Clinical Science (2000) 99, 93–104 (Printed in Great Britain)
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`A guardian angel: the involvement
`of dipeptidyl peptidase IV in
`psychoneuroendocrine function,
`nutrition and immune defence
`
`Martin HILDEBRANDT*, Werner REUTTER†, Petra ARCK*, Matthias ROSE*
`and Burghard F. KLAPP*
`*Charite! Campus Virchow-Klinikum, Medizinische Fakulta$ t der Humboldt-Universita$ t zu Berlin, Medizinische Klinik m.S.
`Psychosomatik, Augustenburgerplatz 1, D-13353 Berlin, Germany, and †Fachbereich Humanmedizin der Freien Universita$ t
`Berlin, Institut fu$ r Molekularbiologie und Biochemie, Arnimallee 22, D-14195 Berlin, Germany
`
`A B S T R A C T
`
`Dipeptidyl peptidase IV (DPP IV, also known as CD26 ; EC 3.4.14.5) is a non-integrin receptor
`glycoprotein with multiple functions, including cell adhesion, cellular trafficking through the
`extracellular matrix and co-stimulatory potential during T cell activation. By virtue of its
`exopeptidase activity, DPP IV plays a key regulatory role in the metabolism of peptide
`hormones. Based on data emerging from different biomedical specialties, it appears worthwhile
`to highlight the different facets of DPP IV in nutrition, immune responses and peptide hormone
`metabolism. The presentation of the complex regulatory circuits in which DPP IV appears to be
`involved may also serve as a note of caution, in view of attempts to apply selective inhibitors of
`DPP IV enzymic activity for the treatment of disease, e.g. Type II diabetes.
`
`INTRODUCTION
`
`Dipeptidyl peptidase IV (DPP IV; EC 3.4.14.5) has a
`unique enzymic activity, cleaving dipeptides from pep-
`tides and proteins carrying proline in their penultimate
`position, a feature which protects peptides from being
`digested by non-specific proteases [1]. DPP IV is
`associated with the plasma membrane of a variety of cells,
`including the venous portion of capillary endothelial cells
`[2], hepatocytes [3,4], enterocytes [5,6] and cells of the
`renal glomeruli and proximal tubules [7,8]. Expression of
`DPP IV defines a higher degree of cell maturation and
`differentiation [9,10]. The functional specificity of DPP
`IV is defined by the site of DPP IV expression and the
`substrates available [11]. Organ-specific functions and
`regulatory circuits appear to have an impact on DPP
`IV expression, such as a proline-rich diet in the intes-
`tinal epithelium [12], interferon-c in kidney [13] and
`
`antigenic [14,15] or mitogenic [16] stimulation of T
`lymphocytes.
`Since its first description in 1966 [17], DPP IV served
`primarily as a target for studies in membrane protein
`biochemistry. DPP IV was shown to be an example for
`mechanisms of membrane protein turnover
`[4,18],
`glycosylation events [19,20], membrane polarization [21]
`and organ-specific differences in the regulation of protein
`expression [22]. Reports on an involvement of DPP IV in
`cell adhesion [23] and immune function [24] allowed a
`first glimpse of the many biological processes in which
`DPP IV appears to be involved.
`
`SUBSTRATES
`
`Many neuropeptides, immunopeptides and peptide hor-
`mones share the feature of having proline residues at
`specific positions in their sequence, which fulfill two
`
`Key words: dipeptidyl peptidase IV, nutrition, peptide hormone metabolism, psychiatric disorders.
`Abbreviations: ADA, adenosine deaminase; DPP IV, dipeptidyl peptidase IV; NPY, neuropeptide Y; PEP, prolyl endopeptidase.
`Correspondence: Dr Martin Hildebrandt, Department of Internal Medicine, Charite! Campus Virchow-Klinikum, Medical Faculty
`of the Humboldt University Berlin, Augustenburger Platz 1, D-13353 Berlin, Germany (e-mail hildebra!charite.de).
