review article diabetes andthe role of inositol-containing

Molecular Medicine 5: 505-514, 1999Molecular Medicine© 1999 The Picower Institute Press

Review Article

Diabetes and the Role of Inositol-ContainingLipids in Insulin Signaling

David R. Jones' and Isabel Varela-Nieto2'Department of Immunology and Oncology, Centro Nacional deBiotecnologia, Consejo Superior de Investigaciones Cientificas,Cantoblanco, Madrid, Spain2Department of Cell Signaling, Instituto de InvestigacionesBiomedicas, Consejo Superior de Investigaciones Cientificas, Madrid,SpainAccepted May 18, 1999.

Abstract

Among metabolic diseases, diabetes is consideredone of the most prevalent throughout the world.Currently, statistics show that over 10% of theworld's aged population (60 years and older) suffersfrom diabetes. As a consequence, it consumes aconsiderable proportion of world health expendi-ture. This review considers both past and currentresearch into the molecular basis of insulin resis-tance found in type II diabetes and focuses on therole of inositol-containing phospholipid metabo-lism. It has been firmly established that the activa-tion of phosphatidylinositol 3-kinase (PI3-K) is im-portant for the propagation of the metabolic actionsof insulin. In addition to the 3-phosphorylatedphosphatidylinositols formed via the action ofPI3-K, the glycosyl-phosphatidylinositol/inositol

phosphoglycan (GPI/IPG) signaling component isalso strongly implicated in mediating numerousmetabolic actions of insulin. Although all the ele-ments within the type II diabetes phenotype havenot been fully defined, it has been proposed thatdefects in insulin transmembrane signaling throughmalfunction of inositol-containing phospholipidmetabolism and absenteeism of the generation ofphospholipid-derived second messengers may be as-sociated with the appearance of the type II diabeticphenotype. Pharmaceutical approaches using syn-thetically produced IPG analogues, which them-selves mimic insulin's actions, alone or in combina-tion with other drugs, may lead the way towardintroducing alternative therapies for type II diabetesin the coming years.

IntroductionDiabetes mellitus is the term used to define theabsolute or partial impairment of insulin action

Address correspondence and reprint requests to: Dr. DavidR. Jones, Laboratory 414, Centro Nacional de BiotecnologfaC.S.I.C., Universidad Aut6noma de Madrid, Campus deCantoblanco, 28049 Madrid, Spain. Phone: 34-91-585-4665; Fax: 34-91-372-0493; E-mail: [email protected]

on the cells of target tissues that leads to abnor-mally elevated blood glucose concentrations. Apositive diagnosis of diabetes is made when afasting plasma glucose level is >7.8 mM or aplasma glucose level is >1 1.1 mM during an oralglucose tolerance test (measured at two timepoints, before 2 hr and at 2 hr) (1-3). The con-sequence of such impairment of insulin functionis the tendency toward the onset of chronic com-

506 Molecular Medicine, Volume 5, Number 8, August 1999

plications such as premature cardiovascular dis-ease, retinopathy, neuropathy, nephropathy,and macrovascular disease (4,5). When no treat-ment against diabetes is administered, severe or-gan damage gradually leads to the death of af-flicted subjects.

There are two types of diabetes. Type I dia-betes (also termed insulin-dependent diabetesmellitus) is the result of autoimmune destructionof the pancreatic beta cells, which gives rise to adeficiency in insulin secretion (6-8). To circum-vent this lack of insulin secretion, insulin is usu-ally self-administered subcutaneously to main-tain an approximately normal blood glucoseconcentration. Type II diabetes (also known asnon-insulin-dependent diabetes mellitus) iscaused by both insulin deficiency and insulinresistance. This insulin resistance, particularlywithin skeletal muscle, liver, and adipose tissue,leads to pronounced hyperglycemia as glucosetransport into cells is severely reduced (3,9). De-spite the fact that the etiology of type II diabetesis less well understood than that of type I, avail-able evidence points to both genetic and nutri-tional/behavioral factors (type of diet, obesity,physical exercise) as possible regulators of theonset of type II diabetes (10, 11).

