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Regulation o f LPL is complex and shoWs tissue specificity, eg. heart and adipose tissue respond differentially to the same physiologic and hormonal signals (Cryer 1981, Gcirfinkel and Schotz 1987). There are a number o f extrinsic factors that regulate LPL activity, eg. obesity, alcohol intake and hormones (reviewed in Taskinen 1987). Nutritional status was the first physiological regulator of LPL activity to be studied in detail, and seems to influence both LPL expression and synthesis. The distribution o f enzyme between adipose tissue and muscle appears to be dependent on nutritional status (Lithell et al. 1978). Postprandially, LPL activity is elevated in adipose tissue compared with heart and muscle tissue, resulting in the channelling of circulating fatty acids produced as a result o f TG hydrolysis into lipid deposits in adipose tissue. In the fasting state, the reverse situation occurs, with high LPL activity in heart and muscle tissues re-directing fatty acids towards these tissues, away from TG deposits in adipose tissue (Cryer 1981, Nilsson-Ehle 1982). M aintainence o f TG homeostasis requires the co-ordinated regulation of LPL activity in tissues that store fatty acids and tissues that oxidize fatty acids during feeding/fasting. The increased TG deposition in obesity may be due to overexpression of LPL activity in adipose tissue (Eckel 1987).

The decrease in LPL activity observed in fat pads of fasting guinea pigs exceeded the decrease in LPL synthesis and LPL mRNA levels (Semb & Olivecrona 1986, 1989), while glucose refeeding intiated a rapid increase in LPL activity with a smaller increase in LPL mRNA (Semb & Olivecrona 1986). Similar results were obtained in human adipose tissue after a carbohydrate-rich meal (Ong & Kern 1989). This suggests that changes in LPL activity upon fasting/feeding are regulated at both transcriptional and post-translational levels (Doolittle et al. 1990). Doolittle and coworkers observed that the reduction in LPL activity in adipocytes o f fasting rats occured without a concurrent change in total LPL mass. They

suggested a ’sliding* mass model in which LPL is redistributed from an active endothelial cell pool to an inactive pool in adipocytes, and in which redistribution in the fasting state proceeds by diverting LPL from a secretory pathway to an intracellular degradative pathway. Increased synthesis o f inactive LPL precursor would maintain total LPL mass at a constant level. This nutrition-state regulated balance between inactive (high mannose forms) LPL and active LPL does not occur in the cld/cld mouse, which exhibits lipase deficiency not due to mutations in the genes for LPL or HL. Brown adipocytes in these mice synthesize substantial amounts o f inactive LPL-protein, which is mostly in the high-mannose form and not secreted (Mashes et al. 1990, Davis et al. 1990). The cld mutation is thought to interfere with LPL and HL oligosaccharide processing, or with the transport o f enzyme from the rough ER to the Golgi-system, confirming the importance o f glycosylation on LPL generation and activity.

A number o f studies have consistently shown significant increases in LPL activity with exercise in human subjects (reviewed in Huttunen 1982). Significant increases in both muscle, adipose and post-heparin LPL activity resulted from endurance training in human subjects (reviewed in Nikkila 1987), with post-training LPL activities up to 10 times greater than basal LPL activities. Many studies however did not control for weight loss during exercise; weight loss independently causes an increase in adipose LPL activity (Kern et al. 1990). Recently, Simsolo et al. (1993) demonstrated a significantly increased ratio o f adipose tissue/muscle LPL activity (which may be an important indicator o f the tendency for storage o f circulating lipids in adipose tissue) following detraining o f athletes (short-term changes in exercise). This was due to a decrease in muscle LPL and an increase in adipose tissue LPL, both o f which occured through post-translational changes, yielding a condition favouring storage o f fatty acids in adipose tissue. The regulation o f LPL in adipose tissue and muscle has important implications for the disposal of TGs from lipoproteins. LPL is able to divert TGs to storage in fat or to oxidation in muscle, therefore conditions that tend to increase adipose LPL activity and/or decrease muscle LPL activity would result in a shunting o f circulating lipid towards storage in adipose and hence favour the development o f obesity (Simsolo et al. 1993). Exercise is believed to be essential for weight-maintenance after weight loss and this could be mediated via a decrease in the adipose/muscle LPL ratio, which would divert more TGs towards muscle for oxidation and less towards adipose for

storage (Simsolo et al. 1993).