`
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`Table 1 Known substrates for DPP IV
`
`Substrate
`
`Effect of modulation by DPP IV
`
`Consequences
`
`Psychoneuroendocrine implications
`
`Growth hormone releasing factor [25,31]
`
`Degradation
`
`Glucagon-like peptide 1 [32], glucagon-like
`peptide 2 [33] and gastric inhibitory
`peptide [26]
`
`Degradation
`
`Procolipase [34]
`
`Partial activation
`
`Fibrinogen a chain [35]
`
`Kentsin [36]
`
`Hydrolysis
`
`Degradation
`
`Decreased induction of growth hormone
`release
`
`Inhibition of energy consumption;
`catabolic effect
`
`Loss of potent insulinotropic and blood
`glucose-normalizing effect
`
`Catabolic (and diabetogenic)
`effect
`
`Breakdown of lipids in the digestive
`tract; release of enterostatin
`
`Modulation of satiety
`
`Inhibition of fibrinogen polymerization
`
`Relevance unknown
`
`Enhanced nociception
`
`Loss of (a) anovulatory effect, (b)
`inhibitory effect on intestinal transit,
`and (c) potent analgetic effect (opiate-
`receptor independent, naloxone-
`sensitive)
`
`Enterostatin [37,38]
`
`Degradation and inactivation
`
`Loss of inhibitory effect on caloric intake
`
`Inhibition of satiety
`
`Human chorionic gonadotropin [39]
`
`Degradation
`
`Unknown
`
`Modulation of satiety
`
`N-Procalcitonin [40]
`
`Trypsinogen [34]
`
`Potent bone-cell mitogen
`
`Unknown; presumably inactivation
`
`Degradation
`
`Unknown
`
`Unknown
`
`Unknown
`
`Table 2 Substrates for DPP IV in inflammatory responses
`tumour necrosis factor-a ;
`interleukin; TNF-a,
`Abbreviations: SDF-1a, stromal-cell-derived factor 1a ; CXCR, C-X-C chemokine receptor type 4 ;
`IL,
`interferon-inducible protein 10 ; RANTES, regulated on activation, normal T cell expressed and secreted ; CCR, C-C chemokine receptor.
`
`IP-10,
`
`Effect of modulation by DPP IV
`
`Consequences
`
`Immunological implications
`
`Substrate
`
`SDF-1a [60]
`
`Degradation to SDF-1a-(3–68)
`
`Monocyte chemotactic protein [64]
`
`Degradation, inactivation
`
`Loss of monocyte chemotactic fuction
`
`RANTES [63,65]
`
`Altered receptor specifity: RANTES-
`(3–68) does not bind to CCR1, but
`still binds to CCR5; no increase in
`cytosolic Ca2+ in monocytes
`
`Inhibition of monocyte chemotaxis with
`simultaneous enhancement of T cell
`migration
`
`major tasks. First, they determine the properties of the
`secondary structure of the peptides, necessary for their
`biological activity, e.g. membrane passage, receptor
`
`binding. Secondly, these residues serve as cleavage points
`for proline-specific peptidases such as DPP IV [1]. In
`consequence, modification of peptide substrates [25] and
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`Lymphotoxin (aa 1–5), IL-2 fragments,
`murine IL-6 (aa 1–12), IL-1 (aa 1–6)
`[61]
`
`Eotaxin [62]
`
`TNF-a [56]
`
`Degradation
`
`Inactivation
`
`Degradation
`
`IP-10 [63]
`
`Inactivation
`
`Loss of lymphocyte chemotactic activity;
`SDF-1a-(3–68) blocks effect of intact
`SDF-1a by occupying CXCR-4
`
`Abolished competition with full-length
`peptide for receptor binding (not
`proven)
`
`Modulation of lymphocyte chemotaxis;
`inhibition of HIV entry via CXCR-4
`
`Reversed inhibitory effect on
`inflammatory response (not proven)
`
`Th2-chemokine; loss of ability to attract
`eosinophils
`
`Inhibition of inflammatory Th2
`responses
`
`Monocytes; main source of TNF-a; main
`target for interferon-c secretion by
`CD26-positive cells
`
`Loss of chemotactic ability for CD4-
`positive T cells
`
`Inhibition of monocyte participation in
`inflammatory responses
`
`Inhibition of chemotactic attraction of
`CD4-positive cells to inflammatory
`sites of the skin
`
`Inhibition of monocyte participation in
`inflammatory responses
`
`RANTES-(3–68) is a chemotaxis
`inhibitor, but protects monocytes
`from cytopathic effects of HIV-1
`infection
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`Multiple functions of dipeptidyl peptidase IV
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`95
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`the use of inhibitors of DPP IV [26] have both been
`shown to prolong the biological half-lives of substrates,
`with potential clinical and pharmaceutical implications
`[27]. The number of substrates for DPP IV is even larger
`when a joint effect of DPP IV and other peptidases, such
`as aminopeptidase N (CD13), is taken into account,
`leading to the hydrolysis of peptides carrying proline at
`the third or a later position from the N-terminus. For
`instance, sequential N-terminal degradation of brady-
`kinin has been proven to involve DPP IV [28,29]. In
`Table 1, potent bioactive peptides that are metabolized
`primarily by DPP IV are listed. An overview of im-
`munologically relevant substrates for DPP IV (Table
`2) further expands the presumptive role of DPP IV in
`immune responses to a complex immunomodulatory
`function.
