Maturation and secretion of lipoprotein lipase in cultured adipose cells. I. Intracellular activation of the enzyme

In vitro and ex vivo models of adipocytes

Adipocytes are specialized cells with pleiotropic roles in physiology and pathology. Several types of fat cells with distinct metabolic properties coexist in various anatomically defined fat depots in mammals. White, beige, and brown adipocytes differ in their handling of lipids and thermogenic capacity, promoting differences in size and morphology. Moreover, adipocytes release lipids and proteins with paracrine and endocrine functions. The intrinsic properties of adipocytes pose specific challenges in culture. Mature adipocytes float in suspension culture due to high triacylglycerol content and are fragile. Moreover, a fully differentiated state, notably acquirement of the unilocular lipid droplet of white adipocyte, has so far not been reached in two-dimensional culture. Cultures of mouse and human-differentiated preadipocyte cell lines and primary cells have been established to mimic white, beige, and brown adipocytes. Here, we survey various models of differentiated preadipocyte cells and primary mature adipocyte survival describing main characteristics, culture conditions, advantages, and limitations. An important development is the advent of three-dimensional culture, notably of adipose spheroids that recapitulate in vivo adipocyte function and morphology in fat depots. Challenges for the future include isolation and culture of adipose-derived stem cells from different anatomic location in animal models and humans differing in sex, age, fat mass, and pathophysiological conditions. Further understanding of fat cell physiology and dysfunction will be achieved through genetic manipulation, notably CRISPR-mediated gene editing. Capturing adipocyte heterogeneity at the single-cell level within a single fat depot will be key to understanding diversities in cardiometabolic parameters among lean and obese individuals.

Calreticulin promotes folding/dimerization of human lipoprotein lipase expressed in insect cells (sf21)

Lipoprotein lipase (LPL) is a non-covalent, homodimeric, N-glycosylated enzyme important for metabolism of blood lipids. LPL is regulated by yet unknown post-translational events affecting the levels of active dimers. On co-expression of LPL with human molecular chaperones, we found that calreticulin had the most pronounced effects on LPL activity, but calnexin was also effective. Calreticulin caused a 9-fold increase in active LPL, amounting to about 50% of the expressed LPL protein. The total expression of LPL protein was increased less than 20%, and the secretion rates for active and inactive LPL were not significantly changed by the chaperone. Thus, the main effect was an increased specific activity of LPL both in cells and media. Chromatography on heparin-Sepharose and sucrose density gradient centrifugation demonstrated that most of the inactive LPL was monomeric and that calreticulin promoted formation of active dimers. Higher oligomers of inactive LPL were present in cell extracts, but only monomers and dimers were secreted to the medium. Interaction between LPL and calreticulin was demonstrated, and the effect of the chaperone was prevented by castanospermine, an inhibitor of N-glycan glucose trimming. Our data indicate an important role of endoplasmic reticulum-based chaperones for the folding/dimerization of LPL.

11Beta-hydroxysteroid dehydrogenase type 1 in differentiating omental human preadipocytes: from de-activation to generation of cortisol

In humans, glucocorticoids are important regulators of adipose tissue distribution and function but circulating cortisol concentrations are normal in most patients with obesity. However, intracellular glucocorticoid levels can be modified by a microsomal enzyme 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) expressed mainly in the liver and adipose tissue. Locally generated cortisol within human adipose tissue can induce preadipocyte differentiation, but the relationship between 11beta-HSD1 expression and adipogenesis is unknown. Our present study has shown that in intact, undifferentiated omental (OM) but not subcutaneous (SC) preadipocytes, 11beta-HSD1 acts primarily as a dehydrogenase inactivating cortisol to cortisone. When preadipocytes become "committed" to adipocyte differentiation, oxo-reductase activity predominates generating cortisol. Since glucocorticoids are not only essential for OM preadipocyte differentiation but also inhibit cell proliferation, we postulate that 11beta-HSD1 dehydrogenase activity in "uncommitted" OM preadipocytes may provide an autocrine mechanism to protect preadipocytes from differentiation, in turn facilitating their proliferation. Once early differentiation is initiated, a "switch" to 11beta-HSD1 oxo-reductase activity generates cortisol, thus promoting adipogenesis. The differences in set-point of 11beta-HSD1 activity between OM and SC human adipose tissue may be an important factor in the pathogenesis of visceral obesity.

Maturation of lipoprotein lipase in the endoplasmic reticulum. Concurrent formation of functional dimers and inactive aggregates

The maturation of lipoprotein lipase (LPL) into a catalytically active enzyme was believed to occur only after its transport from the endoplasmic reticulum (ER) to the Golgi apparatus. To test this hypothesis, LPL located in these two subcellular compartments was separated and compared. Heparin affinity chromatography resolved low affinity, inactive LPL displaying ER characteristics from a high affinity, active fraction exhibiting both ER and Golgi forms. The latter forms were further separated by beta-ricin chromatography and were found to have comparable activities per unit of LPL mass. Thus, LPL must reach a functional conformation in the ER. Active LPL, regardless of its cellular location, exhibited the expected dimer conformation. However, inactive LPL, found only in the ER, was highly aggregated. Kinetic analysis indicated a concurrent formation of LPL dimer and aggregate and indicated that the two forms have dissimilar fates. Whereas the dimer remained stable even when confined to the ER, the aggregate was degraded. Degradation rates were not affected by proteasomal or lysosomal inhibitors but were markedly reduced by ATP depletion. Lowering the redox potential in the ER by dithiothreitol caused the dimer to associate with calnexin, BiP, and protein-disulfide isomerase to form large, inactive complexes; dithiothreitol removal induced complex dissociation with restoration of the functional LPL dimer. In contrast, the LPL aggregate was only poorly associated with ER chaperones, appearing to be trapped in an irreversible, inactive conformation destined for ER degradation.

