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The diversity among cyclic nucleotide phosphodiesterases provides multiple mechanisms for regulation of cAMP and cGMP in the cardiovascular system. Here we report that a calmodulin-stimulated phosphodiesterase (PDE1C) is highly expressed in proliferating human arterial smooth muscle cells (SMCs) in primary culture, but not in the quiescent SMCs of intact human aorta. High levels of PDE1C were found in primary cultures of SMCs derived from explants of human newborn and adult aortas, and in SMCs cultured from severe atherosclerotic lesions. PDE1C was the major cAMP hydrolytic activity in these SMCs. PDE expression patterns in primary SMC cultures from monkey and rat aortas were different from those from human cells. In monkey, high expression of PDE1B was found, whereas PDE1C was not detected. In rat SMCs, PDE1A was the only detectable calmodulin-stimulated PDE. These findings suggest that many of the commonly used animal species may not provide good models for studying the roles of PDEs in proliferation of human SMCs. More importantly, the observation that PDE1C is induced only in proliferating SMCs suggests that it may be both an indicator of proliferation and a possible target for treatment of atherosclerosis or restenosis after angioplasty, conditions in which proliferation of arterial SMCs is negatively modulated by cyclic nucleotides. ( J. Clin. Invest. 1997. 100:2611-2621.)

apeutic intervention for diseases such as atherosclerosis, pancreatitis, or dyslipidemia associated with metabolic syndrome or type II diabetes ( 1-3 ). Central to triglyceride metabolism is lipoprotein lipase (LPL), an extracellular enzyme primarily located in the vascular beds of many tissues ( 3,4 ). LPL catalyzes the hydrolysis of the triglyceride component of chylomicrons (CM) and VLDL, which constitute the major forms of triglycerides in plasma ( 3, 5 ). Although LPL is expressed in many different tissues, the enzyme is expressed at high levels in metabolically active tissues, such as adipose, cardiac muscle, and skeletal muscle, where fatty acids released by the action of LPL are stored or used ( 4 ).LPL appears to be regulated by a variety of mechanisms. Several apolipoproteins associated with CM and VLDL, including apolipoprotein CII (APOC2) and apolipoprotein AV (APOA5), stimulate LPL activity ( 6-9 ) apparently by increasing its V max ( 10,11 ). In contrast, apolipoproteins CI (APOC1) and CIII (APOC3) can inhibit LPL activity ( 7,12 ). LPL is inherently unstable and proteins or other factors that either stabilize or destabilize LPL are likely to play a role in regulating its in vivo activity ( 13 ). The active form of LPL exists as a head-totail homodimer, which dissociates into metastable monomers. These monomers can reassociate to form catalytically active LPL or they can undergo conformational changes, forming inactive, stable monomers. The spontaneous in- Our understanding of how triglyceride (TG) metabolism is regulated is essential for designing avenues of ther-

Angiopoietin-like 3 (ANGPTL3) and angiopoietin-like 4 (ANGPTL4) are secreted proteins that regulate triglyceride (TG) metabolism in part by inhibiting lipoprotein lipase (LPL).Recently, we showed that treatment of wild-type mice with monoclonal antibody (mAb) 14D12, specific for ANGPTL4, recapitulated the Angptl4 knock-out (؊/؊) mouse phenotype of reduced serum TG levels. In the present study, we mapped the region of mouse ANGPTL4 recognized by mAb 14D12 to amino acids Gln 29 -His 53 , which we designate as specific epitope 1 (SE1). The 14D12 mAb prevented binding of ANGPTL4 with LPL, consistent with its ability to neutralize the LPL-inhibitory activity of ANGPTL4. Alignment of all angiopoietin family members revealed that a sequence similar to ANGPTL4 SE1 was present only in ANGPTL3, corresponding to amino acids Glu 32 -His 55 . We produced a mouse mAb against this SE1-like region in ANGPTL3. This mAb, designated 5.50.3, inhibited the binding of ANGPTL3 to LPL and neutralized ANGPTL3-mediated inhibition of LPL activity in vitro. Treatment of wild-type as well as hyperlipidemic mice with mAb 5.50.3 resulted in reduced serum TG levels, recapitulating the lipid phenotype found in Angptl3 ؊/؊ mice. These results show that the SE1 region of ANGPTL3 and ANGPTL4 functions as a domain important for binding LPL and inhibiting its activity in vitro and in vivo. Moreover, these results demonstrate that therapeutic antibodies that neutralize ANGPTL4 and ANGPTL3 may be useful for treatment of some forms of hyperlipidemia.Lipoprotein lipase (LPL) 5 plays a pivotal role in lipid metabolism by catalyzing the hydrolysis of plasma triglycerides (TGs).LPL is likely to be regulated by mechanisms that depend on nutritional status and on the tissue in which it is expressed (1-3). Two secreted proteins, angiopoietin-like 3 (ANGPTL3) and angiopoietin-like 4 (ANGPTL4), play important roles in the regulation of LPL activity (4, 5). ANGPTL3 and ANGPTL4 consist of a signal peptide, an N-terminal segment containing coiled-coil domains, and a C-terminal fibrinogen-like domain. The N-terminal segment as well as full-length ANGPTL3 and ANGPTL4 have been shown to inhibit LPL activity, and deletion of the N-terminal segment of ANGPTL3 and ANGPTL4 resulted in total loss of LPL-inhibiting activity (6, 7). These observations clearly indicate that the N-terminal region of ANGPTL4 contains the functional domain that inhibits LPL and affects plasma lipid levels. The coiled-coil domains have been proposed to be responsible for oligomerization (8); however, it is not known whether the coiled-coil domains directly mediate the inhibition of LPL activity.To define the physiological role of ANGPTL4 more clearly, we characterized the pharmacological consequences of ANGPTL4 inhibition in mice treated with the ANGPTL4-neutralizing monoclonal antibody (mAb) 14D12 (9). Injection of mAb 14D12 significantly lowered fasting TG levels in C57BL/6J mice relative to levels in C57BL/6J mice treated with an isotypematched anti-KLH control (KLH) mAb (9). These reduced TG...