`
`INVOLVEMENT IN IMMUNE FUNCTION
`
`The role of DPP IV in immune function has been
`reviewed in detail [41–43]; only some aspects shall be
`addressed here. The T lymphocyte antigen, CD26, has
`been shown to have DPP IV activity. High expression of
`the antigen defines a distinct subset of T lymphocytes
`with memory cell capacity [43,44]. In a human umbilical
`cord endothelial cell monolayer model, CD26bright
`lymphocytes predominantly transmigrate monolayers
`without a chemokine gradient, in contrast to CD26-
`negative cells [45,46]. Given the memory cell phenotype
`of these cells, early tissue invasion may be important for
`the initiation of inflammatory processes wherever ap-
`propriate.
`Like other ectopeptidases involved in immunologically
`relevant functions, such as CD10 and CD13 (amino-
`peptidase N), the expression of CD26}DPP IV is strict-
`ly developmentally regulated [47]. During thymocyte
`maturation, CD26-associated enzymic activity is onto-
`genically controlled and may be involved in thymic
`deletion of emerging T cell clones [9]. Surface CD26
`antigen expression is important for T cell activation and
`co-stimulation. Since CD26 has only a six-amino-acid
`membrane-anchoring domain, signal transduction must
`be mediated by other cell membrane components. In fact,
`CD26 has been shown to co-precipitate the tyrosine
`kinase CD45 [43]. Other authors suggest that CD26-
`mediated signal transduction occurs via the CD3 (T cell
`receptor-associated complex) f chain [48]. These data
`suggest a complex interaction between CD26, CD45 and
`the CD3 f chain, as discussed [49,50]. Only CD26-
`positive T lymphocytes appear to be capable of pro-
`ducing interferon-c [51]. DPP IV enzymic activity is
`capable of augmenting the cellular responses of CD26-
`transfected Jurkat cells to external stimuli mediated by
`CD26 and}or the CD3–T-cell-receptor complex, leading
`
`to enhanced interleukin-2 production [52]. However,
`the enzymic activity is not mandatory for T cell activation
`via CD26 [53]. CD26 serves as the membrane-anchoring
`protein for ecto-adenosine deaminase (ADA) [54] which,
`in addition to its cell-protective effect of detoxifying
`extracellular adenosine or 2«-deoxyadenosine, interacts
`with different cell surface proteins [55]. The capacity of
`CD26 to bind to ADA further adds to the importance
`of this membrane antigen in T cell protection, adhesion
`and activation.
`Monocytes have been reported to have a surface
`peptidase with a substrate specificity and sensitivity to
`inhibitors of enzymic activity identical with those of
`DPP IV [56]. However, monocytes and cells from the
`monocytic cell line U937 known to carry the enzyme
`were not detected by two antibodies known to recognize
`DPP IV. The nature of this ‘DPP IV-like enzyme’ [56]
`remains to be elucidated. The presence of DPP IV-like
`enzymic activity on the surface of monocytes involves
`DPP IV in the degradation of components of the
`extracellular matrix, implying a role for DPP IV in tissue
`invasion. In addition, Bauvois and colleagues [56] also
`showed degradation of tumour necrosis factor-a by a
`DPP IV-like enzyme and tripeptidyl endopeptidase on
`monocytes, which are the main source of this cytokine
`[57,58]. Interestingly, surface expression of DPP IV on
`lymphoblastic HL-60 cells is enhanced upon cytokine-
`induced differentiation into macrophages, but lost upon
`differentiation into neutrophils [59], suggesting selective
`expression of DPP IV on macrophages, with potential
`relevance for tissue invasion as pointed out above.
