CPA4

CPA4 is a secreted zinc metallocarboxypeptidase of the M14A subfamily that is processed from a proenzyme form by endoproteases and preferentially removes hydrophobic C-terminal residues from peptide substrates including neuropeptides such as neurotensin and opioid peptides (PMID:20385563). Structurally, the mature enzyme adopts a canonical α/β-hydrolase fold whose active-site funnel is occluded by the pro-domain in the zymogen and by the endogenous inhibitor latexin upon binding, both employing analogous β-sheet loop contacts (PMID:15738388, PMID:16091843). CPA4 is a maternally imprinted gene at chromosome 7q32 whose transcription is induced downstream of histone deacetylase inhibition through a p21(WAF1/CIP1)-dependent pathway (PMID:10383164, PMID:12552318). In cancer cells, CPA4 promotes proliferation, epithelial–mesenchymal transition, and drug resistance primarily through activation of the PI3K–AKT–mTOR signaling axis, with which it physically associates, and in the tumor microenvironment it participates in a positive feedback loop with M2-polarized macrophages (PMID:32922037, PMID:38666696).

1. 1999 Medium

   Identification of CPA4 as an HDAC-inhibitor-responsive gene downstream of p21 established its transcriptional regulation and predicted it to be a secreted zinc carboxypeptidase, opening the question of its enzymatic specificity and biological function.

   Evidence Differential display, antisense mRNA inhibition, and sequence analysis in prostate cancer cells

   PMID:10383164

   Open questions at the time

   • Enzymatic activity not directly demonstrated
   • Substrate specificity unknown
   • Mechanism linking p21 to CPA4 transcription undefined
2. 2003 Medium

   Demonstration that CPA4 is maternally imprinted at 7q32—uniquely among the CPA gene cluster—suggested a distinct developmental role and dosage sensitivity for this metallocarboxypeptidase.

   Evidence Allele-specific RT-PCR with SNP markers, pyrosequencing, and replication-timing assays across fetal tissues

   PMID:12552318 PMID:12676894

   Open questions at the time

   • Functional consequence of monoallelic expression unknown
   • Mechanism of imprinting (ICR, methylation) not defined
   • Biallelic expression in fetal brain unexplained
3. 2005 High

   Crystal structures of both the mature CPA4–latexin complex and the pro-CPA4 zymogen revealed that active-site occlusion by the pro-domain and by latexin proceeds through structurally analogous β-sheet loop mechanisms, defining the molecular basis of CPA4 activation and endogenous inhibition.

   Evidence X-ray crystallography of hCPA4–latexin and pro-CPA4, with molecular modeling

   PMID:15738388 PMID:16091843

   Open questions at the time

   • In vivo protease(s) responsible for pro-domain removal not identified
   • Physiological contexts where latexin regulates CPA4 unclear
4. 2010 High

   Biochemical and peptidomic characterization established CPA4 as a secreted, trypsin-activatable carboxypeptidase with strong preference for hydrophobic C-terminal residues, identifying neuropeptides (neurotensin, granins, opioid peptides) as substrates and implicating CPA4 in extracellular peptide processing.

   Evidence Secretion assay, chromogenic substrate kinetics, and quantitative peptidomics on >100 mouse brain peptides

   PMID:20385563

   Open questions at the time

   • In vivo neuropeptide processing by CPA4 not confirmed
   • Tissue-specific activation mechanism unknown
   • Relative contribution versus other MCPs in brain undetermined
5. 2012 High

   Structural characterization of exogenous inhibitors NvCI and SmCI bound to CPA4 revealed two distinct inhibitory strategies—C-terminal substrate-like insertion and N-terminal groove occupation—broadening the understanding of metallocarboxypeptidase inhibition mechanisms.

   Evidence X-ray crystallography at 1.7 Å (NvCI) and mutagenesis-validated structure (SmCI)

   PMID:22294694 PMID:23746805

   Open questions at the time

   • Therapeutic utility of these inhibitors against CPA4 untested in vivo
   • Endogenous N-terminal-type inhibitors of CPA4 not identified in mammals
6. 2016 Medium

   CPA4 was identified as a negative regulator of adipogenesis, showing that its biological roles extend beyond peptide processing to differentiation control in mesenchymal lineages.

   Evidence siRNA knockdown enhancing human preadipocyte differentiation, supported by RNA-Seq and qRT-PCR

   PMID:28209092

   Open questions at the time

   • Substrate or signaling target mediating anti-adipogenic effect not identified
   • Single knockdown approach without rescue
   • Relevance to in vivo adipose biology unconfirmed
7. 2019 Medium

   Loss- and gain-of-function studies in NSCLC cells demonstrated that CPA4 promotes proliferation and suppresses apoptosis via the AKT/c-MYC axis, establishing a pro-oncogenic signaling role validated by xenograft experiments.

   Evidence shRNA knockdown, ectopic overexpression, in vivo xenograft, Western blot pathway analysis

   PMID:31397502

   Open questions at the time

   • Whether enzymatic activity is required for AKT activation unknown
   • Direct versus indirect mechanism of AKT engagement not resolved
8. 2020 Medium

   CPA4 was shown to physically associate with AKT by co-immunoprecipitation and to activate the full PI3K–AKT–mTOR cascade, driving EMT and drug resistance in pancreatic cancer, while separately promoting cardiomyocyte hypertrophy through the same pathway.

   Evidence Co-IP in pancreatic cancer cells, PI3K inhibitor rescue, overexpression/knockdown in neonatal cardiomyocytes with in vivo mouse ISO model

   PMID:32347291 PMID:32922037

   Open questions at the time

   • CPA4–AKT interaction demonstrated by single co-IP without reciprocal pulldown or domain mapping
   • Whether CPA4 enzymatic activity is required for PI3K–AKT activation untested
   • Structural basis of CPA4–AKT association unknown
9. 2024 Medium

   Proteomics identified CPA4 as an M2 macrophage-secreted factor that promotes anaplastic thyroid cancer proliferation and EMT while feeding back to sustain M2 polarization, establishing CPA4 as a mediator of tumor–stroma crosstalk via STAT3, ERK, and AKT/mTOR.

   Evidence LC-MS/MS of conditioned medium, coculture, siRNA/shRNA knockdown, xenograft model

   PMID:38666696

   Open questions at the time

   • Receptor or mechanism of CPA4 uptake by cancer cells undefined
   • Whether CPA4 enzymatic activity or a non-catalytic surface mediates macrophage polarization unknown
   • Feedback loop not confirmed beyond single cell line system

• It remains unknown whether CPA4's pro-oncogenic and hypertrophic signaling functions depend on its carboxypeptidase catalytic activity or on a non-enzymatic scaffolding/binding role, and no in vivo physiological substrate for CPA4 has been confirmed.
• Catalytic-dead mutant studies needed to dissect enzymatic vs. non-catalytic roles
• In vivo substrates not validated
• Knockout or conditional KO mouse phenotype not reported