Proteolytic release of human angiotensin-converting enzyme expressed in Chinese hamster ovary cells is enhanced by phorbol ester

Localization of an N-domain region of angiotensin-converting enzyme involved in the regulation of ectodomain shedding using monoclonal antibodies

ACE chimeric proteins and N domain monoclonal antibodies (mAbs) were used to determine the influence of the N domain, and particular regions thereof, on the rate of ACE ectodomain shedding. Somatic ACE (having both N and C domains) was shed at a rate of 20%/24 h. Deletion of the C domain of somatic ACE generated an N domain construct (ACEDeltaC) which demonstrated the lowest rate of shedding (12%). However, deletion of the N domain of somatic ACE (ACEDeltaN) dramatically increased shedding (212%). Testicular ACE (tACE) having 36 amino acid residues (heavily O-glycosylated) at the N-terminus of the C domain shows a 4-fold decrease in the rate of shedding (49%) compared to that of ACEDeltaN. When the N-terminal region of the C domain was replaced with the corresponding homologous 141 amino acids of the N domain (N-delACE) the rate of shedding of the ACEDeltaN was only slightly decreased (174%), but shedding was still 3.5-fold more efficient than wild-type testicular ACE. Monoclonal antibodies specific for distinct, but overlapping, N-domain epitopes altered the rate of ACE shedding. The mAb 3G8 decreased the rate of shedding by 30%, whereas mAbs 9B9 and 3A5 stimulated ACE shedding 2- to 4-fold. Epitope mapping of these mAbs in conjunction with a homology model of ACE N domain structure, localized a region in the N-domain that may play a role in determining the relatively low rate of shedding of somatic ACE from the cell surface.

Phorbol 12-myristate 13-acetate-induced ectodomain shedding and phosphorylation of the human meprinbeta metalloprotease

Shedding of proteins localized at the cell surface is an important regulatory step in the function of many of these proteins. Human meprin (N-benzoyl-l-tyrosyl-p-aminobenzoic acid hydrolase, PPH, EC 3.4.24.18) a zinc-metalloendopeptidase of the astacin family is an oligomeric protein complex of alpha- and beta-subunits and is expressed abundantly in the intestine and kidney as well as in leukocytes of the lamina propria and in cancer cells. In transfected cells intracellular proteolytic removal of the membrane anchor results in the secretion of the meprin alpha-subunit. In rats and mice, the beta-subunit exists in a membrane-anchored form. In contrast, human meprinbeta is constitutively converted into a secretable form. We now show that phorbol 12-myristate 13-acetate (PMA) stimulates an increased release of hmeprinbeta from transfected COS-1 cells, whereas hmeprinalpha secretion is not influenced. This stimulatory effect is inhibited by the protein kinase C (PKC) inhibitor staurosporine, suggesting that activation of PKC mediates PMA-induced hmeprinbeta shedding. The use of different protease inhibitors shows that two different metalloprotease activities are responsible for the constitutive and the PMA-stimulated hmeprinbeta shedding. We identified tumor necrosis factor alpha-converting enzyme (TACE or ADAM17) as the protease that mediates the PMA-induced release. We also demonstrate that hmeprinbeta is phosphorylated by PMA treatment on Ser687 within a PKC consensus sequence in the cytosolic domain of the protein. This phosphorylation of hmeprinbeta is not, however, implicated in the enhanced secretion by PMA treatment.

CK2 phosphorylates the angiotensin-converting enzyme and regulates its retention in the endothelial cell plasma membrane