`
`CLINICAL ASPECTS OF DPP IV
`
`Inflammatory/autoimmune diseases and
`AIDS
`In cases of allograft rejection, the number of CD26-
`positive lymphocytes and DPP IV activity in serum
`showed sharp increases that were reversible by immuno-
`suppression. Inhibition of DPP IV enzymic activity led
`to a delay in allograft rejection [66]. In patients with
`systemic lupus erythematosus, DPP IV activity in serum
`was shown to be markedly decreased, with DPP IV
`activity on lymphocytes only being decreased in patients
`with active disease [67]. Similar observations were made
`in animal models of systemic lupus erythematosus [68].
`In synovial fluid from patients with rheumatoid arthritis,
`DPP IV activity showed a decrease, while the activities of
`other peptidases, namely proline endopeptidase and
`lysosomal dipeptidyl peptidase II, were increased [69].
`Specific inhibition of DPP IV activity suppressed
`alkylamine- and adjuvant-induced arthritis
`[70,71],
`pointing to a role for DPP IV enzymic activity in the
`pathogenesis of experimentally induced arthritis. Plas-
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`Table 3 DPP IV in disease
`
`Disease/condition
`
`Allograft rejection [79]
`Systemic lupus erythematosus [67]
`
`Rheumatoid arthritis [69]
`Pregnancy [80]
`AIDS [78]
`
`Major depression [81]
`Schizophrenia [82]
`Fibromyalgia [83]
`Anorexia nervosa [84]
`
`DPP IV activity
`# in serum
`$ in serum in all patients
`$ in lymphocytes of patients
`with active disease
`$ in synovial fluid
`$ in serum
`Normal
`
`$ in serum
`$ in serum
`Normal
`# in serum
`
`DPP IV/CD26 expression
`# on lymphocytes
`No data available
`
`No data available
`No data available
`$ no. of DPP IV-positive
`lymphocytes
`No data available
`No data available
`No data available
`$ no. of DPP IV-positive
`lymphocytes
`
`minogen and streptokinase have both been shown to
`bind to DPP IV expressed on rheumatoid synovial
`fibroblasts [72]. Interestingly, fibronectin, a ligand for
`DPP IV [73,74], competes with streptokinase, since both
`proteins bind to DPP IV via the amino acid sequence
`Lys-Thr-Ser-Arg-Pro-Ala, common to both ligands [72].
`Binding of streptokinase to DPP IV resulted in a rise in
`intracellular calcium in fibroblasts and in concomitant
`plasminogen activation. Thus the role of DPP IV in the
`pathogenesis of arthritis is not only confined to its
`enzymic activity.
`In the course of HIV infection, the surface expression
`of the HIV envelope protein gp120}gp41 complex is not
`only responsible for the initiation of cell-to-cell mem-
`brane fusion leading to the formation of syncytia, but
`also initiates apoptosis in CD4-positive cells. Jacotot et
`al. [75] showed that increased expression of CD26 on
`CD4-positive T cells led to an enhanced induction of
`apoptosis by the gp120}gp41 complex. Apparently,
`signalling via CD26, usually leading to T cell activation
`[76],
`is modified following HIV infection,
`involving
`CD26 in the mechanism of triggering apoptosis. In
`contrast, transfection studies using wild-type CD26 and
`mutant CD26 devoid of DPP IV activity [77] suggested
`that
`the presence of DPP IV activity reduces the
`efficiency of HIV infection, whereas the absence of DPP
`IV activity correlates with a higher susceptibility to
`apoptosis, apparently due to an enhanced expression of
`CD95 (Apo-1}Fas). The overall number of CD26-
`positive memory T cells has been shown to be signifi-
`cantly lower in HIV-infected subjects [78], which may be
`the result of apoptotic death induced by the gp120}gp41
`complex. Interestingly, DPP IV activity in the serum of
`these patients was normal, allowing for the hydrolysis of
`RANTES (regulated on activation, normal T cell
`expressed and secreted) and SDF-1a (stromal-cell-de-
`rived factor 1a) mentioned above and, consequently,
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`resulting in a protective effect against HIV entry. This,
`however, remains speculative and awaits further results.
`Table 3 gives an overview of the changes in DPP IV
`activity and expression observed in various diseases,
`including psychiatric disorders.
