Insulin and phorbol ester stimulate phosphorylation of acetyl-CoA carboxylase at similar sites in isolated adipocytes. Lack of correspondence with sites phosphorylated on the purified enzyme by protein kinase C

The BCKDH Kinase and Phosphatase Integrate BCAA and Lipid Metabolism via Regulation of ATP-Citrate Lyase

Branched-chain amino acids (BCAA) are strongly associated with dysregulated glucose and lipid metabolism, but the underlying mechanisms are poorly understood. We report that inhibition of the kinase (BDK) or overexpression of the phosphatase (PPM1K) that regulates branched-chain ketoacid dehydrogenase (BCKDH), the committed step of BCAA catabolism, lowers circulating BCAA, reduces hepatic steatosis, and improves glucose tolerance in the absence of weight loss in Zucker fatty rats. Phosphoproteomics analysis identified ATP-citrate lyase (ACL) as an alternate substrate of BDK and PPM1K. Hepatic overexpression of BDK increased ACL phosphorylation and activated de novo lipogenesis. BDK and PPM1K transcript levels were increased and repressed, respectively, in response to fructose feeding or expression of the ChREBP-β transcription factor. These studies identify BDK and PPM1K as a ChREBP-regulated node that integrates BCAA and lipid metabolism. Moreover, manipulation of the BDK:PPM1K ratio relieves key metabolic disease phenotypes in a genetic model of severe obesity.

Calcineurin-mediated Dephosphorylation of Acetyl-coA Carboxylase is Required for Pheromone Biosynthesis Activating Neuropeptide (PBAN)-induced Sex Pheromone Biosynthesis in Helicoverpa armigera

Chemical signaling plays a critical role in the behavior and physiology of many animals. Female insects, as many other animals, release sex pheromones to attract males for mating. The evolutionary and ecological success of insects therefore hinges on their ability to precisely mediate (including initiation and termination) pheromone biosynthesis. Pheromone biosynthesis activating neuropeptide (PBAN) acts directly on pheromone glands to regulate sex pheromone production using Ca2+ and cyclic-AMP as secondary messengers in the majority of species. However, the molecular mechanism downstream of the secondary messengers has not yet been elucidated in heliothine species. The present study shows that calcineurin, protein kinase A (PKA) and acetyl-coA carboxylase (ACC) are key components involved in PBAN-induced sex pheromone biosynthesis in Helicoverpa armigera using PBAN-dependent phosphoproteomics in combination with transcriptomics. RNAi-mediated knockdown and inhibitor assay demonstrated that calcineurin A is required for PBAN-induced ACC activation and sex pheromone production. Calcineurin-dependent phosphoproteomics and in vitro calcineurin phosphorylation assay further revealed that calcineurin regulated ACC activity by dephosphorylating ser84 and ser92. In addition, PKA-dependent phosphoproteomics and activity analysis revealed that PKA reduces the activity of AMP-activated protein kinase (AMPK), a negative regulator of ACC by phosphorylating the conserved ser92. Taken together, our findings indicate that calcineurin acts as the downstream signal of PBAN/G-protein receptor/Ca2+ to activate ACC through dephosphorylation while inactivating AMPK via PKA to reduce ACC phosphorylation, thus facilitating calcineurin activation of ACC.

Dual role of insulin-like growth factor-1 in acetyl-CoA carboxylase-alpha activity in human colon cancer cells HCT-8: downregulating its expression and phosphorylation

