and late events of T-cell activation are discussed. We particularly focus on how these fatty acids can modulate the production of diacylglycerol and the activation of protein kinase C, mitogen activated protein kinase, calcium signalling and translocation of transcriptional factors, implicated in the regulation of gene transcription in T-cells.

Implication of acyl chain of diacylglycerols in activation of different isoforms of protein kinase C

We synthesized diacylglycerols (DAGs) containing omega-6 or omega-3 polyunsaturated fatty acids [i.e., 1-stearoyl-2-arachidonoyl-sn-glycerol (SAG), 1-stearoyl-2-docosahexaenoyl-sn-glycerol (SDG), and 1-stearoyl-2-eicosapentaenoyl-sn-glycerol (SEG)] and assessed their efficiency on activation of conventional (alpha, beta I, gamma) and novel (epsilon, delta) protein kinase C (PKC). SAG exerted significantly higher stimulatory effects than SDG and SEG on activation of PKC alpha and PKC delta. Activation of PKC beta I by SEG and SDG was higher than that by SAG. Activation of PKC gamma did not differ significantly among DAG molecular species. Addition of SAG to assays containing SEG and SDG exerted additive effects on activation of alpha and epsilon, but not on beta I and gamma, isoforms of PKC. SDG- and SEG-induced activation of PKC delta was significantly curtailed by the addition of SAG. Three DAG species significantly curtailed the PMA-induced activation of beta Iota, gamma, and delta, but not of alpha and epsilon, isoforms of PKC. Our study demonstrates for the first time that in vitro activation of different PKC isoenzymes vary in response to different DAG species, and one can envisage that this differential regulation may be responsible for their in vivo effects on target organs.

Transport and metabolism of polyamines in wild and multidrug resistant human leukemia (K 562) cells

Multidrug resistance (MDR) can be defined as the resistance of cancer cells not just to chemotherapeutic agents to which they have been exposed but also to other apparently unrelated compounds. This MDR phenotype is commonly associated with the high expression of levels of 170 kDa P-glycoprotein, encoded by MDR genes. In the present study, the uptake kinetics of polyamines and their biosynthesis were studied in wild and multidrug resistant (MDR) K 562 cells in culture. The rate (Vmax) of polyamine uptake was significantly lower in MDR cells than that in wild type cells, whereas the Km for the uptake was not significantly different in these cells, suggesting that polyamine transporter is not modified in MDR cells, though their different physiological state influences the uptake process. In a 32 h chase, the transported radioactive polyamines were gradually interconverted. [14C]putrescine was converted into [14C]spermidine following between 15 min and 32 h of culture, and into [14C]-spermine after 16 h of culture, in both the cell types; however, the levels of interconverted radioactive polyamines were always lower in MDR cells as compared with wild type cells. Similarly, internalized [14C]spermidine was converted into [14C]spermine, but not into [14C]putrescine in both the cells types. [14C]spermidine is metabolized into [14C]spermine after 4 h of culture in wild type cells, whereas in MDR cells the interconversion of [14C]spermidine into [14C]spermine is seen only after 16 h of culture. Blocking of the transmembrane drug efflux pump, expressed in the MDR cells, by preincubation in the presence of verapamil, did not influence the uptake of either of the two polyamines (putrescine and spermidine) by MDR cells. On the contrary, this kind of preincubation of wild type cells in the presence of verapamil significantly increased the uptake of these two polyamines. The levels of intracellular polyamine contents in MDR cells were always lower than those in the parental cell line. These results demonstrate that MDR cells are defective in both the uptake of polyamines and their biosynthesis as compared with wild type cells.