Molecular Mechanisms of InsulinAction: Via Proteins andPhospholipidsInsulin delivers its signal through binding to itshigh-affinity cell surface receptor. This protein isa tetrameric complex consisting of two a and two,B subunits. The intracellular domains of the 1Bsubunits contain intrinsic protein tyrosine kinaseactivity and are involved in the initiation of in-sulin-dependent transmembrane signalingevents. The early signal transduction events in-volve the autophosphorylation of its receptor ontyrosine residues and of the insulin receptor sub-strates 1, 2, and 3 (IRS-1, -2, and -3), leading tothe generation of docking sites on both the insu-lin receptor and the IRS proteins for SH2 andSH3 domain-containing proteins. These pro-teins, which include both enzymes and adapters,initiate a multitude of downstream signals thatregulate the phosphorylation state of a wide va-riety of proteins and some phospholipids (12-14).

Despite our vast knowledge of the role ofenhanced protein phosphorylation/dephosphor-ylation in response to insulin, relatively little is

known about insulin's effects on phospholipidmetabolism and its possible role in explaininginsulin resistance in type II diabetes. The connec-tion between insulin-mediated signal transduc-tion and phospholipids received its breakthroughin 1986 when Cuatrecasas' laboratory providedevidence that the previously described "insulin-stimulated water-soluble modulators of enzymeactivity" were in fact derived from glycolipids(15). This led to the rapid appearance of numer-ous reports within the literature confirming thatinsulin, and other growth factors and classicalhormones, stimulated the hydrolysis of similarglycolipids (glycosyl-phosphatidylinositol, nowtermed GPI) generating water-soluble secondmessengers termed inositol phosphoglycan (IPG)(16,17). This process, drawn schematically inFigure 1, indicates GPI phospholipid hydrolysisleading to the generation of IPG. As interest inthe GPI/IPG signaling system during insulin sig-nal transduction grew, investigators in diabeticresearch attempted to understand the metaboli-cally altered phenotype by considering phospho-lipid-derived second messengers and their role inthe control of intermediary metabolism (18).

IPG: A Requirement for FunctionalMetabolic Actions of Insulin?Positive insights into a relationship between theinsulin GPI/IPG signaling system and type II di-abetes soon became apparent after examiningdifferences between insulin-stimulated GPI hy-drolysis in cells isolated from diabetic and normalrats. Three independent investigations revealedthat, in addition to less GPI being present, insu-lin-stimulated GPI hydrolysis in cells exhibitinginsulin resistance was absent or severely im-paired (19-21). IPG, the product of GPI hydro-lysis, is considered a potential insulin secondmessenger on the basis of three lines of evidence:(1) by itself it can mimic a large number of themetabolic actions of insulin (both in vivo and invitro, see later and Table 1), (2) anti-IPG anti-bodies can attenuate several in vivo actions ofinsulin, and (3) synthetically produced chemicalIPG analogues exhibit in vitro and in vivo insu-lin-like activities (see later and ref. 17). The firstof these three lines of evidence has been widelyused to investigate differences in gluconeogene-sis and lipogenesis between normal and diabeticexperimental models. In doing so, the resultshave underlined the importance of this novelinsulin-stimulated signaling pathway. At least

D. R. Jones and I. Varela-Nieto: Insulin-Signaling in Diabetes 507

PLD

PLC

°4w2I01 2

OH

IPG

IPG-1,2-(cydic phosphate)

PA

OH

DAG

Fig. 1. Structure and hydrolysis of a GPI phos-pholipid by phospholipases D (PLD) and C(PLC) generating IPG type A and PA, and IPG-1,2-(cyclic phosphate) type A and DAG. Origi-nally it was proposed that the responsible enzymewas a PLC, however, recent studies suggest that PLDis also capable of generating biologically active IPGtype A from GPI (24,25). Ins, myo-inositol; GlcN,glucosamine; PA, phosphatidic acid; DAG, diacylglycol.

