F2R

F2R (PAR1) is a cell-surface G protein-coupled receptor whose signaling intersects coagulation-associated, inflammatory, and oncogenic programs, with its activity dictated by an N-terminal tethered-ligand motif (LDPRSFLL) (PMID:35256589). This motif mediates a direct interaction with the cytotoxic-lymphocyte protease GZMA, and engagement of F2R by GZMA activates JAK2/STAT1 signaling to promote tumor cell apoptosis and T cell–mediated killing (PMID:35256589). In bone, F2R acts as a negative regulator of osteoclastogenesis upstream of both Akt and NFκB signaling, restraining osteoclast formation, acidification, and bone resorption (PMID:32226307). In several cancer contexts F2R instead drives proliferation and invasion: it activates a FAK/PI3K/AKT axis through the downstream effector COL8A1 in prostate cancer (PMID:42126351) and feeds into p38 MAPK signaling as a downstream effector of POLR1F in anaplastic thyroid cancer (PMID:40250711). F2R expression is transcriptionally controlled at its promoter, both by Ets-1 binding at a functional -506 insertion/deletion polymorphism that tunes promoter activity under hypoxia (PMID:17347481) and by POLR1F-dependent restriction of KDM5C binding that raises H3K4me3 at the F2R promoter (PMID:40250711). The tethered-ligand motif also renders F2R an internalizable receptor exploitable for ligand-directed drug delivery (PMID:41608127).

1. 2007 Medium

   Establishing how F2R abundance is set, a functional promoter polymorphism was shown to control transcription factor binding and responsiveness to hypoxia, linking F2R expression to vascular stress.

   Evidence EMSA with nuclear extracts and reporter transfection assays in HUVECs under hypoxia

   PMID:17347481

   Open questions at the time

   • Direct identification of Ets-1 as the bound factor at -506 is inferred from consensus, not confirmed by supershift or ChIP in this finding
   • Downstream physiological consequence of altered F2R levels not measured
   • No link to receptor signaling outputs
2. 2020 Medium

   Beyond vascular biology, reciprocal gain- and loss-of-function placed F2R as a negative regulator of osteoclastogenesis upstream of Akt and NFκB, revealing a role in bone homeostasis.

   Evidence Lentiviral shRNA knockdown and overexpression in mouse bone marrow cells with Western blot signaling readouts and bone resorption/F-actin ring assays

   PMID:32226307

   Open questions at the time

   • Ligand driving F2R signaling in osteoclasts not identified
   • Whether the effect requires the tethered-ligand motif untested
   • In vivo bone phenotype not demonstrated
3. 2022 Medium

   Defining a direct immune ligand, the N-terminal LDPRSFLL motif was shown to mediate F2R interaction with GZMA, coupling cytotoxic lymphocyte attack to JAK2/STAT1-driven tumor apoptosis.

   Evidence In vivo and in vitro co-interaction assays, single-cell sequencing, and motif-specific functional studies

   PMID:35256589

   Open questions at the time

   • Structural basis of GZMA–motif interaction not resolved
   • Whether GZMA cleaves or otherwise activates the tethered ligand unclear
   • Reported at abstract-level mechanistic detail only
4. 2025 Medium

   Connecting transcriptional control to oncogenic output, POLR1F was shown to raise F2R expression by limiting KDM5C-mediated demethylation, with F2R then driving p38 MAPK signaling in thyroid cancer.

   Evidence RNA-seq after POLR1F knockdown, ChIP for H3K4me3 and KDM5C at the F2R promoter, plus xenograft and zebrafish tumor models

   PMID:40250711

   Open questions at the time

   • Direct vs indirect nature of POLR1F effect on the promoter not fully separated
   • Mechanism coupling F2R to p38 MAPK not defined
   • Generalizability beyond anaplastic thyroid cancer untested
5. 2026 Low

   Extending F2R's pro-tumor signaling, knockdown and overexpression showed it promotes prostate cancer proliferation and invasion via a FAK/PI3K/AKT axis through COL8A1.

   Evidence F2R overexpression/knockdown in prostate cancer cell lines, proliferation/invasion/apoptosis assays, and xenograft models with bioinformatics-inferred COL8A1 link

   PMID:42126351

   Open questions at the time

   • COL8A1 as the key effector is bioinformatics-inferred, not biochemically validated
   • Whether F2R acts through canonical receptor signaling or a non-canonical route unclear
   • Single lab, abstract-level detail
6. 2026 Low

   Demonstrating translational utility, a tethered-ligand-mimetic peptide enhanced nanoparticle uptake and cytotoxicity, validating F2R as an internalizable surface target for ligand-directed delivery.

   Evidence In vitro cellular uptake and cytotoxicity assays in an ovarian cancer cell line with peptide-conjugated versus unconjugated lipid nanoparticles

   PMID:41608127

   Open questions at the time

   • Single cell line, in vitro only, no in vivo targeting data
   • Internalization mechanism not directly characterized
   • Selectivity over other receptors not assessed

• How F2R integrates its opposing roles — pro-apoptotic in immune killing yet pro-proliferative in tumor cells, and anti-osteoclastogenic in bone — through distinct ligands, G protein couplings, and cell contexts remains unresolved.
• G protein/effector coupling for each context not defined in the corpus
• No structural model of receptor states
• Determinants of context-dependent outcome unknown