`
`PSYCHOMODULATORY ASPECTS
`
`Substrates for DPP IV
`Many of the peptide hormones and proteins that have
`been shown (or are assumed) to be substrates for DPP IV
`have, in fact, been at the centre of psychoneuroendocrine
`research in past years (for a review, see [85]). Taking the
`potency of peptide hormones such as neuropeptide Y
`(NPY) or Substance P into account, the impact of DPP
`IV on their biological activity and, similarly, changes in
`DPP IV activity in some psychiatric or psychosomatic
`diseases may have been underestimated in recent years. In
`addition to the substrates described in Tables 1 and 2,
`those with primarily neuroendocrine and}or psycho-
`modulatory function are listed in Table 4.
`
`Changes in DPP IV serum activity in
`psychiatric disorders
`The analysis of DPP IV activity in sera from patients with
`psychiatric or psychosomatic disorders has revealed
`distinct changes in a variety of diseases. However, these
`changes are, in some cases, difficult to appreciate and may
`be subject to misinterpretation.
`
`Diseases in which a decreased serum activity of DPP IV has
`been shown
`Maes et al. [81] have performed extensive studies on DPP
`IV activity in sera from patients with major depression
`and schizophrenia. These patients showed a decrease in
`
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`Multiple functions of dipeptidyl peptidase IV
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`Table 4 Substrates for DPP IV with neuroendocrine and psychomodulatory function
`
`Substrate
`
`Endomorphin-1 [86]
`
`b-Casomorphin [87,88]
`
`Effect of modulation by DPP IV
`
`Degradation and inactivation
`
`Degradation and inactivation
`
`Kentsin [36]
`
`Degradation
`
`Peptide YY [89], NPY
`(analogue of peptide Y)
`
`Modulation of receptor specificity/loss of Y1-receptor-
`mediated functions
`
`Substrate P [91]
`
`Degradation to a more potent heptapeptide
`
`Consequences
`
`Loss of potent l-agonistic effect
`
`Loss of analgesic (naloxone-sensitive) and stimulatory effect on
`dietary intake
`
`Loss of (a) anovulatory effect, (b) inhibitory effect on intestinal
`transit, and (c) potent analgetic effect (opiate-receptor
`independent, naloxone-sensitive)
`
`Phase-shift in the endogenous circadian rhythm of thalamic
`neurons; blood pressure recovery during endotoxic and
`haemorrhagic shock [90]
`
`More profound effect on transmission of nociception, depression of
`blood pressure and relaxation of smooth muscle
`
`serum DPP IV activity as compared with healthy
`controls, a change that was apparently independent of
`anti-depressants and anti-psychotic drugs [82]. The
`assumption that these changes may reflect a certain
`degree of immunosuppression was not correlated with
`changes in lymphocyte subsets or altered lymphocyte
`transformation tests.
`
`Diseases with increased activity of DPP IV in serum
`Patients with hyporectic eating disorders show an in-
`crease in DPP IV serum activity and a decrease in the
`proportion of peripheral blood lymphocytes expressing
`CD26 [84]. This finding sheds new light on the changes
`of immune function in patients with eating disorders,
`with regard to the notion that patients with eating
`disorders, especially anorexia nervosa, often remain
`immunocompetent [92].
`
`Diseases without alterations in DPP IV activity in serum
`In fibromyalgia, changes in DPP IV serum activity,
`presumed to exist because of (a) the presence of de-
`pressive symptoms and (b) the role of DPP IV in the
`degradation of collagen and other components of the
`extracellular matrix, could not be observed [83]. Rather,
`the serum activity of another peptidase, prolyl endo-
`peptidase (PEP), was shown to be decreased in patients
`with fibromyalgia. Since PEP is known to be involved in
`post-proline cleavage, with Substance P as a substrate, the
`authors concluded that a decreased serum activity of PEP
`may be related to aberrant pain perception and depressive
`symptoms. The same authors showed a higher PEP
`serum activity related to stress-induced anxiety, whereas
`DPP IV activity in serum was not altered [93]. In contrast
`with DPP IV, PEP in serum has been reported not to
`share its membrane-bound counterpart’s substrate spe-
`cificity [94]. Thus changes in DPP IV activity may be
`clinically more relevant for the metabolism of Substance
`P than changes in PEP activity in serum.