two types of IPG exist. The structures of the IPGshave been refined but not totally elucidated. IPGtype A contains myo-inositol and glucosamine(22), whereas IPG type P contains a-pinitol (3-0-methyl D-chiro-inositol) and galactosamine as

core components (23). Both types of IPG havebeen shown to additionally contain neutral sugarsand phosphate residues. The origin of IPG type A isthought to be myo-inositol-containing GPI, as bothphospholipase C (PLC)- and phospholipase D(PLD)-mediated hydrolysis of GPI yield biologicallyactive IPG molecules (17,22,24,25). The same can-

not be said, however, for IPG type P, as importantdifferences are evident. Stereochemically, the

favored configuration of chiro-inositol within phos-phoinositides for phospholipase C-mediated hy-drolysis is that of the L-diastereomer (26). How-ever, Lamer's group has claimed that IPG type Pcontains D-chiro-inositol (23,27), thereby putting indoubt its phospholipid origin as D-chiro-inositol-containing phosphoinositide-specific PLCs havethus far remained elusive. Within a few years ofthe publication of a preliminary structure of GPI,a synthetic chemical analogue of IPG type A (1-D-6-0- (2-amino-2-deoxy-a-D-glucopyranosyl) -myo-inositol 1,2-(cyclic phosphate)) was reported. Thiscompound exhibited both insulin-mimetic proper-ties (stimulation of lipogenesis in rat adipocytes,ref. 28) and insulin-like growth factor I (IGF-I)-mimetic properties (stimulated mitogenesis andJun and Fos oncogene expression in the developingchicken inner ear, refs. 29-31; I. Varela-Nieto,D. R. Jones, Y. Leon, H. N. Caro, T. W. Radema-cher, unpublished observations). It appears thatthe myo-inositol 1,2-(cyclic phosphate) portion ofthe above IPG type-A analogue is important, as(1 -D-6-0-(2-amino-2-deoxy-a-D-glucopyranosyl) -myo-inositol 1-phosphate is an inactive IPG type-Aanalogue (32). The opening of a myo-inositol 1,2-(cyclic phosphate) moiety to form a myo-inositol1-phosphate within IPG type A could be a methodfor the cessation of its biological activity in vivo.This decyclization could be achieved by the actionof the enzyme cyclic inositol phosphohydrolase(33). In addition to I-D-6-0-(2-amino-2-deoxy-a-D-glucopyranosyl) -myo-inositol 1,2- (cyclic phos-phate), the synthesis of 6-0-(2-amino-2-deoxy-a-D-glucopyranosyl)-D-chiro-inositol- 1-phosphate, ananalogue of IPG type P, has been reported. Thislatter compound also shows both insulin-mi-metic (34) and distinct IGF-I-mimetic (e.g., dif-ferentiation of the cochleovestibular ganglion inthe developing chicken inner ear, ref. 35) at-tributes. A variation on the structure of theabove-mentioned IPG type-P analogue has re-cently been reported. The pseudodisaccharideconsisting of a-pinitol linked to galactosamine,termed INS-2, is able to stimulate testosteroneproduction in human thecal cells to an extentsimilar to that of insulin (36,37). Furthermore,synthetic inositol-glycan moieties mimic insulinaction, as discussed in detail below. An inspec-tion of the biological properties of IPG type-Aand type-P analogues reveals that they showcontrasting biological effects that correspond tothose of purified IPG subtypes (17,38,39). Theseobservations reinforce the claim that IPG is a truesecond messenger and is essential in many met-abolic, mitogenic, and differentiation processes.


