KLK14

KLK14 is a secreted trypsin-like serine protease that operates in proteolytic cascades governing seminal clot liquefaction, epidermal desquamation, and PAR2-mediated signaling (PMID:11352573, PMID:18482984, PMID:22505524). In semen, it drives liquefaction by directly cleaving semenogelins I and II and by activating KLK3 (PSA) and other kallikreins in a cascade, while also self-limiting through internal cleavage that inactivates KLK3 at higher concentrations and through Zn2+-mediated inhibition that semenogelins themselves relieve (PMID:18482984). KLK14 acts as a biased agonist of proteinase-activated receptor 2 (PAR2), cleaving and unmasking its tethered activating sequence to trigger calcium transients, MAPK activation, β-arrestin recruitment, and receptor internalization—an activity that distinguishes it from KLK8, which cannot signal through human PAR2 (PMID:22505524). This PAR2 axis is co-opted in cervical carcinogenesis, where KLK14 acts downstream of KLK5 and KLK7 to drive RhoA and NF-κB signaling, such that ablation of KLK5/KLK7 attenuates the HPV-dependent phenotype by limiting KLK14 activation (PMID:40753921). In skin, KLK14 activity is restrained by LEKTI fragments through pH-dependent inhibition: the inhibitory complex dissociates at acidic pH to release active protease, coupling the epidermal pH gradient to controlled desquamation (PMID:17596512).

1. 2001 Medium

   Established the molecular identity of KLK14 as a secreted protein with predicted trypsin-like specificity, providing the foundation for all functional study of the enzyme.

   Evidence In vitro translation, Northern blot, in situ hybridization, and genomic structure analysis of the chromosome 19q13.4 gene

   PMID:11352573

   Open questions at the time

   • Protease activity inferred from sequence, not demonstrated enzymatically
   • No substrates or physiological role established
   • Tissue distribution not linked to function
2. 2007 High

   Resolved how KLK14 activity is restrained in skin by showing LEKTI fragments inhibit it in a pH-dependent manner, linking the epidermal pH gradient to controlled desquamation.

   Evidence Biochemical inhibition assays and kinetics with recombinant LEKTI fragments plus pH-dependent release experiments

   PMID:17596512

   Open questions at the time

   • In vivo desquamation role of KLK14 specifically not isolated from KLK5/KLK7
   • Downstream substrates in skin not defined here
   • Physiological LEKTI fragment concentrations not established
3. 2008 High

   Defined KLK14's physiological role in semen liquefaction, showing it both initiates a kallikrein cascade and is feedback-regulated, answering how seminal clot breakdown is controlled.

   Evidence Ex vivo semen liquefaction assays with a specific synthetic inhibitor, recombinant enzyme addition, activity assays, and Western blot for semenogelin cleavage

   PMID:18482984

   Open questions at the time

   • Quantitative hierarchy among activated KLKs not fully resolved
   • In vivo fertility consequences not tested
   • Structural basis of Zn2+ inhibition not determined
4. 2012 High

   Identified PAR2 as a signaling receptor activated by KLK14, establishing the enzyme as a biased agonist and distinguishing its signaling from other family members.

   Evidence Calcium, MAPK, β-arrestin, and internalization assays in HEK/KNRK cells expressing human or rat PAR2, plus PAR2-peptide cleavage, with direct comparison to KLK8

   PMID:22505524

   Open questions at the time

   • Physiological tissue context of PAR2 activation not addressed
   • Downstream transcriptional consequences not mapped here
   • Endogenous KLK14 concentrations relative to signaling threshold unknown
5. 2025 Medium

   Placed the KLK14-PAR2 axis in a disease pathway, showing KLK14 acts downstream of KLK5/KLK7 to drive RhoA and NF-κB signaling in HPV-dependent cervical carcinogenesis.

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

   PMID:40753921

   Open questions at the time

   • Direct demonstration that KLK14 alone is sufficient for the tumorigenic phenotype not shown
   • Mechanism by which KLK5/KLK7 activate KLK14 in vivo not detailed
   • Single-lab finding

• How KLK14 activity is integrated and prioritized across its distinct contexts—semen, skin, and tumor signaling—and what determines substrate versus receptor engagement in vivo remains unresolved.
• No structural model of the active enzyme with substrate or inhibitor in the corpus
• In vivo loss-of-function phenotype of KLK14 itself not characterized
• Regulatory determinants selecting cascade activation vs PAR2 signaling unknown