Insulin-like growth factor-1 (IGF-1) plays the role in cellular lipid synthesis and cell proliferation. However, the role of IGF-1 on the growth of colon cancer cell line HCT-8 is not clear. In this study, HCT-8 cells were exposed to IGF-1 at 0, 10, 50, or 100 ng/ml in serum-free medium. Fatty acid/lipid synthesis in HCT-8 cells was examined by 2-14C-acetate incorporation. HCT-8 cell growth and proliferation were determined by MTT assay and Trypan blue exclusive viable cell counting. We found that in serum starvation conditions, IGF-1 at 10-100 ng/ml induced dose-dependent down regulation of both the ACCα expression and the phosphorylation in HCT-8 cells, maintaining a balance in ACCα activity and lipid synthesis. IGF-1 reduced p-ATM, p-AMPK, and then p-ACCα protein levels in HCT-8 cells. IGF-1 increased p-Akt levels, but decreased p-ERK1/2 levels, leading to the decrease in ACCα protein and mRNA levels. Similarly, ERK1/2 inhibitor PD98059 reduced ACCα expression. IGF-1 influences neither HCT-8 cell growth nor their p53 protein levels and PARP cleavage. In a word, IGF-1 reduced ACCα phosphorylation via an ATM/AMPK signaling pathway and suppressed ACCα expression through an ERK1/2 transduction, playing a dual role in regulating ACCα activity and lipogenesis. This may render a cell with survival advantages under a serum starvation crisis, representing a novel mitogenic role of IGF-1.

Structure and regulation of acetyl-CoA carboxylase genes of metazoa

Acetyl-CoA carboxylase (ACC) plays a fundamental role in fatty acid metabolism. The reaction product, malonyl-CoA, is both an intermediate in the de novo synthesis of long-chain fatty acids and also a substrate for distinct fatty acyl-CoA elongation enzymes. In metazoans, which have evolved energy storage tissues to fuel locomotion and to survive periods of starvation, energy charge sensing at the level of the individual cell plays a role in fuel selection and metabolic orchestration between tissues. In mammals, and probably other metazoans, ACC forms a component of an energy sensor with malonyl-CoA, acting as a signal to reciprocally control the mitochondrial transport step of long-chain fatty acid oxidation through the inhibition of carnitine palmitoyltransferase I (CPT I). To reflect this pivotal role in cell function, ACC is subject to complex regulation. Higher metazoan evolution is associated with the duplication of an ancestral ACC gene, and with organismal complexity, there is an increasing diversity of transcripts from the ACC paraloges with the potential for the existence of several isozymes. This review focuses on the structure of ACC genes and the putative individual roles of their gene products in fatty acid metabolism, taking an evolutionary viewpoint provided by data in genome databases.

Promoter I of the ovine acetyl-CoA carboxylase-alpha gene: an E-box motif at -114 in the proximal promoter binds upstream stimulatory factor (USF)-1 and USF-2 and acts as an insulin-response sequence in differentiating adipocytes

Acetyl-CoA carboxylase-alpha (ACC-alpha) plays a central role in co-ordinating de novo fatty acid synthesis in animal tissues. We have characterized the regulatory region of the ovine ACC-alpha gene. Three promoters, PI, PII and PIII, are dispersed throughout 50 kb of genomic DNA. Expression from PI is limited to adipose tissue and liver. Sequence comparison of the proximal promoters of ovine and mouse PIs demonstrates high nucleotide identity and that they are characterized by a TATA box at -29, C/EBP (CCAAT enhancer-binding protein)-binding motifs and multiple E-box motifs. A 4.3 kb ovine PI-luciferase reporter construct is insulin-responsive when transfected into differentiated ovine adipocytes, whereas when this construct is transfected into ovine preadipocytes and HepG2 cells the construct is inactive and is not inducible by insulin. By contrast, transfection of a construct corresponding to 132 bp of the proximal promoter linked to a luciferase reporter is active and inducible by insulin in all three cell systems. Insulin signalling to the -132 bp construct in differentiated ovine adipocytes involves, in part, an E-box motif at -114. Upstream stimulatory factor (USF)-1 and USF-2, but not sterol regulatory element-binding protein 1 (SREBP-1), are major components of protein complexes that bind this E-box motif. Activation of the 4.3 kb PI construct in differentiated ovine adipocytes is associated with endogenous expression of PI transcripts throughout differentiation; PI transcripts are not detectable by RNase-protection assay in ovine preadipocytes, HepG2 cells or 3T3-F442A adipocytes. These data indicate the presence of repressor motifs in PI that are required to be de-repressed during adipocyte differentiation to allow induction of the promoter by insulin.