Table 1. Insulin mimetic effects of IPG type A (A), IPG type P (P), and their analogues from differentsources

Whole Cells Cell Extracts

Enzymatic Activity orBiological Activity Effect Phosphorylation Effect

Lipolysis (A, A*, A')Lipogenesis (A, A*, A')GPAT (A*, A')Phospholipid methyltransferase (A)Steroidogenesis (A, P*)Glucose transport (A, A*, A")GLUT4 translocation (A*, A')Acetyl-CoA carboxylase (A)Glycogen phosphorylase a (A)Pyruvate kinase (A)Glucose oxidation (A)Glucose production (A)Lactate accumulation (A)Glycogen synthesis (A, P, A*, A')Glycogen synthase kinase-3 (A*, A")Tyrosine aminotransferase (A)Protein phosphorylation (A, A*, A")

cAMP levels (A)PI 3-kinase (A*, A-)Myelin basic protein kinase (A)Mitogen activated kinase (A')Protein kinase B phosphorylation (A')Fructose-2,6-P2 levels (A)Ion channels (A)Ca2+_Mg2+ ATPase (A)Ca2+ entry (A)Amino acid transport (A)Protein synthesis (A, A')Specific mRNA levels (A)

DNA and RNA synthesis (A)Cell proliferation (A, P)Insulin secretion (A)Cell differentiation (neurogenesis) (P)

InhibitionStimulationStimulationInhibitionStimulationStimulationStimulationStimulationInhibitionStimulationStimulationInhibitionStimulationStimulationInhibitionNo effectIStimulationInhibitionInhibitionStimulationStimulationStimulationStimulationStimulationModulationStimulationInhibitionStimulationStimulationIStimulationInhibitionStimulationStimulationInhibitionStimulation

cAMP phosphodiesterase (A)Pyruvate dehydrogenase (A)PDH phosphatase (P)Glucose-6-phosphatase (A)Fructose-1,6-biphosphatase (A)Adenylate cyclase (A)cAMP-kinase (A)Casein kinase II (A)Glycerol-3P acyltransferase (P)ATP citrate lyase (A)Galactolipid sulfotransferase (A)Protein phosphatase 2C (P)

Adapted and updated from Jones and Varela-Nieto (17). A*, A phosphoinositolglycan-peptide (PIG-P) (57,58,61); A', variouschemically synthesized PIG-P analogues (38,39); GPAT, glycerol-3-phosphate acyltransferase P*, an analog of IPG type P (INS-2)consisting of a-pinitol and galactosamine (36) PDH, pyruvate dehydrogenase.

Despite the uncertainty of the precursor

and/or origin of IPG type P, the role of chiro-inositol and IPG type P in type II diabetes has

been the subject of multiple and enlighteninginvestigations. Kennington and colleagues re-

ported that in patients suffering from type II

StimulationStimulationStimulationInhibitionInhibitionInhibitionInhibitionBiphasicStimulationStimulationInhibitionStimulation

D. R. Jones and I. Varela-Nieto: Insulin-Signaling in Diabetes

diabetes and in diabetic Rhesus monkeys therewas a decreased urinary excretion of chiro-inosi-tol compared to that seen in healthy controls.Furthermore, chiro-inositol in IPG type-P prepa-rations from type II diabetic human biopsy mus-cle tissue was undetectable (40). A follow-upreport by Asplin and co-workers indicated that intype II diabetics both the amount of IPG type Pand its bioactivity were reduced in hemodiasy-late, urine, and muscle compared to normal sub-jects (41). These and other reports (which haveincluded results of the analysis of inositol isomerexcretion and of IPG preparations from diabeticsof differing nationalities) tentatively suggest thata decrease in chiro-inositol and IPG type-P bio-synthesis may be underlying features of type IIdiabetes and that the urinary chiro-inositol-to-myo-inositol ratio could be considered a clinicalmarker for predicting the onset of diabetes at anearly stage (42). Another direct report indicatingthe abnormality of IPG type-P production in di-abetics is that from Shaskin and colleagues (43),whose results clearly indicate that in a group ofinsulin-resistant subjects, glucose ingestion failedto increase the serum concentration of IPG typeP compared to that seen in healthy controls. Pakand colleagues have addressed the metabolism ofboth myo- and chiro-inositol in the diabetic Goto-Kakizaki rat compared to its normal healthycounterpart (44). Their findings indicate thatthere exist significant differences in the in vivointerconversion of the inositol isomers in theirfree unconjugated form and at the phosphoino-sitide level between the two rat phenotypes. Thisclearly underlines the importance of correct ino-sitol isomer homeostasis, particularly in insulin-sensitive tissues.