Soluble angiotensin-converting enzyme (ACE) is derived from the membrane-bound form by proteolytic cleavage of its C-terminal domain. Because intracellular events might be involved in the regulation of the cleavage process, we determined whether the cytoplasmic tail of ACE is phosphorylated and whether this process regulates secretion. Immunoprecipitation of ACE (180 kDa) from (32)P-labeled endothelial cells revealed that ACE is phosphorylated. Phosphorylation was not observed in endothelial cells overexpressing a mutant form of ACE (ACEDeltaS, all five cytoplasmic serine residues replaced by alanine). CK2 coprecipitated with ACE from endothelial cells, and CK2 phosphorylated both ACE and a peptide corresponding to the cytoplasmic tail. Mutation of serine(1270) within the CK2 consensus sequence almost abolished ACE phosphorylation. In ACE-overexpressing endothelial cells, ACE was mostly localized to the plasma membrane. However, no ACE was detected in the plasma membrane of ACEDeltaS-overexpressing cells, although a precursor ACE (170 kDa) was prominent in the endoplasmic reticulum and the cell supernatant contained substantial amounts of the soluble protein (175 kDa). A correlation between ACE-phosphorylation and secretion was confirmed in endothelial cells treated with the CK2-inhibitor, 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole, which time-dependently decreased the phosphorylation of ACE and increased its shedding. These results indicate that the CK2-mediated phosphorylation of ACE regulates its retention in the plasma membrane and may determine plasma ACE levels.

Epitope-specific antibody-induced cleavage of angiotensin-converting enzyme from the cell surface

Angiotensin I-converting enzyme (ACE; CD143, EC 3.4.15.1) is a type-1 integral membrane protein that can also be released into extracellular fluids (such as plasma, and seminal and cerebrospinal fluids) as a soluble enzyme following cleavage mediated by an unidentified protease(s), referred to as ACE secretase, in a process known as "shedding". The effects of monoclonal antibodies (mAbs) to eight different epitopes on the N-terminal domain of ACE on shedding was investigated using Chinese hamster ovary cells (CHO cells) expressing an ACE transgene and using human umbilical vein endothelial cells. Antibody-induced shedding of ACE was strongly epitope-specific: most of the antibodies increased the shedding by 20-40%, mAbs 9B9 and 3A5 increased the shedding by 270 and 410% respectively, whereas binding of mAb 3G8 decreased ACE shedding by 36%. The ACE released following mAb treatment lacked a hydrophobic transmembrane domain anchor. The antibody-induced shedding was completely inhibited at 4 degrees C and by zinc chelation using 1,10-phenanthroline, suggesting involvement of a metalloprotease in this process. A hydroxamate-based metalloprotease inhibitor (batimastat, BB-94) was 15 times more efficacious in inhibiting mAb-induced ACE shedding than basal (constitutive) ACE release. Treatment of CHO-ACE cells with BB-94 more effectively prevented elevation in antibody-dependent (but not basal) ACE release induced by 3,4-dichloroisocoumarin and iodoacetamide. These data suggest that different secretases might be responsible for ACE release under basal compared with antibody-induced shedding. Further experiments with more than 40 protease inhibitors suggest that calpains, furin and the proteasome may participate in this process.

Induction of angiotensin I-converting enzyme transcription by a protein kinase C-dependent mechanism in human endothelial cells

Angiotensin I-converting enzyme (ACE) has been implicated in various cardiovascular diseases; however, little is known about the ACE gene regulation in endothelial cells. We have investigated the effect of the protein kinase C activator phorbol 12-myristate 13-acetate (PMA) on ACE activity and gene expression in human umbilical vein endothelial cells (HUVEC). Our results showed a 3- and 5-fold increase in ACE activity in the medium and in the cells, respectively, after 24-h stimulation by PMA. We also observed an increase in the cellular ACE mRNA content starting after 6 h and reaching a 10-fold increase at 24 h in response to 100 ng/ml PMA as measured by ribonuclease protection assay. This effect was mediated by an increased transcription of the ACE gene as demonstrated by nuclear run-on experiments and nearly abolished by the specific PKC inhibitor GF 109203X. Our results indicate that PMA-activated PKC strongly increases ACE mRNA level and ACE gene transcription in HUVEC, an effect associated with an increased ACE secretion. A role for early growth response factor-1 (Egr-1) as a factor regulating ACE gene expression is suggested by both the presence of an Egr-1-responsive element in the proximal portion of the ACE promoter and the kinetics of the Egr-1 mRNA increase in HUVEC treated with PMA.