A switch in dehydrogenase to reductase activity of 11 beta-hydroxysteroid dehydrogenase type 1 upon differentiation of human omental adipose stromal cells

As exemplified in patients with Cushing's syndrome, glucocorticoids play an important role in regulating adipose tissue distribution and function, but circulating cortisol concentrations are normal in most patients with obesity. However, human omental adipose stromal cells (ASCs) can generate glucocorticoid locally through the expression of the enzyme 11 beta-hydroxysteroid dehydrogenase (11 beta-HSD) type 1 (11 beta-HSD1), which, in intact cells, has been considered to be an oxoreductase, converting inactive cortisone (E) to cortisol (F). Locally produced F can induce ASC differentiation, but the relationship between 11 beta-HSD1 expression and adipocyte differentiation is unknown. Primary cultures of paired omental (om) and sc ASC and adipocytes were prepared from 17 patients undergoing elective abdominal surgery and cultured for up to 14 d. Expression and activity of 11 beta-HSD isozymes were analyzed together with early (lipoprotein lipase) and terminal (glycerol 3 phosphate dehydrogenase) markers of adipocyte differentiation. On d 1 of culture, 11 beta-HSD1 activity in intact om ASCs exceeded oxoreductase activity in every patient (78.9 +/- 24.9 vs. 15.8 +/- 3.7 [mean +/- SE] pmol/mg per hour, P < 0.001), and in sc ASCs, relative activities were similar (40.6 +/- 12.2 vs. 36.9 +/- 8.8). Conversely, in freshly isolated om adipocytes, reductase activity exceeded dehydrogenase activity (23.6 +/- 1.5 vs. 6.2 +/- 0.8 pmol/mg per hour, P < 0.01). Following 14 d of culture in serum-free conditions with addition of 10 nM insulin (Ctr) or insulin with 100 nM F (+F), lipoprotein lipase/18S RNA levels increased in both the Ctr- and +F-treated ASCs, but glycerol 3 phosphate dehydrogenase increased only in the +F cultures. In both cases, however, 11 beta-HSD1 oxoreductase activity exceeded dehydrogenase activity (Ctr: 53.3 +/- 9.0 vs. 32.4 +/- 10.5, P < 0.05; +F: 65.6 +/- 15.6 vs. 37.1 +/- 11.5 pmol/mg per hour, P < 0.05), despite no significant changes in 11 beta-HSD1 mRNA levels. In sc ASCs, dehydrogenase activity was similar to reductase activity in both Ctr- and +F-treated cells. Type 2 11 beta-HSD expression was undetectable in each case. These data show that in intact, undifferentiated om ASCs, 11 beta-HSD1 acts primarily as a dehydrogenase, but in mature adipocytes oxoreductase activity predominates. Because glucocorticoids inhibit cell proliferation, we postulate that 11 beta-HSD1 activity in uncommitted ASCs may facilitate proliferation rather than differentiation. Once early differentiation is initiated, a "switch" to 11 beta-HSD1 oxoreductase activity generates F, thus promoting adipogenesis. Site-specific regulation of the set-point of 11 beta-HSD1 activity may be an important mechanism underpinning visceral obesity.

Synthesis of active high mannose-type lipoprotein lipase in human adipose tissues

Lipoprotein lipase (LPL) activity in the retroperitoneal adipose tissue of a patient with Cushing's syndrome and in the subcutaneous adipose tissue of a patient with aseptic necrosis of the femoral head was higher than that in the corresponding tissues of the control subjects. The amount of [35S]methionine incorporated into LPL was also higher in these patients than in control subjects. However, the ratio of activity and amount of radioactivity in the LPL of patients was identical to that of control subjects, indicating that LPL synthesized in the adipose tissues of patients had a normal specific activity. LPL with Mr = 57000 was composed of two types of subunits: one type was partially endo H-sensitive, yielding a product with Mr = 55000, and the other was totally endo H-sensitive, yielding a product with Mr = 52000. Both retroperitoneal and subcutaneous adipose tissues of control subjects contained nearly equal amounts of partially and totally endo H-sensitive subunits. In the retroperitoneal adipose tissue of a patient with Cushing's syndrome, 8% of subunits were partially endo H-sensitive and 92% were totally endo H-sensitive. In the subcutaneous adipose tissue of a patient with aseptic necrosis of the femoral head, 21% of subunits were partially endo H-sensitive and 79% were totally endo H-sensitive. The 24-h treatment of subcutaneous adipose tissue of a control subject with 1 mM 1-deoxymannojirimycin (dMM) caused the synthesis of active, but totally endo H-sensitive, LPL. Thus, in human adipose tissue, the processing of one oligosaccharide chain of an LPL subunit to a complex type chain in the trans Golgi was not necessary for the expression of activity.