Malonyl-CoA-independent acute control of hepatic carnitine palmitoyltransferase I activity. Role of Ca2+/calmodulin-dependent protein kinase II and cytoskeletal components

The mechanism of malonyl-CoA-independent acute control of hepatic carnitine palmitoyltransferase I (CPT-I) activity was investigated. In a first series of experiments, the possible involvement of the cytoskeleton in the control of CPT-I activity was studied. The results of these investigations can be summarized as follows. (i) Very mild treatment of permeabilized hepatocytes with trypsin produced around 50% stimulation of CPT-I activity. This effect was absent in cells that had been pretreated with okadaic acid (OA) and seemed to be due to the action of trypsin on cell component(s) distinct from CPT-I. (ii) Incubation of intact hepatocytes with 3, 3'-iminodipropionitrile, a disruptor of intermediate filaments, increased CPT-I activity in a non-additive manner with respect to OA. Taxol, a stabilizer of the cytoskeleton, prevented the OA- and 3, 3'-iminodipropionitrile-induced stimulation of CPT-I. (iii) CPT-I activity in isolated mitochondria was depressed in a dose-dependent fashion by the addition of a total cytoskeleton fraction and a cytokeratin-enriched cytoskeletal fraction, the latter being 3 times more potent than the former. In a second series of experiments, the possible link between Ca2+/calmodulin-dependent protein kinase II (Ca2+/CM-PKII) and the cytoskeleton was studied in the context of CPT-I regulation. The data of these experiments indicate that (i) purified Ca2+/CM-PKII activated CPT-I in permeabilized hepatocytes but not in isolated mitochondria, (ii) purified Ca2+/CM-PKII abrogated the inhibition of CPT-I of isolated mitochondria induced by a cytokeratin-enriched fraction, and (iii) the Ca2+/CM-PKII inhibitor KN-62 prevented the OA-induced phosphorylation of cytokeratins in intact hepatocytes. Results thus support a novel mechanism of short-term control of hepatic CPT-I activity which may rely on the cascade Ca2+/CM-PKII activation --> cytokeratin phosphorylation --> CPT-I de-inhibition.

Elucidation of a promoter activity that directs the expression of acetyl-CoA carboxylase alpha with an alternative N-terminus in a tissue-restricted fashion

Previous studies in rats and humans have demonstrated that acetyl-CoA carboxylase alpha (ACC-alpha), the principal ACC isoenzyme in lipogenic tissues, is transcribed from two promoters, PI and PII, that operate in a tissue-specific fashion. Each promoter gives rise to ACC-alpha mRNA isoforms that differ in their 5' untranslated regions but essentially encode the same protein product. In the present study we demonstrate that such a pattern of promoter usage is evident in sheep tissues but in addition we have detected the expression of a novel ACC-alpha mRNA isoform that is expressed in a variety of tissues including kidney, lung, liver and mammary gland, where it is markedly induced during lactation. This novel transcript differs from the previously described ACC-alpha mRNA in that exon 5, the primary coding exon in both PI and PII transcripts, is replaced by a 424-nt sequence that seems to represent the 5' terminus of the mRNA. The 424-nt sequence encodes a 17-residue N-terminal region as the N-terminal residue in the deduced sequence is a methionine flanked by several in-frame stop codons. The 5' terminal 424 nt are present as a single exon, which we have termed exon 5A, in the sheep ACC-alpha gene and this is located approx. 15 kb downstream of exon 5 and 5 kb upstream of exon 6. A 1.5 kb HindIII-BglII fragment encompassing the 5' terminus and sequence immediately upstream of exon 5A demonstrates promoter activity when transiently transfected into HepG2 cells and HC11 mouse mammary cells and this is markedly enhanced when insulin is present in the culture medium. Promoter activity is also evident in primary sheep mammary epithelial cells. These results demonstrate the presence of a third promoter, PIII, in the ACC-alpha gene that results in the tissue-restricted expression of an ACC isoenzyme.