The two biochemical assays principally usedto define the biological activity of the IPG sub-types are the IPG type A-mediated inhibition ofprotein kinase A and the stimulation of pyruvatedehydrogenase phosphatase by IPG type P. Asmentioned previously, IPG molecules possess in-sulin-mimetic properties. These properties,which have been extracted from more than adecade of research literature, are listed inTable 1. Clearly, in most of the insulin-mimeticobservations, IPG type A was used because itcould be prepared through the in vitro hydrolysisof purified GPI from various sources (22,45).Also, IPG type P was perhaps viewed at first withcaution, as less information was available con-cerning its structure and its source was unde-fined, thereby limiting its use in experimentalstudies. As techniques have improved for the

purification of both types of IPG, during the last5 years, exciting evidence has shed new light onthe role of IPG subtypes in the mechanism ofinsulin action. The first in vivo effects of IPG inanimal models showed that both IPG subtypeswere able to stimulate glycogen production inthe diaphragm of normal rats and transientlyreduce the elevated blood glucose in streptozo-tocin-diabetic rats (46). This later observationwas reinvestigated using prolonged intravenousdelivery of IPG type P, which was able to nor-malize the plasma glucose level in diabetic rats toan extent to that observed with insulin (23). Twofurther papers from Larner's laboratory have in-dicated that insulin-dependent regulation of glu-cose metabolism through IPG production is in-deed more complicated than first thought. Thisclaim is substantiated by the finding that in theplasma of type II diabetics there exists an ele-vated (compared to control subjects) concentra-tion of an antagonist of insulin-stimulated glu-cose oxidation (47). Structural informationconcerning this antagonist revealed that it con-tained myo-inositol 1,2-(cyclic phosphate), galac-tosamine, and mannose residues, indicating at-tributes very similar to those of the two IPGsubtypes identified thus far. The biological activ-ity of this inhibitor was found to be due to theinhibition of IPG type P-stimulated pyruvate de-hydrogenase phosphatase activity by shifting themagnesium ion requirement of the enzyme tothe right (48).

Evidence suggests that the in vivo regulationof IPG production in diabetic subjects must besomehow perturbed, compared to that in healthycontrols. Directly or indirectly, upstream of insu-lin-stimulatable GPI-hydrolyzing enzymes laythe insulin receptor and the IRS proteins. Point-mutated and kinase-deficient insulin receptorsfail to couple to the generation of IPG type Athrough GPI hydrolysis (49,50), which impliesthat in some way, insulin-stimulated tyrosinephosphorylation events control GPI hydrolysis. Itis not known however, if this is due to a lack oftyrosine phosphorylation of a phospholipase orto malfunction of an adapter protein linking thereceptor to such an enzyme involving a tyrosinephosphorylation event. In the case of insulin,this tyrosine-dependent GPI hydrolysis may berelated to IRS protein expression and function. Adecreased level of IRS protein expression andserine/threonine residue hyperphosphorylationof IRS proteins give rise to signals that attenuateinsulin signal transduction and, furthermore, arecharacteristic of insulin-resistant cells (51, 52). In

509


Insulin

2 2 11 2 2 2

II 4l_ = w_

Diabetic state Normal state

Fig. 2. Schematic indicating a possible routethrough which insulin could stimulate IPGtype-A generation in normal insulin-responsivecells. Points within the proposed route at which ablockade would result in the formation of insulin-resistant cells are indicated. (1) Insulin receptor pro-tein tyrosine kinase (point-mutated, kinase domain-deleted, serine/threonine hyperphosphorylation).(2) IRS proteins (expression levels, phosphorylation