The amino acid sequences of all known cGMP-binding phosphodiesterases (PDEs) contain internally homologous repeats (a and b) that are 80 -90 residues in length and are arranged in tandem within the putative cGMPbinding domains. In the bovine lung cGMP-binding, cGMP-specific PDE (cGB-PDE or PDE5A), these repeats span residues 228 -311 (a) and 410 -500 (b). An aspartic acid (residue 289 or 478) that is invariant in repeats a and b of all known cGMP-binding PDEs was changed to alanine by site-directed mutagenesis of cGB-PDE, and wild type (WT) and mutant cGB-PDEs were expressed in COS-7 cells. Purified bovine lung cGB-PDE (native) and WT cGB-PDE displayed identical cGMP-binding kinetics, with ϳ1.8 M cGMP required for half-maximal saturation. The D289A mutant showed decreased affinity for cGMP (K d > 10 M) and the D478A mutant showed increased affinity for cGMP (K d Ϸ 0.5 M) as compared to WT and native cGB-PDE. WT and native cGB-PDE displayed an identical curvilinear profile of cGMP dissociation which was consistent with the presence of distinct slowly dissociating (k off ‫؍‬ 0.26 h ؊1 ) and rapidly dissociating (k off ‫؍‬ 1.00 h ؊1 ) sites of cGMP binding. In contrast, the D289A mutant displayed a single k off ‫؍‬ 1.24 h ؊1 , which was similar to the calculated k off for the fast site of WT and native cGB-PDE, and the D478A mutant displayed a single k off ‫؍‬ 0.29 h ؊1, which was similar to that calculated for the slow site of WT and native cGB-PDE. These results were consistent with the loss of a slow cGMP-binding site in repeat a of the D289A mutant cGB-PDE, and the loss of a fast site in repeat b of the D478A mutant, suggesting that cGB-PDE possesses two distinct cGMP-binding sites located at repeats a and b, with the invariant aspartic acid being crucial for interaction with cGMP at each site. Cyclic nucleotide phosphodiesterases (PDEs)1 constitute a complex family of enzymes which catalyze the hydrolysis of 3Ј:5Ј-cyclic nucleotides to the corresponding nucleoside 5Ј-monophosphates. The multiple PDEs differ in their substrate specificities, sensitivities to inhibitors, modes of regulation, and tissue distributions. Most PDEs are chimeric multidomain proteins, possessing distinct catalytic and regulatory domains (1). A 250-amino acid segment of sequence, which is conserved among all mammalian PDEs and is located in the more carboxyl-terminal portions of the PDE molecules, contains the catalytic site of these enzymes (1-4). Domains of the PDEs which interact with allosteric/regulatory factors are thought to be located within the more amino-terminal regions (1, 5, 6).The cGMP-binding PDEs comprise a heterogeneous subgroup of PDEs, all of which exhibit allosteric cGMP-binding sites that are distinct from the sites of cyclic nucleotide hydrolysis. This group consists of at least three classes of PDEs: the cGMP-stimulated PDEs (cGS-PDEs, or PDE2s 2 ) (7), the photoreceptor PDEs (rod outer segment PDE (ROS-PDE; PDE6A/B) (8) and cone PDE (PDE6C) (9)), and the cGMP-binding, cGMPspecific PDE (cGB-PDE; PDE5A) (10). The s...