Proliferation and differentiation of stromal-vascular cells in primary culture differ between neonatal pigs consuming maternal or formula milk

Proliferation and differentiation of preadipocytes from 7-d-old pigs consuming maternal or formula milk were examined in primary culture of stromal-vascular (s-v) cells derived from subcutaneous adipose tissue. Unsuckled pigs were bottle-fed isoenergetically with colostrum and then sow's milk (SM) or with formula milk alone (F) from birth to 7 d. Isolated cells were exposed to serum-supplemented medium and serum-free medium to determine proliferation and differentiation, respectively. Proliferation estimated between d 3 and 4 of culture was higher (P<0.05) in cells from F than SM pigs. In addition, the number of s-v cells isolated from 1 g of adipose tissue was higher (P<0.01) in F than SM pigs. Variables assessing differentiation were also affected. The percentage of differentiating cells and lipoprotein lipase (LPL) activity were lower (P<0.05) in F than SM pigs, whereas malic enzyme (ME) activity did not differ significantly between the two groups. In conclusion, formula milk increased the number of s-v cells and their capacity for proliferation, whereas the potential for cell differentiation was lower compared with cells from the maternal milk group. Further studies are required to identify the growth and/or nutritional factors that are implicated in the observed differences and to determine whether subsequent development of adipose tissue is affected.

The effect of GH and IGF-I on preadipocytes from Large White and Meishan pigs in primary culture

Proliferation and differentiation of preadipocytes from 7-day-old Large White (LW) and Meishan (MS) pigs were studied in primary culture. The effects of porcine GH (pGH) and IGF-I as well as the expression of GH (GHR) and IGF-I (IGF-IR) receptors mRNA were examined. Preadipocytes were exposed to serum-supplemented and serum-free medium to determine proliferation and differentiation, respectively. Proliferation was higher in MS than in LW pigs. Treatment with pGH (2 nM) or IGF-I (10 nM) resulted in a similar decrease in proliferation in LW and MS pigs. Parameters assessing differentiation and the effects of pGH and IGF-I on differentiation did not differ between the two breeds. The percentage of differentiating cells and LPL and ME activities were markedly reduced by pGH. IGF-I did not reduce differentiation significantly. Both GHR and IGF-IR mRNA were expressed in adipose tissue, adipocytes, preadipocytes, and 6-day-cultured cells from LW and MS pigs. The similar action of pGH and IGF-I on preadipocyte proliferation and differentiation, associated with the similar expression of GHR and IGF-IR mRNA in LW and MS pigs, suggests that the GH/IGF-I axis is not impaired in MS pigs. The difference in preadipocyte proliferation observed between LW and MS pigs could account for their adiposity difference.

Heparin-releasable lipoprotein lipase activity is increased in cardiomyocytes after culture

The activity of lipoprotein lipase (LPL) in adult rat heart cardiomyocytes after overnight culture on laminin-coated plates for 18-22 h was compared with enzyme activity in freshly isolated cardiomyocytes. LPL activity in cellular homogenates from cultured cardiomyocytes and freshly isolated cells was 240 and 233 nmol oleate released ·h-1·mg-1protein, respectively. LPL specific activity (mU/ng LPL protein) was 0.07 in cultured cells compared with 0.42 in freshly isolated cells, indicating an increased content of inactive LPL mass after overnight culture. The heparin-induced release of LPL activity into the medium of cultured cardiomyocytes (198 nmol ·h-1·mg-1) was much greater than heparin-releasable LPL (HR-LPL) activity (59 nmol ·h-1·mg-1) from freshly isolated cells. HR-LPL activity from cultured cardiomyocytes was dependent on serum (16.3-fold activation) and was inhibited by high ionic strength (1 M NaCl) and by a polyclonal antibody to LPL. Cultured cardiomyocytes also had more immunodetectable LPL on the cell surface compared with freshly isolated cardiomyocytes, consistent with increased HR-LPL activity. Therefore, overnight culture may permit cardiomyocytes time to recover from the stress of isolation by increasing the content of LPL on the cell surface.Key words: lipoprotein lipase, cardiac myocytes.

Effect of carbonyl cyanide m-chlorophenylhydrazone (CCCP) on the dimerization of lipoprotein lipase

Lipoprotein lipase (LPL), an enzyme playing the central role in triglyceride metabolism, is a glycoprotein and a homodimer of identical subunits. Dimerization and proper processing of oligosaccharide chains are important maturation steps in post-translational regulation of enzyme activity. Indirect evidences suggest that dimerization of LPL occurs in endoplasmic reticulum (ER) or Golgi. In this study, we investigated the dimerization status of LPL in 3T3-L1 adipocytes, using sucrose density gradient ultracentrifugation and carbonyl cyanide m-chlorophenylhydrazone (CCCP), an inhibitor of ER-Golgi protein transport. In the presence of CCCP, no increase of cellular LPL activity was detected during 2 b of recovery period after the depletion of LPL, with heparin and cycloheximide. Only endoglycosidase H (endo H)-sensitive subunits were found in CCCP-treated cells after endo H digestion, suggesting that inactive LPL was retained in ER. In the presence of castanospermine, an inhibitor of ER glucosidase I, LPL subunits of both control and CCCP-treated cells had same molecular weight, indicating that complete oligosaccharides were transferred to LPL subunits in the presence of CCCP. In sucrose density gradient ultracentrifugation, all the LPL protein synthesized in the presence of CCCP was found at the dimeric fractions as in control cells. Most of LPL protein in control cells showed high affinity for heparin, and there was no difference between the control and CCCP-treated cells. These results suggest that dimerization and acquisition of high affinity for heparin of LPL can occur in ER of CCCP-treated cells without acquisition of catalytic activity.