`
`NUTRITIONAL ASPECTS
`
`DPP IV degrades peptides and proteins to small peptides
`and amino acids that are suitable for transport and re-
`utilization. Degradation by DPP IV represents a rate-
`limiting step for the intestinal and renal transport of
`proline-containing peptides. Enzymes such as trypsino-
`gen and procolipase are among the many substrates
`described. The insulinotropic hormone, glucagon-like
`peptide 1, has multifaceted actions, which include stimu-
`lation of insulin gene expression, trophic effects on the b-
`cells,
`inhibition of glucagon secretion, promotion of
`satiety, inhibition of food intake and slowing of gastric
`emptying, all of which contribute to normalization of
`elevated glucose levels. By deactivation of glucose-
`dependent
`insulinotropic polypeptide
`[26,95]
`and
`glucagon-like peptide 1 [96], DPP IV abolishes their
`potent insulinotropic effects, so that their activity in
`serum lasts only a few minutes. For this reason, the use of
`inhibitors specific for DPP IV enzymic activity has been
`proposed as a novel strategy to treat Type II diabetes
`[96]. The participation of DPP IV in the reabsorption of
`proline-containing di- and tri-peptides from the renal
`proximal tubuli [97] may be regarded as a safeguard
`mechanism to recover proline.
`NPY is one of the most potent orexigenic peptide
`hormones [98–100] and a known substrate for DPP IV.
`The modulation of receptor specificity for NPY after
`degradation by DPP IV [89] deserves special attention
`with regard to nutritional control. The orexigenic effect
`of NPY appears to be mediated by hypothalamic
`receptors of subtypes Y1 and Y5 [101,102]. Conse-
`quently, the altered receptor specificity of NPY after
`degradation by DPP IV may alter the influence of NPY
`on appetite and satiety. Similarly, the enhanced gastric
`motility induced by binding of NPY to NPY1 receptors
`[103] would be abrogated after peptide degradation by
`DPP IV.
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`SOLUBLE DPP IV
`
`Origin and function
`DPP IV activity is detectable in serum [104], urine,
`seminal plasma [105] and amniotic fluid. The origin of
`soluble DPP IV is not completely understood, although
`inflammatory and malignant processes in tissues ex-
`pressing high amounts of DPP IV have been shown along
`with increased DPP IV activity in serum, probably due to
`tissue disruption. A release or cleavage of membrane-
`bound DPP IV from outer membranes by an active and
`regulated mechanism, although unproven, has been
`suggested [106,107]. Comparison of DPP IV purified
`from human serum showed that, apart from the N-
`terminal membrane-anchoring domain that was missing
`from DPP IV from serum, DPP IV from serum and
`kidneywereindistinguishableimmunologically,enzymo-
`logically and with regard to the capacity to bind ADA
`[104].
`Studies of
`cultured peripheral blood T
`lymphocytes from patients with oral cancer [108] have
`led to the assumption that DPP IV from activated T
`lymphocytes is shed into the bloodstream. In hepatoma-
`bearing rats, a loss of DPP IV from the hepatoma plasma
`membrane paralleled an increase in DPP IV serum
`activity, suggesting a transfer of DPP IV into the serum
`[109]. On the other hand, decreased surface expression
`and altered protein processing, resulting in premature
`degradation, may account for the loss of membrane-
`bound DPP IV as well. Studies on lymphoblastic HL-60
`cells suggested that a decrease in DPP IV expression is
`
`not associated with increased cleavage of DPP IV into the
`culture medium [59]. To date, there is no report that
`definitely proves the origin of DPP IV in serum from the
`membrane-bound form of any tissue.
`An analysis of biological variations in serum activity of
`DPP IV has revealed a low intra- and inter-individual
`variability of DPP IV serum activity, a slightly higher
`activity of DPP IV in serum in summer, and an overall
`low amplitude of changes in DPP IV activity in the
`course of 1 year [110]. However, significant changes may
`develop rapidly, for example a decrease in DPP IV serum
`activity within the first weeks of pregnancy (M.
`Hildebrandt, unpublished work).
`
`Function of DPP IV in serum
`Due to the presence of a soluble form of DPP IV in
`serum, any peptide circulating in the blood carrying
`proline in the penultimate N-terminal position is a
`candidate substrate for DPP IV and will be metabolized
`within minutes, resulting in activation, inactivation or
`modulation of its biological effect. Although serum DPP
`IV, like membrane-bound DPP IV, has been shown to
`bind ADA [104], we assume that the main function of
`soluble DPP IV is probably exerted via its enzymic
`activity. Table 5 lists potential substrates for DPP IV; in
`addition to the substrates listed in Tables 1, 2 and 4, these
`may help to elucidate the complex functions of DPP IV
`enzymic activity and, consequently, the broad impli-
`cations that changes in DPP IV activity may have.
