KLK14

KLK14 is a secreted trypsin-like serine protease that functions in skin barrier homeostasis, seminal clot liquefaction, protease-activated receptor signaling, and epithelial tumorigenesis. In skin, KLK14 activity is controlled by the endogenous inhibitor LEKTI in a pH-dependent manner: at neutral pH LEKTI binds KLK14 tightly and irreversibly, whereas acidification in the stratum corneum releases active enzyme (PMID:17596512). In seminal fluid, KLK14 directly cleaves semenogelins I and II and bidirectionally modulates KLK3 (PSA) activity, with Zn²⁺-mediated inhibition reversed by semenogelins providing a substrate-coupled regulatory switch; specific inhibition of KLK14 delays semen liquefaction ex vivo (PMID:18482984). KLK14 activates PAR2 by proteolytic unmasking of its tethered ligand, triggering calcium transients, MAPK activation, and β-arrestin-dependent receptor internalization, and in HPV-driven cervical carcinogenesis it operates downstream of KLK5/KLK7 to promote tumor progression through PAR2-dependent RhoA and NF-κB signaling (PMID:22505524, PMID:40753921).

1. 2001 Medium

   Establishing the identity of KLK14 as a secreted serine protease with trypsin-like specificity expressed in prostatic epithelium filled the gap of whether the KLK14 locus encodes a functional protease and where it is produced.

   Evidence In vitro translation, Northern blot, and in situ hybridization in prostate tissue

   PMID:11352573

   Open questions at the time

   • No endogenous substrates identified
   • Enzymatic activity not demonstrated on native substrates
   • Expression survey limited to prostate
2. 2007 High

   Demonstrating that LEKTI fragments inhibit KLK14 in a pH-dependent manner — tight binding at neutral pH and release at acidic pH — established the first endogenous regulatory mechanism for KLK14 and explained how proteolytic activity is spatially controlled in the stratum corneum.

   Evidence Kinetic inhibition assays, pH-shift binding/release experiments, and LEKTI fragment characterization from cultured keratinocytes and epidermis

   PMID:17596512

   Open questions at the time

   • Downstream skin substrates of released KLK14 not identified
   • In vivo consequences of dysregulated KLK14 in LEKTI-deficient skin not directly tested
3. 2008 High

   Identifying semenogelins and KLK3 as direct substrates/targets of KLK14 and showing that specific KLK14 inhibition delays semen liquefaction established a physiological role for KLK14 in male reproduction and revealed a Zn²⁺/semenogelin regulatory circuit.

   Evidence Ex vivo semen liquefaction assay with specific synthetic inhibitor (ACT-G9) and recombinant KLK14; Zn²⁺ inhibition reversal assay; KLK3 fragmentation analysis

   PMID:18482984

   Open questions at the time

   • Relative contribution of KLK14 versus other seminal KLKs in vivo not quantified
   • No genetic loss-of-function model for reproductive phenotype
4. 2012 High

   Showing that KLK14 proteolytically activates PAR2 to elicit calcium transients, MAPK signaling, β-arrestin recruitment, and receptor internalization identified a cell-surface receptor through which KLK14 can transduce extracellular proteolytic activity into intracellular signaling.

   Evidence Calcium flux, MAPK, β-arrestin interaction, and receptor internalization assays in PAR2-expressing HEK and KNRK cells; peptide cleavage assays

   PMID:22505524

   Open questions at the time

   • Physiological tissue context for KLK14–PAR2 signaling not established
   • Relative potency compared to other PAR2 agonists (thrombin-family proteases) not determined in vivo
5. 2025 Medium

   Genetic epistasis in HPV-driven cervical carcinogenesis placed KLK14 downstream of KLK5/KLK7 and showed it promotes tumorigenesis via PAR2-dependent RhoA and NF-κB pathways, linking the KLK14–PAR2 axis to a disease-relevant signaling cascade.

   Evidence KLK5/KLK7 knockout mice, bulk RNA-seq, RhoA and NF-κB reporter assays, human biopsy expression analysis

   PMID:40753921

   Open questions at the time

   • Direct enzymatic activation of KLK14 by KLK5 or KLK7 not reconstituted in vitro
   • Whether KLK14 cleaves PAR2 in cervical epithelium in vivo not directly shown
   • Contribution of KLK14 independent of other downstream KLK5/KLK7 targets not isolated

• A structural basis for KLK14 substrate selectivity and the atomic details of its interaction with LEKTI remain unresolved, and no Mendelian or engineered KLK14-null phenotype has been reported in vivo.
• No crystal or cryo-EM structure of KLK14 alone or in complex with inhibitors/substrates
• No KLK14 knockout mouse phenotype reported
• Full in vivo substrate repertoire undefined