Developmental expression and functional activity of beta 1- and beta 3-adrenoceptors in murine 3T3-F442A differentiating adipocytes

Beta 1- and beta 3-adrenoceptor mRNA and protein expression, and contribution of each subtype to the catecholamine-sensitive adenylyl cyclase system were studied during the adipose conversion of the murine 3T3-F442A cell line. Northern and reverse transcriptase-polymerase chain reaction analyses indicated that emergence of beta 3-adrenoceptor transcripts was concomittant with that of the gene encoding adipsin, a very late marker of adipose differentiation. Conversely, the induction of the beta 1-adrenoceptor mRNA occurred early after cell commitment towards adipose conversion. Changes in beta-subtype gene expression were accompanied by parallel modifications in receptor expression and function. 125I-cyanopindolol saturation and competition binding experiments showed a 3-fold increase in beta 1-adrenoceptor density in day 3 post-confluent cells. The beta 3-subtype population became detectable later and represented approximately 95% of total beta-adrenoceptors in day 8 and day 12 post-confluent cells. Adenylyl cyclase activity in response to the beta 3-adrenoceptor-selective agonists CGP12177 (4-(3-t-butylamino-2-hydroxypropoxy)-benzimidazol-2-one), ICI201651 ([(R)-4-(2 hydroxy-3-phenoxypropylamino-ethoxy)-N-(2- methoxyethyl)phenoxy-acetamide]) and cyanopindolol was virtually absent in young adipocytes, but dramatically increased in mature cells. The respective contributions of the beta 1- and the beta 3-subtypes to the production of cAMP were resolved by an Eadie-Hofstee computer analysis of isoproterenol and norepinephrine concentration-response curve of adenylyl cyclase activity. Agonist response curves in the presence of beta 1- and beta 2-adrenoceptor antagonist indicated that the beta 1-subtype accounted for the totality of beta-adrenoceptor-mediated adenylyl cyclase activation in young adipocytes. In mature adipose cells approximately 90% of this response was due to an activation of the beta 3-adrenoceptor. In addition, approximately 84% of the maximal norepinephrine-stimulated lipolysis was mediated by the beta 3-adrenoceptor in fully differentiated adipocytes. The differentiation-dependent expression of beta-subtypes in adipocytes is a biphasic process involving an initial and moderate induction of beta 1-adrenoceptors followed by the emergence of a prominent beta 3-adrenoceptor population. Compared analysis of both receptor occupancy and cAMP production shows that the beta 3-subtype is more efficiently coupled to the adenylyl cyclase system than the beta 1-adrenoceptor. Thus in mature adipose cells this receptor subtype represents the core of cAMP-dependent regulation of the lipolytic, antilipogenic and thermogenic effects of catecholamines.

Lipoprotein lipase and glucose-6-phosphate dehydrogenase activities in bovine and ovine adipose tissue incubated for 7 days: effects of insulin and/or dexamethasone

The in vitro regulation of lipoprotein lipase (LPL) and glucose-6-phosphate dehydrogenase (G6PDH) activity in bovine and ovine adipose tissue was investigated. Adult non-lactating non-pregnant cows (n = 5) or ewes (n = 5) were given limited amounts of feed for 8 or 10 days and then overfed for 10 (ewes) or 21 (cows) days. Perirenal adipose tissue explants were incubated for 2, 4 or 7 days. Regardless of the experimental conditions, the activity of LPL and G6PDH after 2 days of incubation was lower than in fresh tissue. Insulin significantly increased LPL activity in bovine but not in ovine adipose tissue, and it had no effect on G6PDH activity in the two species. Dexamethasone addition to the insulin-supplemented medium significantly increased LPL activity in ovine adipose tissue, whereas it was decreased in bovine adipose tissue on days 4 and 7. Moreover, dexamethasone addition to the insulin-supplemented medium did not change G6PDH activity in the two species on day 2, whereas it was increased in bovine and ovine adipose tissue on days 4 and 7. Therefore, the effects of insulin and/or dexamethasone on LDL and G6PDH differed with ruminant species and incubation time.