Purification and characterisation of an insulin-stimulated protein-serine kinase which phosphorylates acetyl-CoA carboxylase

An insulin-stimulated protein kinase specific for acetyl-CoA carboxylase has been purified from rat epididymal adipose tissue using Mono-Q chromatography. The kinase binds to (and phosphorylates) the relatively inactive, dimeric form of acetyl-CoA carboxylase, but not to its active, polymeric form, and this property has been used to purify the kinase. Under the conditions used, phosphorylation by the purified kinase did not result in a detectable increase in acetyl-CoA carboxylase activity. These studies also led to the recognition of an 'activator' protein which is capable of increasing the activity of acetyl-CoA carboxylase without changing its phosphorylation state. It is suggested that this 'activator' protein, together with the insulin-activated acetyl-CoA carboxylase kinase, may play a role in the activation of acetyl-CoA carboxylase by insulin.

Regulation of mammalian acetyl-coenzyme A carboxylase

Long-chain fatty acids are involved in all aspects of cellular structure and function. For controlling amounts of fatty acids, cells are endowed with two acetyl-coenzyme A carboxylase (ACC) systems. ACC-alpha is the rate-limiting enzyme in the biogenesis of long-chain fatty acids, and ACC-beta is believed to control mitochondrial fatty acid oxidation. These two isoforms of ACC control the amount of fatty acids in the cells. Phosphorylation and dephosphorylation of ACC-alpha cause enzyme inactivation and activation, respectively, and serve as the enzyme's short-term regulatory mechanism. Covalently modified enzymes become more sensitive toward cellular metabolites. In addition, many hormones and nutrients affect gene expression. The gene products formed are heterogeneous and tissue specific. The ACC-beta gene is located on human chromosome 12; the cDNA for this gene has just been cloned. The gene for the alpha-isoform is located on human chromosome 17. The catalytic core of the beta-isoform is homologous to that of the alpha-isoform, except for an additional peptide of about 150 amino acids at the N terminus. This extra peptide sequence makes the beta-form about 10,000 daltons larger, and it is thought to be involved in the unique role that has been assigned to this enzyme. The detailed control mechanisms for the beta-isoform are not known.

Effect of oxaloacetate and phosphorylation on ATP-citrate lyase activity

ATP-citrate lyase (CL) catalyzes the conversion of citrate and CoA to oxaloacetate (OA) and acetyl-CoA. As the coupled malic dehydrogenase (MDH) assay is not able either to study the effect of oxaloacetate (OA) on CL activity or to measure accurately CL activity in biological samples, a new assay was developed. The CL-citrate coupled CAT assay measures the amount of acetyl-CoA formed by transferring radiolabeled acetyl-CoA synthesized from [14C]citrate to chloramphenicol with chloramphenicol acetyltransferase (CAT). Employing this assay, the rate of increase in acetyl-CoA synthesis from citrate is linear with respect to added CL. Kinetic values for ATP, CoA and citrate are similar to those obtained using the MDH assay. The effect of CL phosphorylation on enzyme activity was determined. CL phosphorylated by cAMP-dependent protein kinase or by this kinase and glycogen synthase kinase-3 (GSK-3) decreases the apparent Vmax without changing the apparent Km. The effect of OA, a product of the enzyme reaction, on CL activity was also determined. Computational analysis of the data obtained without added OA and at three concentrations of OA indicate that the apparent Km for the substrate is not altered even though the apparent Vmax is decreased. The effect of OA on the activity of phosphorylated enzyme was also determined. OA decreases the apparent Vmax of the phosphorylated enzyme to the same extent as in control CL. This assay is able to measure CL activity in cytosol from 3T3-L1 adipocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin-stimulated hydrolysis of phosphatidylcholine by phospholipase C and phospholipase D in cultured rat hepatocytes