`
`Table 5 Potential substrates for DPP IV among bioactive peptides
`Abbreviations: ACTH, adrenocorticotrophic hormone (corticotropin); CLIP, corticotropin-like intermediary peptide; MIF, melanocyte-stimulating hormone release-
`inhibiting factor 1.
`
`Function
`
`Effect of metabolism by DPP IV
`
`Expression in raphe nuclei: stimulates paradoxical sleep and
`enhances excitability in the hippocampal CA1 region; inhibits
`b-endorphin-triggered prolactin release
`
`Unknown; equal effects of N-terminal CLIP
`fragments 18–24 and 20–24 (result of
`metabolism by DPP IV) on paradoxical sleep
`
`Substrate
`
`ACTH-(18–39) (CLIP) [111]
`
`Gastrin-releasing peptide [39]
`
`Pancreatic polypeptide [112]
`
`Tyr-[Trp2]MIF-1, Tyr-MIF-1,
`Tyr-[Lys2]MIF-1 [113]
`
`Morphiceptin [114]
`
`Substance P-(3–11) [115]
`
`Gastrin release; insulin release; involved in pituitary hormone
`secretion; increased secretion of ACTH via corticotropin releasing
`hormone; suppression of glucose intake
`
`Pancreatic polypeptide is able to raise glucocorticoid secretion
`acting directly on the inner adrenocortical cells
`
`Bioactive peptides isolated from brain tissue; Tyr-MIF-1 and Tyr-
`[Trp2]MIF-1 bind to l opiate receptors: potent analgetic effect
`l Opioid receptor agonist
`
`More potent than Substance P
`
`Inactivation
`
`Inactivation
`
`Inactivation
`
`Inactivation
`
`Unknown
`
`Unknown
`
`Unknown; presumably inactivation
`
`Unknown; presumably inactivation
`
`Brain natriuretic peptide-32 [116]
`
`Diuretic/natriuretic and vasorelaxant peptide
`
`Gly-Pro-Arg-Pro [117]
`
`Inhibitor of fibrin polymerization
`
`His-Pro-Phe-His-Leu-D-Leu-Val-Tyr [118]
`
`Renin inhibitor
`
`# 2000
`
`The
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`Biochemical
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`Society
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`and
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`the Medical
`
`Research
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`Society
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`Page 6 of 12
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`
`Multiple functions of dipeptidyl peptidase IV
`
`99
`
`DPP IV-LIKE PEPTIDASES
`
`Other peptidases have been described that have DPP IV-
`like substrate specificity, T cell activation capacity and
`sensitivity towards DPP IV-specific inhibitors. Duke
`Cohan and colleagues [119] isolated a monomeric form
`of serum DPP IV of molecular mass 175 kDa, in contrast
`with the size of 105–110 kDa reported previously. This
`large serum DPP IV-like peptidase was shown to express
`functional DPP IV activity, with substrate and inhibitor
`specificities and a pH–activity profile similar to those of
`CD26. Analysis of peptides after limiting proteolysis and
`N-terminal sequencing revealed no identity with CD26,
`but
`some identity with other peptidases. Unlike
`CD26, the serum form did not bind ADA-1. The con-
`servation of DPP IV activity and five epitopes specific
`to recombinant soluble CD26 suggest, however, signifi-
`cant structural similarity. Of note, a 60 kDa DPP IV
`protein fragment has been described by several groups
`[120], which has been shown conclusively to represent a
`part of DPP IV protein and which may associate with
`DPP IV to form a 175 kDa protein complex.
`More recently, two enzymes with post-proline cleav-
`ing activity and a catalytic triad similar to that of DPP IV
`have been described. Quiescent cell proline dipeptidase is
`expressed intracellularly in CD26-negative lymphocytic
`cell lines and non-lymphocytic cell lines also, and acts
`over a broad pH range [121]. A 100 kDa protein with
`DPP IV-like activity, termed N-acetylated a-linked
`acidic dipeptidase-like protein, has been isolated from rat
`and human ileum [122,123]. Another member of the same
`gene family that also possesses DPP IV-like activity, N-
`acetylated a-linked acidic dipeptidase II, is expressed in
`testis, ovary and distinct areas of the brain [123]. DPP IV-
`b, an 82 kDa protein with CD26-like DPP activity, has
`been described [124,125]. DPP IV-b and CD26 differ in
`their sensitivity towards inhibitors of DPP IV, allowing
`their distinction. Taken together,
`it appears that,
`in
`addition to the evolutionary conservation of DPP IV
`structure, some redundancy of DPP IV function has been
`provided.