Insulin is a major regulator of LPL activity (Pollare et al. 1991). Insulin deficiency is associated with low LPL activity which is increased by treatment with insulin, however the exact mechanism of insulin action in-vivo still awaits eludication. Hypertriglyceridaemia is a common lipid abnormality in patients with non-insulin dependent diabetes (Garg & Grundy 1990). Numerous studies have described decreases in LPL activity in diabetic individuals who are under poor glycaemic control, subsequent treatment elevates LPL activity with a fall in plasma TGs (reviewed in Simsolo et al. 1992). This increase in LPL activity was accompanied by an increase in LPL mass and synthesis but there was no change in LPL mRNA levels, suggesting that glycémie control of LPL occurs at a translational level or through a possible change in degradation (Simsolo et al. 1992). The effects o f insulin on LPL in adipose tissue in-vitro are well documented; insulin produces an increase in LPL- antigen on the surface o f adipocytes (Ailhaud 1990) and enhances the rate of LPL-release into the medium (Chan e ta l. 1988, Pradines Figueres et al. 1988). Insulin may act by increasing the half-life o f LPL mRNA, since it increases cellular LPL activity, rates of LPL synthesis and LPL mRNA levels in adipocytes without affecting the transcriptional rate (Speake et al. 1985, Raynolds et al. 1990). However posttranslational as well as posttranscriptional mechanisms are implicated by another study which showed that when cultured adipocytes were treated with insulin, LPL activity increased and LPL mass decreased, without a change in LPL mRNA levels (Semenkovich et al. 1989).

LPL displays basal hydrolysing activity, but the presence o f apoCII, a physiological activator o f LPL (La Rosa et al. 1970) significantly stimulates this activity; purified apoCII was found to stimulate LPL activity 5-fold (Faustinella et al. 1992). The requirement of apoCII for meiximal LPL activity probably prevents the expression o f lipase activity at its site of intracellular synthesis (Wang et al. 1992). There is apoCII enrichment o f TG-rich lipoproteins due to transfer from HDL to the surface o f chylomicrons and VLDLs soon after secretion o f lipoproteins into the plasma compartment (Havel et al. 1973). It has been suggested that the lipoprotein particle rich in both apoCII and TGs attaches to the endothelium via its interaction with LPL. Lipolysis may be terminated by the disappearance of apoCII from the TG-rich lipoprotein; this seems to coincide with the appearance o f

lysophosphatidylcholine in the lipoprotein particle and consequently with reduced affinity o f apoCII for the lipoprotein surface (Windier et al. 1986). ApoCII deficiency is clinically indistinguishable from LPL deficiency (Breckenridge et al. 1978). However heterozygotes for apoCII deficiency do not exhibit HTG (Cox et al. 1978). The HTG induced by apoCII deficiency can be relived by supplementation of about 1 0 % o f the normal plasma level o f apoCII (Breckenridge et al. 1978). These observations suggest that apoCII is not rate limiting for LPL-mediated lipolysis in the normal situation. The C-terminal tetrapeptide, Lys-Gly-Glu-Glu, o f apoCII has been implicated in mediating the initial ionic interaction between apoCII and LPL (Cheng et al. 1990). The N-terminus o f LPL has been suggested to be the location o f the apoCII binding domain (Davis et al. 1992). Recently the Lys 147- L ysl48 domain in LPL was implicated to be the ionic apoCII binding site; site-directed mutagenesis o f these two Lys residues to Ala produced a mutant LPL that showed a 35% reduction o f apoCII-activation compared to either wild type LPL or two other mutants in which two adjacent basic residues (at residues 279 + 280 and 445 + 446) were replaced by cdanine (Bruin et al. 1994a).