We have investigated the mechanism of action by which insulin increases phosphatidate (PA) and diacylglycerol (DAG) levels in cultured rat hepatocytes. Insulin initially stimulated phosphatidylcholine-dependent phospholipase D (PC-PLD) with a significant increase in both PA and intracellular as well as extracellular choline. The involvement of phospholipase D was confirmed by the formation of PC-derived phosphatidylethanol in the presence of ethanol. The DAG increase appeared to be biphasic. Only the early phase of DAG production was inhibited by propranolol, an inhibitor of the phosphatidate phosphatase (PAP) responsible for the conversion of PA into DAG, suggesting that initially the DAG increase is due to the PLD-PAP pathway. The delayed DAG increase was in parallel with increased intracellular and extracellular phosphocholine and probably derived directly from PC-PLC activity. Experiments performed in the presence of 1 microM phorbol 12-myristate 13-acetate (PMA) indicated that protein kinase C (PKC) mediated the insulin effect on PC-PLC, but not on PC-PLD. These findings were confirmed using the PKC inhibitors calphostin, H7 and staurosporine. The dual activation of these phospholipases with a biphasic elevation of DAG levels and activation of specific PKC isoenzymes could be necessary to elicit both early and delayed effects of insulin.

Cloning of human acetyl-CoA carboxylase cDNA

Acetyl-CoA carboxylase is the rate-limiting enzyme in the biogenesis of long-chain fatty acids. In order to understand the mechanisms that regulate human acetyl-CoA carboxylase at the gene level, and the relationship between its structure and function, cDNA clones for human acetyl-CoA carboxylase have been isolated and sequenced. Human acetyl-CoA-carboxylase cDNA contains 7020 nucleotides encoding a protein of 2340 amino acids with a calculated relative molecular mass of 264575. The human enzyme shows approximately 85% identity in nucleotide sequence with previously cloned rat acetyl-CoA carboxylase, and shows 90% identity in the amino acid sequence. Two human acetyl-CoA-carboxylase mRNA species, which differ in the 5' untranslated region with the same coding sequence, have been identified. The sequence analysis reveals that type I and type II acetyl-CoA-carboxylase mRNA contain 313- and 173-base-long 5' untranslated regions, respectively. The first 240 nucleotides in the 5' untranslated region of type I acetyl-CoA-carboxylase mRNA replace the first 100 nucleotides of the (G + C)-rich region of the 5' untranslated region of the type II mRNA. These two species of mRNAs are the only species of human ACC mRNA which have been detected compared to at least five species in rat tissues, and they are expressed in a tissue-specific manner.

Stimulation of proliferation of HL60 cells by low concentrations of 12-O-tetradecanoylphorbol-13-acetate and its relationship to the mitogenic effects of insulin

The effects of the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) on the growth and differentiation of cultured human acute promyelocytic leukemia (HL60) cells have been studied using cells growing in a fully defined medium consisting of RPMI 1640 supplemented with selenium dioxide, insulin, and either transferrin or ferric citrate. High concentrations of TPA (greater than 1 nM) cause the expected inhibition of proliferation and induction of macrophage-like differentiation. In contrast, in cells deprived of insulin, which continue to grow at a slow rate, lower concentrations of TPA stimulate proliferation without inducing differentiation. A TPA concentration between 0.03 and 0.3 nM will approximately double the long-term rate of thymidine incorporation into DNA and the rate of increase in cell density. Low-TPA becomes progressively less able to stimulate further proliferation as the insulin concentration is increased and is virtually without effect on cells stimulated by an optimal insulin concentration (5 micrograms ml-1). Insulin itself stimulates proliferation to a greater extent than low-TPA, increasing the long-term rate of thymidine incorporation and the rate of increase in cell density by three- to fourfold. The ability of higher concentrations of TPA to induce differentiation is independent of the presence of insulin. Low-TPA also stimulates the short-term incorporation of thymidine (during a 1-h pulse after 1 or 2 days incubation) by three- to fourfold, as compared to a sevenfold stimulation by insulin. The proliferation response to low TPA concentrations provides a useful model for dissecting the signalling pathways that control cell proliferation following stimulation by insulin and activators of protein kinase C.