`
`OUTLOOK: TOWARDS A MORE
`GENERALIZED VIEW OF DPP IV ENZYMIC
`ACTIVITY
`
`‘…if we break up a living organism by isolating its
`different parts it is only for the sake of ease in analysis and
`by no means in order to consider them separately. Indeed
`when we wish to ascribe to a physiological quality its
`value and true significance we must always refer it to its
`whole and draw our final conclusions only in relation to
`the effects in the whole’ (Claude Bernard).
`How are we to understand the physiological role of
`DPP IV enzymic activity? In fact, some aspects of DPP
`
`IV appear contradictory: some substrates of DPP IV
`exert an effect that directly opposes the effects of other
`substrates of DPP IV. Examples of this include the
`regulation of fat intake, natriuresis and the chemotactic
`ability of immunocompetent cells. However, this view
`does not take into account the fact that these antagonizing
`substrates may differ in (a) speed of metabolism by DPP
`IV, (b) biological importance, and (c) tissue- or organ-
`specific role.
`Some aspects of DPP IV function and substrates,
`however, appear to be clearly defined. Based on the
`effects and substrates described, we assume that DPP IV
`activity enhances nociception by inactivation of potent
`analgesic l-opiate receptor agonists and by processing
`Substance P to a more potent derivative, a process which
`may best be described as an ‘arousal reaction’ and which
`may involve additional circuits via other substrates for
`DPP IV. Furthermore, an elevation of DPP IV activity
`would exert an immunoprotective effect, mainly via
`expansion of T cell activation, but also by an inhibitory
`effect on corticosteroid release. Based on the exper-
`imental and clinical data available, the apparently con-
`tradictory effects of T cell proliferation and immuno-
`protection on the one hand, but chemokine inactivation
`on the other, may be explained by a bimodal action of
`DPP IV in immune function: immune reactions that have
`been initiated by other mechanisms are supported by
`DPP IV enzymic activity, while other immune reactions
`are rather reduced, thus focusing the immunosurveillance
`on processes that are already under way. In fact, in vitro
`studies that deliberately address the effect of inhibition of
`DPP IV on T cell proliferation, thus studying secondary
`responses upon stimulation, may imitate an already active
`inflammatory process rather than a baseline immune
`response. Furthermore, cleavage of chemokines such as
`eotaxin [62] suggests an inhibition of Th2-like cytokine
`responses by DPP IV activity. This adds to the ob-
`servation that high expression of DPP IV}CD26 defines
`a Th1}Th0 phenotype among T cells and correlates with
`enhanced production of Th1-like cytokines, namely
`interferon-c [51]. The net effect of DPP IV activity would
`be a shift towards a Th1 cytokine response, in part
`attributable to a degradation of cytokines involved in
`Th2-like responses.
`The inactivation of potent insulinotropic peptides by
`DPP IV, together with an increase in glucose intake due
`to inactivation of gastrin-releasing peptide, would result
`in an enhanced availability of glucose in blood. We
`assume that an increase in DPP IV activity would result
`in an overall decreased digestive activity, which, together
`with the enhanced availability of blood glucose, may be a
`nutritional status required for an organism’s ‘state of
`alert’. These rather catabolic effects of DPP IV match
`well with the assumption that anabolic effects of insulino-
`tropic peptides, but also of growth hormone releasing
`factor and other mitogens, are abolished by DPP IV.
`
`# 2000
`
`The
`
`Biochemical
`
`Society
`
`and
`
`the Medical
`
`Research
`
`Society
`
`Page 7 of 12
`
`

`
`100
`
`M. Hildebrandt and others
`
`Nutrition:
` enhanced glucose availability
` catabolic nutritional state by:
`• decreased insulinotropic peptides
`• higher glucose intake
`• ambivalent (biphasic ?) effect on
` fat intake
`• reduction of anabolic
` influences
`
`DPP IV
`
`Immune function:
` Focused support of
` inflammatory processes
` (Th1-