Glycosylation and secretion of lipoprotein lipase by 3T3-L1 adipocytes: effects of brefeldin A

Time courses of synthesis and secretion of lipoprotein lipase (LPL) were examined in 3T3-L1 adipocytes. LPL was glycosylated in the endoplasmic reticulum (ER) within 10 min after synthesis, and was transported after 20-30 min to the trans Golgi where it was converted to the mature form with M(r) = 55,000-58,000, which was resistant to endoglycosidase H (endo H). LPL subunits with M(r) = 55,000-58,000 appeared in the medium within 30 min after synthesis. The effects of brefeldin A (BFA), which inhibits transport of glycoproteins in various types of cells, on secretion and glycosylation of LPL were also examined. BFA completely blocked release of LPL activity into the medium, causing accumulation of the activity in cells. The suppressive effect of BFA on release of LPL activity was reversible. BFA-treated cells synthesized LPL with M(r) = 53,000-55,000 consisting of 2 types of subunits, the main type being totally endo H-sensitive and the other partially endo H-sensitive. No LPL were secreted into the medium by BFA-treated cells.

Hypothalamic neuropeptide Y messenger ribonucleic acid levels in pre-obese and genetically obese (fa/fa) rats; potential regulation thereof by corticotropin-releasing factor

Neuropeptide Y (NPY) is a 36 amino-acid peptide. It is localized within the brain but is also present peripherally. It is a well substantiated orexigenic peptide with several other endocrine and behavioural effects. In this study NPY mRNA levels were measured, using the polymerase chain reaction amplification technique, in the hypothalamus of pre-obese (unweaned 13-day-old), young (weaned 28-day-old) and adult (11-week-old) obese fa/fa rats and compared to those of lean age-matched controls. Before weaning, pre-obese pups had the same NPY mRNA levels as controls. After weaning NPY mRNA levels were increased 2-fold in young 28-day-old and 4-fold in adult obese rats, relative to corresponding controls. When adult obese rats were intracerebroventricularly-treated with ovine corticotropin-releasing hormone (oCRF) for 7 days, they stopped gaining body weight relative to vehicle-infused obese controls. Upon measuring NPY mRNA levels in the hypothalamus of these two groups of animals, it was shown that the high NPY mRNA levels of vehicle-treated (control) obese rats were decreased by 3-fold following the intracerebroventricular oCRF administration. It is proposed that: 1) hypothalamic NPY may play a role in the establishment and maintenance of the genetic obesity syndrome of the fa/fa rat, and 2) maintenance of the genetic obesity syndrome of the fa/fa rat, and 2) hypothalamic NPY could be partly regulated by central CRF.

Glycosylation of lipoprotein lipase in human subcutaneous and omental adipose tissues

Human adipose tissues from the abdomen (subcutaneous), thigh (subcutaneous) and omentum were incubated for 2 h with [35S]methionine. Then glycosylation of lipoprotein lipase (LPL) was analyzed by sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of endoglycosidase H (endo H)-digested subunits of the 35S-labeled lipase. Adipose tissues from the abdomen, thigh, and omentum all synthesized LPL subunits with Mr = 57,000 composed of two types of subunits. One type was partially endo H-sensitive yielding a product with Mr = 55,000, indicating that it had one endo H-resistant and one endo H-sensitive oligosaccharide chain. The other type of subunit was totally endo H-sensitive yielding a product with Mr = 52,000. Subcutaneous adipose tissues contained nearly equal amounts of partially and totally endo H-sensitive subunits of LPL, whereas omental adipose tissues contained mainly partially endo H-sensitive subunits of LPL.

Interferon-gamma and interleukin-1 beta inhibit adipoconversion in cultured rodent preadipocytes

Cytokines like tumor necrosis factor (TNF), interferon-gamma (IFN-gamma), and interleukin-1 (IL-1) are known to interfere with the differentiation of cultured cell lines of adipocyte precursors. In the present study, the effect of mouse and rat IFN-gamma, as well as human IL-1 beta, was investigated on rodent preadipocytes in primary cultures, either in the presence of fetal bovine serum (FBS, 10%) or in serum-free defined medium. IFN-gamma exerted an antiproliferative action that was more pronounced when cells reached confluency than during the growth phase of the culture. Morphological observation and quantifications of undifferentiated and differentiating cells revealed that IFN-gamma caused a decrease in the proportion of cells devoid of lipid droplets which would correspond to fibroblast-like cells, whereas preadipocytes remained unaffected. IFN-gamma induced a marked retardation of adipoconversion, resulting in a partial inhibition of lipoprotein lipase (LPL) activity and a severe decrease in glycerol-3-phosphate dehydrogenase (GPDH) activity. The antiproliferative and anti-LPL effects of IFN-gamma were neutralized by adding anti-IFN-gamma antibodies, while these antibodies prevented only partially the depressing effect of IFN-gamma on GPDH activity. Contrary to IFN-gamma, IL-1 beta slightly enhanced the proliferation in preadipocyte cultures. IL-1 beta also depressed adipoconversion, inhibited markedly LPL activity, and partially reduced GPDH activity. These results show that the influence of cytokines on adipoconversion observed in preadipocyte cell lines can be found in normal preadipocytes in culture.