Identification of a major maturation-activated acetyl-CoA carboxylase kinase in sea star oocytes as p44mpk

Maturation-activated protein-serine/threonine kinases were investigated in the high-speed supernatant fractions from sea-star oocytes harvested at the time of germinal vesicle breakdown. One of the major stimulated protein kinases able to phosphorylate acetyl-CoA carboxylase in these extracts was found to co-purify with a 44 kDa myelin basic protein kinase (p44mpk) that is activated with a similar time course during oocyte maturation. Purified sea-star oocyte p44mpk phosphorylated acetyl-CoA carboxylase (purified from rat liver) predominantly on serine and to a small extent on threonine. Furthermore, the phosphorylation of acetyl-CoA carboxylase occurred principally on a tryptic phosphopeptide which displayed electrophoretic and chromatographic properties very similar to those of the peptide that has previously been shown to undergo increased phosphorylation in response to insulin in rat adipocytes [Brownsey & Denton (1982) Biochem. J. 202, 77-86]. The acetyl-CoA carboxylase was phosphorylated at a similar rate and to a similar extent by casein kinase II, which was also purified from maturing sea-star oocytes. Although casein kinase II was also activated approximately 3-fold near the time of nuclear envelope breakdown, it was responsible for only a minor component of the total enhanced acetyl-CoA carboxylase kinase activity measured in the soluble extracts from maturing oocytes. Acetyl-CoA carboxylase was a relatively poor substrate for the major S6 peptide kinase activity that was also stimulated during resumption of meiosis in the oocytes. The properties of the p44mpk are reminiscent of those of a microtubule-associated protein 2 (MAP-2) kinase that is activated in response to insulin and other mitogens in mammalian cells [Ray & Sturgill (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 3753-3757; Hoshi, Nishida & Sakai (1988) J. Biol. Chem. 263, 5396-5401]. It is intriguing that several of the mammalian protein kinases that are acutely activated after mitogenic prompting of quiescent mouse fibroblasts (i.e. G0 to G1 transition), such as MAP-2 kinase, casein kinase II and S6 kinase II, have counterparts that are activated during M-phase in maturing sea star oocytes.

Protein-serine kinase from rat epididymal adipose tissue which phosphorylates and activates acetyl-CoA carboxylase. Possible role in insulin action

1. Most of the cyclic-nucleotide-independent acetyl-CoA carboxylase kinase activity in an extract of rat epididymal adipose tissue was evaluated from a Mono Q column by 0.175 M-NaCl at pH 7.4. The activity of the kinase in this fraction (fraction 1) was increased after exposure of intact tissue to insulin. 2. Incubation of purified adipose-tissue acetyl-CoA carboxylase with [gamma-32P]ATP and samples of fraction 1 led to the incorporation of up to 0.4 mol of 32P/mol of enzyme subunit. Most of the phosphorylation was on serine residues within a single tryptic peptide. This peptide, on the basis of two-dimensional t.l.c. analysis, h.p.l.c. and Superose 12 chromatography, appeared to be the same as the acetyl-CoA carboxylase peptide ('I'-peptide) which exhibits increased phosphorylation in insulin-treated tissue. 3. Phosphorylation of purified acetyl-CoA carboxylase by the kinase in fraction 1 was found to be associated with a parallel 4-fold increase in activity. However, increases in both phosphorylation and activity were much diminished if fraction 1 was treated by Centricon centrifugation to remove low-Mr components. Among these components was a potent inhibitor of acetyl-CoA carboxylase activity which appeared to be necessary for the kinase in fraction 1 to be fully active. 4. The inhibitor remains to be identified, but inhibition requires MgATP, although the inhibitor itself does not cause any phosphorylation of the carboxylase. No effects of insulin were observed on the activity of the inhibitor. 5. It is concluded that the kinase probably plays an important role in the mechanism whereby insulin brings about the well-established increases in phosphorylation and activation of acetyl-CoA carboxylase in adipose tissue.

