FBLN5

FBLN5 (fibulin-5/DANCE/EVEC) is a secreted, calcium-dependent extracellular matrix glycoprotein that serves as a central organizer of elastic fiber assembly while also functioning as an integrin ligand that couples the matrix to intracellular signaling (PMID:12021267, PMID:17371835). As an elastogenic scaffold, FBLN5 promotes coacervation and alignment of tropoelastin on microfibrils and tethers the lysyl oxidase-like enzymes LOXL1, LOXL2, and LOXL4 to drive tropoelastin crosslinking; age-associated truncated FBLN5 loses this elastogenic capacity (PMID:17371835). Its deposition onto fibrillin-1 microfibrils is regulated through direct binding to LTBP-2, which controls whether FBLN5 and elastin assemble in a fibrillin-1-dependent or -independent manner (PMID:17581631). Through its RGD motif, FBLN5 engages integrins (including αvβ3) to mediate cell adhesion and activate FAK-MAPK signaling, and its overexpression potentiates ERK1/2, p38 MAPK, and AP-1 activity downstream of TGF-β via a Smad3-independent route (PMID:10428823, PMID:12021267, PMID:33614230). The requirement for FBLN5 in human elastogenesis is established genetically: a homozygous S227P missense mutation in its fourth calcium-binding EGF-like domain causes autosomal recessive cutis laxa type I, and a heterozygous tandem duplication adding EGF-like motifs causes autosomal dominant cutis laxa (PMID:12189163, PMID:12618961). FBLN5 expression is regulated by TGF-β and IL-1β, by HIF1α downstream of mitochondrial ROS, and by multiple microRNAs (miR-200c, miR-370-3p, miR-138) that directly target its 3'UTR, linking its levels to proliferation, migration, and NF-κB signaling in cancer and smooth muscle contexts (PMID:12021267, PMID:12909585, PMID:34475958, PMID:37644092, PMID:22685266).

1. 1999 Medium

   Established FBLN5 as a secreted vascular ECM protein and integrin ligand, answering whether it had a cell-interactive function rather than being purely structural.

   Evidence Recombinant DANCE protein in endothelial cell adhesion assays plus in situ hybridization in developing and injured vasculature

   PMID:10347091 PMID:10428823

   Open questions at the time

   • Which specific integrin heterodimers engage the RGD motif was not resolved
   • Downstream signaling consequences of adhesion not defined
2. 2002 Medium

   Defined FBLN5 as a dual elastin- and integrin-binding protein and placed it upstream of MAPK signaling, connecting matrix scaffolding to intracellular pathways.

   Evidence Recombinant calcium-dependent elastin-binding assays, cell adhesion assays, and ERK1/2, p38, and AP-1 activation epistasis using dominant-negative Smad3, MKK1, and p38α

   PMID:12021267

   Open questions at the time

   • Mechanistic link between integrin engagement and MAPK activation not directly demonstrated
   • Smad3-independent TGF-β transcriptional mechanism not identified
3. 2002 Medium

   Demonstrated that FBLN5 function is genetically required for human elastogenesis, moving it from a candidate elastic-fiber protein to a disease gene.

   Evidence Homozygous S227P missense mutation in a calcium-binding EGF-like domain identified in a consanguineous cutis laxa family; heterozygous EGF-domain duplication in dominant cutis laxa

   PMID:12189163 PMID:12618961

   Open questions at the time

   • How the specific domain alterations disrupt elastogenesis biochemically was inferred from structure, not measured
   • Genotype-phenotype basis for recessive vs dominant inheritance not mechanistically resolved
4. 2003 Medium

   Showed FBLN5 is coordinately co-regulated with elastin and is susceptible to elastase, linking its abundance to elastic fiber turnover and inflammation.

   Evidence Northern, Western, and immunohistochemistry in lung fibroblasts with elastase and IL-1β treatment

   PMID:12909585

   Open questions at the time

   • Transcriptional regulators mediating co-regulation with elastin not identified
   • Functional consequence of elastase cleavage products not tested
5. 2007 High

   Reconstituted the core elastogenic mechanism, showing FBLN5 organizes tropoelastin on microfibrils and recruits LOXL enzymes for crosslinking, and is integrated onto fibrillin-1 via LTBP-2.

   Evidence In vitro elastogenesis assay with recombinant full-length vs truncated FBLN5 and LOXL co-localization; Co-IP identifying LTBP-2, with siRNA knockdown and recombinant rescue in skin fibroblasts

   PMID:17371835 PMID:17581631

   Open questions at the time

   • Stoichiometry and structural basis of the FBLN5-tropoelastin-LOXL assembly not resolved
   • How LTBP-2 binding switches deposition modes mechanistically unclear
6. 2021 Medium

   Connected the elastogenic and signaling roles in a tumor context, showing LOXL1-FBLN5 physical interaction and RGD-dependent αvβ3 integrin engagement activating FAK-MAPK to drive angiogenesis.

   Evidence Reciprocal Co-IP, RGD-dependent binding, and FAK-MAPK activation assays with overexpression/depletion in cholangiocarcinoma cells

   PMID:33614230

   Open questions at the time

   • Whether LOXL1 binding and integrin signaling are mechanistically coupled not shown
   • Generalizability beyond the cancer cell context not tested
7. 2012 Medium

   Identified post-transcriptional control of FBLN5 by microRNAs, establishing that its expression is tunable and linking it to proliferative phenotypes.

   Evidence 3'UTR luciferase reporter assays and gain/loss-of-function with miR-200c, miR-370-3p, and miR-138 across smooth muscle, cancer, and stem cell models

   PMID:22685266 PMID:33460398 PMID:34475958

   Open questions at the time

   • Relative contribution of each miRNA in physiological tissue not established
   • Whether FBLN5 repression by miRNAs acts through elastogenesis or signaling differs by context
8. 2023 Medium

   Placed FBLN5 in a regulatory network downstream of transcriptional and signaling inputs, identifying HIF1α and UBE2T-linked pathways controlling its expression.

   Evidence Loss-of-function of NDUFS1 with ROS measurement and HIF1α epistasis in gastric cancer; UBE2T knockdown with ChIP/EMSA/GST pull-down and double-knockdown rescue in lung adenocarcinoma

   PMID:35543375 PMID:37644092

   Open questions at the time

   • Direct HIF1α occupancy of the FBLN5 promoter vs indirect regulation not fully separated
   • How FBLN5 mediates downstream ERK/GSK3β/β-catenin effects mechanistically unclear

• How FBLN5's distinct roles as an elastogenic scaffold versus an integrin-signaling ligand are coordinated within a single tissue context remains unresolved.
• No structural model of the FBLN5-tropoelastin-LOXL-LTBP2 assembly
• Whether signaling and elastogenic functions use the same or separable protein interfaces is unknown