Secretion-coupled increase in the catalytic activity of rat hepatic lipase

Freshly isolated rat hepatocytes synthesize and secrete hepatic lipase (HL). Comparison of secreted HL with intracellular HL indicates a secretion-linked increase in the specific enzyme activity. (a) Immunotitration with polyclonal anti-HL showed a 3-5-fold lower specific enzyme activity of intracellular HL than of secreted HL. This was confirmed by ELISA using a mixture of monoclonal anti-HL's. (b) After isolation on Sepharose-heparin, a similar difference in specific enzyme activity was observed, whereas the apparent Km for glyceroltrioleate was not different. (c) HL activity secreted in the absence of protein de novo synthesis was 5-fold higher than was accounted for by the fall in intracellular activity, whereas HL protein lost from the cells was near-completely recovered in the extracellular medium. (d) The presence of inactive HL protein was demonstrated in cells treated with castanospermine, which inhibits secretion of newly synthesized HL by interfering with maturation at an early stage of N-linked oligosaccharide processing. Upon removal of castanospermine, secretion of HL activity recovered, even when protein de nove synthesis was inhibited, strongly suggesting that part of the inactive HL was mobilized and became activated. This secretion-coupled increase in HL activity in the absence of protein synthesis suggests the existence of inactive precursor within rat hepatocytes. The catalytic activity of HL becomes apparent upon maturation of the protein after oligosaccharide processing by the rough endoplasmic reticulum glucosidases.

Cytoplasmic proteins of porcine adipocytes: identification with monoclonal antibodies

In the present study, monoclonal antibodies were produced using porcine adipocyte extracts as the immunogen. Two of the monoclonal antibodies, designated CB6 and IB4, exhibited reactivity toward only cells containing lipid in stromal-vascular cell cultures. The antigens recognized by the CB6 and IB4 monoclonal antibodies were 50 kD and 55 kD proteins, respectively. In vivo, IB4 immunoreactivity was detected only in lipid-containing cells, whereas immunofluorescence using CB6 was also detectable around muscle fiber bundles underlying the subcutaneous mesenchyme. In fetal subcutaneous mesenchyme, CB6 and IB4 immunoreactivities toward lipid-containing cells increased with developmental age, but each was not detectable in cells containing the smallest lipid droplets. In stromal-vascular cultures containing adipocytes, 48 hour treatment with the anti-lipogenic agent, growth hormone, only slightly altered CB6 immunoreactivity, whereas IB4 immunoreactivity was reduced by more than sixfold. The exact identity of the CB6 and IB4 antigens was not determined, but each may be useful as markers for studying regulation of adipocyte metabolism.

Cellular and secreted lipoprotein lipase revisited

Lipoprotein lipase (LPL) of adipose cells is present only in membrane compartments, mainly in the Golgi apparatus. LPL is a typical secretory protein which appears to be active as a homodimer. The process of LPL synthesis and maturation requires multiple steps. LPL is synthesized in the endoplasmic reticulum as an inactive monomer of Mr 51,000; a high-mannose, inactive monomer of Mr 55,500 is then formed. An active homodimer form, bearing two complex oligosaccharide chains per monomer of Mr 58,000, forms in the Golgi apparatus. This mature form, present in secretory vesicles, can be secreted constitutively or after exposure to heparin. A model is proposed in which LPL is present in secretory vesicles in a potentially active, condensed, or "polymerized" form. This model, which applies to various LPL-containing tissues in different species - including human - would explain the "activation" of LPL.

Increasing effect of vanadate on lipoprotein lipase activity in isolated rat fat pads

Vanadate increased lipoprotein lipase (LPL) activity in the isolated fat pads in a time- and dose-dependent manner. The increasing effect of vanadate was inhibited by amiloride, similar to that of insulin, and it also was not additive to that of insulin. Although the increasing effects of vanadate and insulin were preserved in K(+)-free medium, appreciable decreases in both effects were observed by replacement of Na+ with choline ion or omission of Ca2+ in the medium. Vanadate showed the full effect in the presence of cycloheximide at concentrations that inhibited protein synthesis of the fat pads, suggesting that the action of vanadate is not due to the increase in protein synthesis. Tetrakis (acetoxymethyl) ester of quin 2 at 50 microM concentration never inhibited the action of vanadate though it showed a little inhibition at a concentration of 300 microM. No inhibition of the action of vanadate was observed with ruthenium red. These results suggest that vanadate increases the LPL activity via a process less sensitive to the intracellular Ca2+ concentration. Adrenaline, dibutyryl cyclic AMP, and 3-isobutyl-1-methylxanthine all inhibited the action of vanadate, suggesting that the action is inhibited with increase in the intracellular concentration of cyclic AMP. Monensin and carbonyl cyanide m-chlorophenylhydrazone inhibited the action of vanadate. In contrast, the action of insulin was never inhibited by monensin. Tunicamycin and 2-deoxyglucose, at rather high concentrations, inhibited both actions. These findings suggest that vanadate increases the LPL activity through mechanisms of action involving amiloride- and monensin-sensitive pathways dependent on energy.

Effect of starvation on lipoprotein lipase activity in the liver of developing rats

Liver lipoprotein lipase activity in neonatal (1- and 5-day-old) rats was 2-3-times than in the liver of adult rats. In mid-suckling (15-day-old) or weaned (30-day-old) animals, it was not significantly different from the low activity detected in adult rats. Starvation resulted in a 3-fold increase of lipoprotein lipase activity in the neonatal liver, but did not affect the activity in the liver of mid-suckling, weaned or adult rats. When isolated livers from both 1- and 5-day-old pups were perfused with heparin, a sharp peak of lipoprotein lipase activity appeared in the perfusate. In fed neonates, the peak area accounted for about 70% of the total (released + non-releasable) activity. In starved neonates, the proportion of heparin-releasable activity increased up to about 90%. These results indicate that neonatal rat liver lipoprotein lipase activity is markedly affected by changes in the nutritional status of the animal, and the effect is restricted to the vascular pool of the enzyme, as was reported in extrahepatic tissues from adult rats.