Insulin, oxytocin, and vasopressin stimulate protein kinase C activity in adipocyte plasma membranes

Incubation of isolated rat adipocytes with insulin, vasopressin, or oxytocin increased plasma membrane-bound protein kinase C (PKC) activity by 100-400%. PKC activity was assayed by a procedure that is virtually background-free, thus permitting assay of protein kinase activity in highly diluted samples of solubilized membranes. Hormone-dependent increases in PKC activity were limited to plasma membranes. Stimulation of the kinase was half-maximal with 70 pM insulin, and the hormone effect was rapid. Oxytocin and vasopressin produced effects on PKC similar to insulin, but the magnitude of the vasopressin stimulation exhibited seasonal variations. Treatment of cells with phorbol 12-myristate 13-acetate (PMA) resulted in a loss of PKC activity from the cytosol and a gain in plasma membrane activity, indicative of translocation of the enzyme. With activity measurements it was not possible to determine if insulin stimulated a translocation of the kinase. However, Western blot analysis of plasma membranes with polyclonal antibodies directed against PKC suggest that at least some of the insulin-stimulated PKC activity resulted from enzyme translocation.

Location and function of three sites phosphorylated on rat acetyl-CoA carboxylase by the AMP-activated protein kinase

1. We have sequenced two tryptic/chymotryptic peptides (TC3 and TC3a) containing a third site phosphorylated on rat acetyl-CoA carboxylase by the AMP-activated protein kinase. Comparison with the complete sequence of rat acetyl-CoA carboxylase predicted from the cDNA sequence [López-Casillas et al. (1988) Proc. Natl Acad. Sci. USA 85, 5784-5788] shows that this site corresponds to Ser1215. 2. Comparison of the cDNA sequence with previous amino acid sequence data identifies the other two sites for the AMP-activated protein kinase as Ser79 and Ser1200. A total of eight serine residues phosphorylated in vitro by six protein kinases can now be identified: six of these (Ser23, Ser25, Ser29, Ser77, Ser79 and Ser95) are clustered in the amino terminal region, while two (Ser1200 and Ser1215) are located in the central region. 3. Prior phosphorylation of Ser77 and Ser1200 by cyclic-AMP-dependent protein kinase prevents subsequent phosphorylation of Ser79 and Ser1200, but not Ser1215, by the AMP-activated protein kinase. Phosphorylation of Ser1215 under these conditions is not associated with a change in enzyme activity. 4. Limited trypsin treatment of native acetyl-CoA carboxylase selectively cleaves off the highly phosphorylated amino-terminal region containing Ser79. 5. Phosphorylation at Ser79 and Ser1200 by the AMP-activated protein kinase dramatically decreases Vmax and increases the A0.5 for citrate. Phosphorylation at Ser77 and Ser1200 by cyclic-AMP-dependent protein kinase causes more modest changes in the A0.5 for citrate and the Vmax. Dephosphorylation, or removal of the amino-terminal region containing Ser77/79 using trypsin, reverses all of these effects. 6. These results suggest that the effects of the AMP-activated protein kinase on acetyl-CoA carboxylase activity are mediated entirely by phosphorylation of Ser79, and not Ser1200 and Ser1215. The smaller effects of cyclic-AMP-dependent protein kinase are mediated by phosphorylation of Ser77.

Effects of the tumour promoter okadaic acid on intracellular protein phosphorylation and metabolism

Okadaic acid is a polyether derivative of 38-carbon fatty acid, and is implicated as the causative agent of diarrhetic shellfish poisoning. It is a potent tumour promoter that is not an activator of protein kinase C, but is a powerful inhibitor of protein phosphatases-1 and -2A (PP1 and PP2A) in vitro. We report here that okadaic acid rapidly stimulates protein phosphorylation in intact cells, and behaves like a specific protein phosphatase inhibitor in a variety of metabolic processes. Our results indicate that PP1 and PP2A are the dominant protein phosphatases acting on a wide range of phosphoproteins in vivo. We also find that okadaic acid mimics the effect of insulin on glucose transport in adipocytes, which suggests that this process is stimulated by a serine/threonine phosphorylation event.