In individuals with hyperlipidaemia, apoCIII levels were found to be significantly postively correlated with the plasma LPL-inhibition activity and apoCIII was identified as one o f the most important plasma factors involved in the regulation of LPL activity (Wang et zil. 1985). M ore recently, studies showed that overexpression o f apoCIII was a direct cause o f the hypertriglyceridaemia observed in transgenic mice (Ito et al. 1990), further substantiating the suggestion o f a direct relationship between apoCIII levels and HTG and a modulating effect o f apoCIII on LPL activity. The concentration of apoCIII required for 50% inhibition o f LPL activity in-vitro decreased when apoCII was also present, suggesting that apoCII actually increased the affinity between LPL and apoCIII, without involving apoCIII competing for the activator site of apoCII (McConathy et al. 1992), consistent with previous observations that apoCIII is a non-competitive inhibitior o f LPL. ApoE has also been found to exhibit inhibitory activity (Wang et al. 1981, McConathy & Wang 1989), but its correlation with plasma LPL inhibitory activity may be masked by the stronger correlation between apoCIII and LPL activity (Wang et al. 1985). The requirem ent for high concentrations o f apoCIII and apoE to inhibit LPL (Wang et al. 1985) is likely to ensure the unhindered lipolysis o f TG-rich lipoproteins in the vascular com partment o f normolipidaemic

individuals and apoCIII and apoE will exert their effect only when plasma TG concentration is high (Wang et al. 1992). Since the increase o f TG is associated with increased levels o f apoC III and apoB, it is likely that these apolipoproteins are the signals and modulators responsible for the in-vivo inhibition o f LPL activity (Wang et al. 1992). A study on transgenic mice provided direct evidence that overexpression of apoCIII can be a primary cause o f hypertriglyceridaemia.

The binding o f LPL to the endothelium is thought to be weakened by local fatty acid accum ulation resulting from lipolysis of plasma triglycerides. This has been indicated in a study o f healthy men, in whom the lipolytic system has been overloaded by infusion o f triglyceride emulsions, resulting in a substantial increase in levels o f free fatty acids in the plasm a o f some o f the individuals, which correlated closely to a rise in plasma LPL activity (O livecrona & Bengtsson-Olivecrona 1990, Peterson et al 1990). Peterson et al. (1990) suggested that in situations when free fatty acids are generated more rapidly by LPL than they are used by the local tissue, these fatty acids cause dissociation o f the enzyme from its binding to endothelial heparan sulphate, releasing the LPL enzyme-lipoprotein complex into the circulation. Excess of free fatty acids released from TG-rich lipoproteins have therefore been proposed to exert feed-back control of lipolysis, due to dissociation of LPL from its endothelial binding sites. These suggestions were supported by experiments on cultured endothelial cells (Saxena et al. 1989). Significant positive correlations have been found between postprandial LPL response and free fatty acids derived directly from lipolysis of postprandial TG-rich lipoproteins, indicating that the magnitude of response was similar for LPL and postprandially derived fatty acids (Karpe et al. 1992). This is evidence for a simultaneous and possibly coordinated metabolism and supports the hypothesis of free fatty acid control o f endothelial LPL under physiological conditions in humans (rate o f lipolysis o f TG -rich lipoproteins can be partly regulated by accumulating free fatty acids that dissociate LPL from its endothelial binding sites). The synergistic effects o f apoCII and albumin allow the optimium activiation of LPL for the hydrolysis o f long-chain triacylglycerols (Wang et al. 1993), due to the combination o f apoCII-mediated activiation o f LPL (La Rosa et al. 1970) and the transfer o f fatty acids to albumin in plasma. The requirem ent for serum albumin in the LPL-catalyzed reaction is well documented (Wang et al. 1990) and the rate o f transfer of fatty acid to albumin has been shown to be much faster

than the chemical cleavage step o f catalysis (Foster & Berman 1981, Wang et al. 1990).