Inhibition by serum components of the expression of lipoprotein lipase gene upon stimulation by growth hormone

Growth hormone regulates in a positive way the expression of the lipoprotein lipase gene at a transcriptional level in preadipocyte Ob1771 cells. Inhibition by serum components of this expression was investigated upon stimulation by growth hormone. Low-molecular weight, lipid-soluble components (a serum lipid extract, corticosteroids and oleic acid) and high-molecular weight, hydrophilic components (TGF-beta and those present in delipidated serum) were inhibitory. Inhibition of the expression of LPL mRNAs and that of LPL activity were parallel. It is concluded that the regulation of the expression of LPL gene occurs likely at a transcriptional level and that a balance between multiple effectors present in serum are active in an opposite manner.

Stimulatory release of lipoprotein lipase activity from rat fat pads by vanadate

Vanadate stimulated the release of lipoprotein lipase (LPL) activity from rat fat pads into the medium in a time- and dose-dependent manner. It exerted the synergetic effect with heparin. The stimulatory effects of vanadate and heparin were decreased by incubation in Na+- or Ca2+-free media but were well preserved in K+-free medium. Amiloride inhibited the vanadate-stimulated release of LPL activity in a dose-dependent manner, but did not inhibit the heparin-stimulated release of LPL activity. Colchicine, antimycin A, and carbonyl cyanide m-chlorophenylhydrazone suppressed the stimulatory effect of vanadate, but cycloheximide did not. Preincubation of the fat pads with the tetrakis (acetoxymethyl) ester of quin 2 (quin 2-AM) inhibited the vanadate-stimulatory release of LPL activity without affecting basal activity. The concentration required for half-maximal inhibition of the action of vanadate by quin 2-AM was calculated to be 39 microM, suggesting that the action of vandate was dependent on intracellular Ca2+ concentration. The heparin-stimulated release, on the other hand, was not inhibited even at higher concentrations of quin 2-AM (up to 200 microM). These findings suggest that vanadate stimulates the release of LPL activity through mechanisms of action involving amiloride-sensitive and calcium-dependent pathways with a requirement of metabolic energy.

Rapid modulation of rat adipocyte lipoprotein lipase: effect of calcium, A23187 ionophore, and thrombin

The effect of calcium ions on the lipoprotein lipase (LPL) activity in rat adipocytes has been investigated. Incubation of the cells in the absence of extracellular calcium produced a rapid decline of LPL activity in the cells. The enzyme, however, could be immediately reactivated in less than 3 min by the addition of calcium. The degree of reactivation was proportional to the concentration of extracellular calcium. alpha 1 agonists phenylephrine and methoxamine affected LPL activity only slightly, as did vasopressin and angiotensin II. In contrast, calcium ionophore A23187 elicited a quick and transient enzyme activation which reached its peak 4 min after the addition of the drug. Thrombin (0.1 U/ml) produced the most rapid and intense response. The effect of thrombin was already evident 10 s after its addition, and the enzyme activity almost doubled above the basal level. Extracellular calcium was necessary to achieve thrombin activation. Contrary to previous thought, these data support the conclusion that LPL may undergo rapid activation, and that calcium ions are critically involved in this activation process. Thrombin rapidly raises LPL activity and may be one of its physiological activators in vivo.

Adrenergic blocking agents and lipoprotein lipase activity

Lipoprotein lipase activity, total serum cholesterol and triglycerides, HDL cholesterol were determined before and after two weeks of treatment with a low dose of an alpha 1-blocking agent (prazosin) or of a beta-adrenergic-blocking drug (metoprolol). Lipoprotein lipase activity was almost doubled after prazosin (p less than 0.02) and practically unchanged after metoprolol, at a time and at a drug dosage when only minor changes in blood pressure and serum lipids were detectable. HDL cholesterol was slightly but significantly increased after prazosin (p less than 0.05). Heart rate was increased after prazosin (p less than 0.05) and decreased after metoprolol (p less than 0.01).

Short-term stimulation by insulin of lipoprotein lipase secretion in adipose cells

The spontaneous secretion of lipoprotein lipase has been examined in adipose cells of mouse Ob17, Ob17SA and 3T3-F442A clonal lines as well as in rat adipose cells in primary culture. Striking differences are observed both in serum-free and serum-supplemented media, rat adipose cells and 3T3-F442A cells being the most active. Insulin from 10(-11) M to 10(-9) M was able to modulate the rate of LPL secretion from 2- to 4-fold. The stimulatory effect of insulin on this process occurred within 30 min in cells treated or not with cycloheximide. It is concluded that insulin is able to modulate the rate of LPL secretion independently of the synthesis of new enzyme molecules on a short-term basis and within a physiological range of concentrations.