Proteolytic release of membrane proteins: studies on a membrane-protein-solubilizing activity in CHO cells

Diverse membrane proteins are solubilized by a specific proteolytic cleavage in the stalk sequence adjacent to the membrane anchor, with release of the extracellular domain. Examples are the amyloid precursor protein, membrane-bound growth factors and angiotensin-converting enzyme (ACE). The identities and characteristics of the responsible proteases remain elusive. We have studied this process in Chinese hamster ovary (CHO) cells stably expressing wild-type ACE (WT-ACE) or juxtamembrane (stalk) deletion or chimaera mutants. Determination of the C termini (i.e. the cleavage sites) of released, soluble wild-type and mutant ACE by MALDI-TOF mass spectrometry indicated that the membrane-protein-solubilizing protease (MPSP) in CHO cells is not constrained by a particular cleavage site motif or by a specific distance from the membrane, but instead may position itself with respect to the putative proximal, folded extracellular domain adjacent to the stalk. Nevertheless, kinetic analyses of release rates indicated that a minimum distance from the membrane must be preserved. Interestingly, soluble full-length (anchor-plus) WT-ACE incubated with fractions of, or intact, CHO cells was not cleaved. In all cases, release was stimulated by a media change or by the addition of phorbol ester, with rate enhancements of 5- and 50-fold, respectively, for WT-ACE. The phorbol ester effect was abolished by staurosporine, a protein kinase C (PKC) inhibitor. We propose that the CHO cell MPSP that solubilizes ACE: (1) only cleaves proteins embedded in a membrane; (2) requires an accessible stalk and cleaves at a minimum distance from both the membrane and proximal extracellular domain; (3) positions itself primarily with respect to the proximal extracellular domain and (4) is regulated in part by a PKC-dependent mechanism.

Genetic factors in lung disease. Part II: Lung cancer and angiotensin converting enzyme gene

The recent progress in molecular biology has led to the elucidation of pathogenesis of lung cancer. The development of a lung cancer requires multiple genetic changes, consisting of the activation of oncogenes, including the K-ras and myc genes, and of inactivation of tumour suppressor genes, including the Rb, p53 and CDKN2 genes. Knowing the specific genes undergoing such changes should be useful as biomarkers for the early detection of cells destined to become malignant. Moreover, such genetic changes could be targets of newly designed drugs and gene-based therapy. Although the angiotensin I-converting enzyme was originally discovered in equine plasma, it has been recognized in various organs and cells other than vascular endothelial cells. This enzyme is also known to have wide substrate specificity to many peptides. The definite roles of angiotensin converting enzyme (ACE) in the respiratory system are largely unknown. Recent progress in molecular biology of the ACE, however, gives us a good chance to look over the significance of ACE in respiratory diseases as well as cardiovascular disorders. In this review, we show the recent advances in the basic studies of the ACE and refer to its clinical application.

Cleavage-secretion of angiotensin I-converting enzyme in yeast

Angiotensin I-converting enzyme (ACE) is a type I transmembrane protein composed of two domains (N and C domains) which undergoes a post-translational proteolytic cleavage in mammalian cells to release the soluble ectodomain. The protease involved in ACE cleavage-secretion (ACE-secretase) is not well characterised and eludes isolation: the presence of a yeast homologue, thus more amenable to genetic manipulation, would facilitate its identification. We have expressed a secreted form of the ACE C domain, lacking the C-terminal membrane anchor (C domain(deltaCOOH)), and the membrane-anchored C domain (C domain) in the yeast Pichia pastoris by fusion to prepro-alpha-factor. Immunofluorescent labelling localises the ACE C domain to the periphery of yeast cells but not C domain(deltaCOOH), however, expression of both C domain and C domain(deltaCOOH) produced soluble enzymes in the culture medium. Immunocharacterisation of the two soluble forms of the C domain indicates a proteolytic cleavage of the membrane-bound C domain to produce the soluble counterpart. Thus ACE undergoes a proteolytic cleavage in yeast.