5 I

1t

I I

I I

Metabolic effects

state). (3) Phospholipases C (PLC) or D (PLD) (ex-pression levels, coupling). (4) GPI in the plasmamembrane (quantity, localization). (5) IPG type-Atransporter/receptor IPG type-A signal "transducingprotein" (expression levels, functionality). The opensymbols represents IPG derived from GPI-PLD-medi-ated GPI hydrolysis. The closed symbols representIPG derived from GPI-PLC-mediated GPI hydrolysis.

addition, and equally intriguing, in IRS-2-knock-out mice the type II diabetic phenotype was ob-served, suggesting that insulin signaling throughthis molecule is important for avoiding extracel-lular hyperglycemia (53).

One of the signal transduction pathways thatis rapidly initiated by insulin is the activation ofPI3-K through its interaction with the IRS pro-teins (54,55). The intricacies of this signalingpathway are further complicated by the very re-cent finding that insulin-dependent activation ofP13-K may be compartmentalized within cells.This subcellular activation of P13-K in distinct cellcompartments appears to be dependent on thelocalized expression of IRS proteins, whose pat-tern is determined by the metabolic status of thecell; this, in turn, is inferred by the diet of theanimal (56). Interestingly, an IPG type A-likeglycopeptide termed PIG-P, isolated from Saccha-romyces cerevisiae, was able to elevate both IRS- 1-associated P13-K activity and the tyrosine phos-phorylation state of IRS- 1 without increasinginsulin receptor phosphorylation, indicating aclear bypass of the insulin receptor (57). This wasa significant advance, in that it described a de-

fined signal transduction pathway for PIG-P,which led to a physiological response: the trans-location of GLUT4 glucose transporters to theplasma membrane for the uptake of glucose fromthe extracellular space (58). These GLUT4 glu-cose transporters may be localized in membranecaveolae (59), from which GPI and IPG havebeen isolated (60). The mechanism by whichPIG-P exerts its insulin mimetic properties hasbeen investigated. A membrane-spanning pro-tein labile to high-concentration salt, tryptic ac-tion, and N-ethylmaleimide may act as a "trans-ducer protein" with the cooperation of accessoryproteins, including caveolin, to cause the activa-tion of a cytoplasmic tyrosine kinase that in turncould phosphorylate the IRS proteins, therebyleading to the early activation of PI3-K. ThisPI3-K activation may represent the point atwhich insulin-stimulated signal transduction viaits receptor and PIG-P action through the "trans-ducer protein" converge (61). A series of back-to-back reports immediately followed which ex-amined the structural requirements for PIG-Pbiological activity through the use of an exten-sive range of chemically synthesized analogues.