Tissue-specific regulation of guinea pig lipoprotein lipase; effects of nutritional state and of tumor necrosis factor on mRNA levels in adipose tissue, heart and liver

Levels of mRNA for lipoprotein lipase (LPL) in guinea pig epididymal adipose tissue, heart and liver were determined by dot blot analysis of total RNA using a cDNA probe complementary to the coding region, and compared to the LPL activity. For adipose tissue we also measured the incorporation of radioactivity into immunoprecipitable LPL after pulse-labeling with [35S]methionine. LPL activity was 93%, LPL mRNA 82% and LPL synthesis 85% lower in epididymal fat pads from animals fasted for 48 h compared to rigorously fed animals. In contrast, neither LPL activity nor LPL mRNA levels differed in heart. A single dose of tumor necrosis factor (TNF) decreased LPL activity and LPL mRNA in fat pads with no effects in heart. In the liver, TNF caused a marked increase in LPL mRNA levels, which are normally very low. Northern-blot analysis confirmed a previous observation that the patterns of mRNA species differ between heart, in which a 3.8-kb mRNA dominates, and adipose tissue, in which the LPL mRNAs of 3.3 and 2.1 kb occur in similar abundance as the 3.8-kb species.

Lipoprotein lipase, hepatic lipase, and carnitine in premature infants

Twenty six preterm infants were studied at the age of 2, 7, and 26 days. The activities of lipoprotein and hepatic lipase in plasma taken 15 minutes after a heparin bolus of 100 IU/kg had been given and the concentrations of carnitine in serum and urine were measured. The mean gestational age was 31 weeks (range 26-35 weeks) and birth weight 1580 g (range 840-2280 g). Thirteen infants weighed under 1500 g at birth (very low birth weight), 20 were of appropriate weight for gestational age and six were small for gestational age. Lipoprotein lipase activity was higher in the preterm infants of appropriate weight than in the infants of very low birth weight and those who were small for gestational age. At the age of 2 or 7 days the activity of lipoprotein lipase in the preterm infants (mean (SEM) 46.2 (4.3) mumol free fatty acid/ml/hour) was, however, higher than in term infants and adults. Multivariate regression analyses showed that weight and relative birth weight together explained 58% of the variance of lipoprotein lipase activity but only 3% of the variance of hepatic lipase activity. Serum carnitine concentration was lower in the preterm infants than in term infants. Urinary excretion of carnitine increased progressively with age but was independent of serum concentration and carnitine intake. Urinary excretion of total carnitine was significantly greater in the infants who were small for gestational age (mean (SEM) 754 (203) nmol/mg of creatinine, n = 6) than in the infants of appropriate weight (161 (22.0) nmol/mg of creatinine, n = 12) but acyl/free carnitine ratio was smaller in the infants who were small for gestational age than in infants of appropriate weight (0.56 v 5.5). The results indicate that the slow elimination of fat from the circulation in preterm infants less mature than 32 weeks of gestation can hardly be explained by low lipoprotein lipase activity.

Secretion of lipoprotein lipase from myocardial cells isolated from adult rat hearts

Heparin (5 U/ml) induced the release of LPL into the incubation medium of cardiac myocytes isolated from adult rat hearts. The secretion of LPL occurred in two phases: a rapid release (5-10 min of incubation with heparin) that was independent of protein synthesis followed by a slower rate of release that was inhibited by cycloheximide. The rapid release of LPL induced by heparin likely occurs from sites that are at or near the cell surface. LPL secretion could also be stimulated by heparan sulfate and dermatan sulfate, but not by hyaluronic acid, chondroitin sulfate or keratan sulfate. Heparin-releasable LPL activity measured in short-term incubations represented a large fraction (40-50%) of the initial LPL activity associated with myocytes, but the fall in cellular LPL activity following heparin was less than the amount of LPL activity secreted into the incubation medium. This discrepancy was not due to latency of LPL in the pre-heparin cell homogenates, but in part could be due to a three-fold greater affinity of the heparin-released enzyme for substrate as compared to LPL in post-heparin myocyte homogenates.

Mechanisms for turnover of lipoprotein lipase in guinea pig adipocytes

Guinea-pig adipocytes released lipoprotein lipase activity to the medium without depletion of cell-associated lipoprotein lipase activity. Heparin caused immediate release of 20-25% of the lipase activity to the medium, and also enhanced the continued release. After addition of cycloheximide, cell-associated lipoprotein lipase activity decreased rapidly. Release of lipase activity to the medium continued unabated for about 30 min, but there was little release thereafter. The release accounted for only about 25% of the initial lipoprotein lipase activity in the absence and about 50% in the presence of heparin. In pulse-chase experiments with [35S]methionine, labeled lipoprotein lipase appeared in the medium within 40 min, and most of the release occurred during the first h of chase. In a 4-h chase the total (cells + medium) amount of labeled lipase decreased to 34%. Thus, degradation was a main fate of the lipase. Heparin markedly increased the amount of labeled lipase that was released to the medium and decreased the amount that was degraded. Heparin did not change the time-course for the release, and the amount of labeled lipase degraded was proportional to the amount not released to the medium, indicating that the effect of heparin was primarily on release, not on degradation as such. This study demonstrates that adipocytes synthesize lipoprotein lipase in excess of what is being released, and that the excess is rapidly degraded.