Polyamine transport regulation by calcium and calmodulin: role of Ca(2+)-ATPase

The study was conducted on human leukemia (K 562) cells to characterize the mechanisms implicated in the regulation of the polyamine spermidine (Spd) transport process. The antagonists of calmodulin, trifluoperazine (TFP), W-7 (N-[6-aminohexyl]-5-chloro-1-naphthelenesulfonamide), or mellitin inhibited significantly polyamine Spd uptake in these cells. The translocation of calmodulin towards plasma membrane and a concomitant decrease in its contents in cytosol were directly correlated with the time course increases similar to that of Spd uptake, indicating that calmodulin is recruited towards plasma membrane during the Spd transport process. Diminution of free intracellular calcium, (Ca2+)i, by preincubating the cells in BAPTA (bis[2-amino-5-methylphenoxyl]-ethane-N,N,N',N',-tetraacetate) buffer inhibited Spd transport significantly. Addition of lanthanum (LAN), a molecule known to inhibit Ca2+ efflux via Ca(2+)-ATPase, curtailed Spd uptake by these cells. LAN inhibited Vmax, but not the Km, of Spd uptake, indicating that the former does not directly interact with the polyamine transporter; rather it regulates the transport process, probably via its action on Ca(2+)-ATPase. Calmodulin-stimulated uptake of 45Ca2+ by inside-out vesicles of K 562 cells, a measure of Ca(2+)-ATPase activity. Furthermore, addition of LAN inhibited both basal and calmodulin-stimulated activity of Ca(2+)-ATPase. Thapsigargin (THAP), a molecule known to elevate (Ca2+)i due to its action on the endoplasmic reticulum, increased Spd transport whereas addition of LAN inhibited THAP-stimulated Spd transport activity. THAP increased free (Ca2+)i in these cells, and a pre-addition of LAN to these cells curtailed the THAP-stimulated increases of (Ca2+)i concentrations. Addition of Spd brought about elevations in (Ca2+)i contents. Caffeine also increased (Ca2+)i in these cells; however, it failed to stimulate significantly the Spd uptake process, indicating that (Ca2+)i which is involved in the regulation of polyamine transport pathways does not belong to the calcium-induced calcium-release (CICR) pool. Replacement of Ca2+ from the incubation medium (i.e., 0% Ca2+) resulted in higher uptake activity as compared to that in 100% Ca2+ medium, demonstrating that in 100% Ca2+ medium the calcium efflux process is quickly compensated by calcium refilling/influx from the extracellular medium, while in 0% Ca2+ medium there is perpetual efflux of (Ca2+)i which contributes to higher Spd uptake process. The results of this study suggest that an increase in free (Ca2+)i and its release from the cells via Ca(2+)-ATPase, and concomitant activation of calmodulin, which controls Ca(2+)-pump activity, are involved in the regulation of the Spd uptake process in human leukemia cells.

Phorbol esters augment polyamine transport by influencing Na(+)-K+ pump in murine leukemia cells

In this report, we elucidate the role of Na(+)-K+ pump in the regulation of polyamine spermidine (Spd) transport in murine leukemia (L 1210) cells in culture. Ouabain, known to bind extracellularly to the alpha-subunit of the Na(+)-K+ pump, inhibits the pump activity. The L 1210 cells were found to possess ouabain binding sites at 7.5 fmol/10(6) cells. Ouabain significantly inhibited the Spd uptake in a dose-dependent manner. The maximum inhibition of Spd uptake by ouabain was observed beyond 200 microM. Spd transport was inversely correlated with the [3H]ouabain binding to L 1210 cells: an increase in the saturation of ouabain binding to L 1210 cells resulted in a decrease of the Spd uptake process. Treatment of L 1210 cells with protein kinase C activator phorbol esters increased the Spd transport and, also, ouabain-sensitive 86Rb+ uptake, a measure of the activity of the Na(+)-K+ pump. H-7, a protein kinase C inhibitor, significantly inhibited the ouabain-sensitive 86Rb+ uptake by L 1210 cells. Phorbol esters stimulated the level, but not the rate, of 22Na+ influx. Addition of H-7 to L 1210 cells inhibited the 22Na+ influx process. A concomitant phorbol ester-induced increase in 22Na+ influx, [14C]Spd uptake, together with the functioning of Na(+)-K+ pump, indicates the role of the "Na+ cycle" in the regulation of the polyamine transport process.

Exogeneous diacylglycerols downregulate the activity of Na(+)-K+ pump in Xenopus laevis oocytes

In this study, cell permeable diacylglycerols, sn-1,2-dioctanoglycerol (DiC8), and sn-1-oleoyl-2-acetylglycerol (OAG) were found to downregulate the activity of Na(+)-K+ pump in Xenopus laevis oocytes. Both DiC8 and OAG decreased the binding of [3H]ouabain to intact oocytes while phorbol esters did not appreciably influence the same. These diacylglycerols inhibited the amiloride-sensitive 22Na+ influx and ouabain-sensitive 86Rb+ uptake in the oocytes. Furthermore, DiC8 prevented the 22Na+ efflux from the oocytes preloaded with 22Na+. Addition of H-7 to DiC8- and OAG-treated oocytes stimulated the pump activity curtailed by the two latters. The impairment of Na(+)-K+ pump activity by diacylglycerols suggests that protein kinase C activators may stimulate endocytosis of membrane-coupled Na(+)-K+ ATPase.