- 'V3

D. R. Jones and I. Varela-Nieto: Insulin-Signaling in Diabetes

Interestingly, 1 -D-6-0- (2-amino-2-deoxy-a-D-glucopyranosyl) -myo-inositol 1,2- (cyclic phos-phate) -containing analogues exhibited the great-est insulin-mimetic properties in rat adipocytesand diaphragms (stimulation of P13-K, stimula-tion of PKB/Akt, stimulation of glucose trans-port, inhibition of glycogen synthase kinase-3activity, stimulation of glycogen synthase, stim-ulation of the MAP kinase pathway, and stimu-lation of protein synthesis), which underline theimportance of this structure (38,39). Earlierstudies with an IPG-like molecule (similar to thatisolated from Saccharomyces cerevisiae) derivedfrom the GPI anchor of the variant surface gly-coprotein of Trypanosoma brucei demonstrated in-sulin-mimetic characteristics (inhibition of lipol-ysis in rat adipocytes and gluconeogenesis inhepatocytes) (62). This compound, again resem-bling that of IPG type A, was subsequentlyshown to cause the activation of the proteinserine/threonine phosphatase type 1 in rat adi-pocytes (63). New light has recently been shedon the mechanism of insulin-mediated activa-tion of protein serine/threonine phosphatases.Results concerning insulin-mediated activationof glycogen synthase and inhibition of glycogenphosphorylase in young, lean, adult Rhesusmonkey liver have implicated the involvementof activated protein serine/threonine phospha-tases types 1 and 2C. These activations may bemediated by IPG type P (64,65). At the molecularlevel, activation of protein phosphatase type 1may be more complex because of the dual con-trol exerted by both IPG type A and type P. Whenthreonine phosphorylated, DARRP-32 andINH- 1 were potent inhibitors of protein phos-phatase type 1. Very recently, IPG type A wasfound to inhibit the PKA-mediated phosphory-lation of DARRP- 32 and INH- 1, whereas IPGtype P was observed to directly stimulate proteinphosphatase-2C, thereby causing the dephos-phorylation of phosphorylated DARRP-32 andINH- 1 (66). With all the information available inthe literature, we were able to tentatively align apathway of molecular events, starting at the in-sulin receptor and culminating at the point ofIPG generation in normal insulin-responsivecells. This chain of events is illustrated inFigure 2. Within the proposed scheme, possiblepoints, including the insulin receptor protein ty-rosine kinase, IRS proteins, phospholipase type C(or D), membrane GPI, and the IPG transporteror IPG "transducer protein" at which defectscould exist, are indicated. A malfunction of any

of these components could be associated with theonset of type II diabetes.

Closing RemarksWithout doubt, insulin signaling is a complexevent that involves the divergence of multiplepathways downstream from its activated recep-tor. Some of these signaling cascades converge tocontrol enzymes directly involved in intermedi-ary metabolism, whereas others lead to the con-trol of protein translation (67). Although theroles of IPG types A and P are not completelyunderstood, we believe that they play importantroles in insulin signaling. Perhaps the most directevidence comes from the findings that, if theenzyme responsible for the synthesis of myo-ino-sitol-containing GPI is knocked out, then insulinfails to stimulate glycogen, suggesting that thepresence of the GPI/IPG signaling system is aprerequisite for the process in cells that use IPGtype A for glycogen production (68). However,because IPG type P has been shown to stimulateglycogen production in vivo in other cell types(23,46), it seems plausible to suggest that thereare important cell-to-cell type differences in boththe generation and utilization of IPG subtypes tomediate, in some cases, the same metabolic ef-fects (36,69; I. Varela-Nieto, D. R. Jones, Y. Leon,H. N. Caro, T. W. Rademacher, unpublished ob-servations). In terms of metabolic control, insulinelicits many of its effects through the activationof PI3-K (54,5 5). In light of recent work in whichinsulin-mimetic PIG-P and synthetic inositol-gly-can analogues have been shown to activate var-ious enzymes through the obligatory participa-tion of P13-K, it could be possible that the type IIdiabetic phenotype may manifest itself when thelevels of P13-K or its functionality are belowthose in normal models (70-72).

It is hoped that in the near future, furtherinsights into the GPI/IPG signaling pathway willemerge to allow us to extend our knowledge ofhow diabetes causes its unwelcomed aberationsof intermediary metabolism and, in particular,how chemically defined agents, such as IPG an-alogues, may provide a possible starting point forthe design of drugs to combat type II diabetes.

AcknowledgmentsWe thank Dr. J.E. Feliu for critical reading of thismanuscript and for helpful suggestions. D.R.J.

511


holds a postdoctoral fellowship from the Associ-ation for International Cancer Research. The De-partment of Immunology and Oncology wasfounded and is supported by the C.S.I.C. andPharmacia and Upjohn. This study was sup-ported by grants from Direccion General de In-vestigacion, Ciencia y Tecnologia PM96-0075,and Europharma (Boehringer Ingelheim group)to I.V.-N.

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