Membrane protein secretases

A diverse range of membrane proteins of Type 1 or Type II topology also occur as a circulating, soluble form. These soluble forms are often derived from the membrane form by proteolysis by a group of enzymes referred to collectively as 'secretases' or 'sheddases'. The cleavage generally occurs close to the extracellular face of the membrane, releasing physiologically active protein. This secretion process also provides a mechanism for down-regulating the protein at the cell surface. Examples of such post-translational proteolysis are seen in the Alzheimer's amyloid precursor protein, the vasoregulatory enzyme angiotensin converting enzyme, transforming growth factor-alpha, the tumour necrosis factor ligand and receptor superfamilies, certain cytokine receptors, and others. Since the proteins concerned are involved in pathophysiological processes such as neurodegeneration, apoptosis, oncogenesis and inflammation, the secretases could provide novel therapeutic targets. Recent characterization of these individual secretases has revealed common features, particularly sensitivity to certain metalloprotease inhibitors and upregulation of activity by phorbol esters. It is therefore likely that a closely related family of metallosecretases controls the surface expression of multiple integral membrane proteins. Current knowledge of the various secretases are compared in this Review, and strategies for cell-free assays of such proteases are outlined as a prelude to their ultimate purification and cloning.

Metabolism of Bradykinin by Peptidases in Health and Disease

This chapter provides an overview of the metabolism of bradykinin (BK) by peptidases in health and disease. The enzymatic breakdown of kinins affects the duration of their biological actions as the plasma half-life of intravenously injected BK is in the range of seconds. Kinins are cleaved in vitro and in vivo by enzymes that belong to families, such as zinc-metallopeptidases, astacin-like metallopeptidases, and catheptic enzymes. Vane noted the importance of the pulmonary circulation in the metabolism of vasoactive substances, such as BK as well as angiotensin 1 and 5- hydroxytryptamine. It is clear after decades of research that angiotensin 1-converting enzyme (ACE) on the vascular endothelial cell surface is the most important inactivator of blood-borne BK. BK may act primarily in an autocrine and paracrine fashion, establishing the importance of local regulation of its activity by enzymes on cell surfaces. Thus, the assortment of other enzymes that can inactivate BK is important in a variety of physiological and pathological situations. Most physiological systems have redundant pathways of metabolism so that the abolishment of one pathway is compensated for by the presence of others. This is demonstrated by the pharmacological inhibition of ACE in hypertension.

Phosphorylation of the cytosolic domain of peptidylglycine alpha-amidating monooxygenase

Peptidylglycine alpha-amidating monooxygenase (PAM) is a bifunctional enzyme that catalyzes the COOH-terminal alpha-amidation of neural and endocrine peptides through a two-step reaction carried out sequentially by its monooxygenase and lyase domains. PAM occurs in soluble and integral membrane forms. Metabolic labeling of stably transfected hEK-293 and AtT-20 cells showed that [32P]PO4(3-) was efficiently incorporated into Ser and Thr residues of membrane PAM but not into soluble PAM. Truncation of integral membrane PAM proteins (which terminate with Ser976) at Tyr936 eliminated their phosphorylation, suggesting that the COOH-terminal region of the protein was the site of phosphorylation. Recombinant PAM COOH-terminal domain was phosphorylated on Ser932 and Ser937 by protein kinase C (PKC). PAM-1 protein recovered from different subcellular fractions of stably transfected AtT-20 cells was differentially susceptible to calcium-dependent, staurosporine-inhibitable phosphorylation catalyzed by endogenous cytosolic protein kinase(s). Although phorbol ester treatment of hEK-293 cells expressing PAM-1 stimulated the cleavage/release of a bifunctional 105-kDa PAM protein, the effect was an indirect one since it was also observed in hEK-293 cells expressing a truncated PAM-1 protein that was not phosphorylated. AtT-20 cells expressing PAM-1 lacking one of the PKC sites (PAM-1/Ser937-->Ala) exhibited an altered pattern of PAM.PAM antibody internalization, with the mutant protein targeted to lysosomes upon internalization. Thus, phosphorylation of Ser937 in the COOH-terminal cytosolic domain of membrane PAM plays a role in a specific step in the targeting of this protein.