Background/Purpose: Endothelial progenitor cells (EPCs) are known to induce vasculoproliferative responses within rheumatoid arthritis (RA) synovial tissue (ST). Inhibitor of DNA-Binding 1 (Id1) is a transcription factor shown to be actively transcribed in only progenitor cells, making it a possible marker for identifying EPCs in neovascular responses in RA ST. We previously showed that Id1 is not only expressed in RA STs, but is also secreted and upregulated in RA synovial fluid (SF). The identification of Id1 in RA tissues indicates progenitor cells are active in the RA joint, and may serve as a potential target for limiting neovascularization in the RA ST. This study further examines the cellular sources, angiogenic and chemotactic potential, and cell signaling properties of Id1.

Methods: Severe combined immunodeficient (SCID) mice were implanted with RA ST and allowed to engraft for six weeks. Mice were injected i.v. with fluorescently dye-tagged EPC’s while receiving simultaneous intragraft injections with either human Id1 (10 nM) or PBS. Another group of SCID mice grafted and injected similarly, received intragraft injections of RA SF immunodepleted with either non-specific IgG (“sham depleted”), or Id1 immunodepleted with a neutralizing antibody. For signal transduction analysis, human dermal microvascular endothelial cells (HMVECs) and EPCs were plated and stimulated with human Id1. Cell lysates were made and Western blot analysis was used to determine the kinetics of protein kinase expression. We also examined whether Id1 induces HMVECs to form blood vessels, measured as an increase in plug hemoglobin (Hb) concentration normalized to plug weight, using the mouse Matrigel plug angiogenesis assay. Finally, to determine the cells secreting Id1, we cultured monocytes, HMVECs, EPCs and fibroblasts and measured the supernatants for Id1 expression by ELISA.

Results: There was a significant increase in EPC migration to engrafted RA ST in the SCID mouse chimera to intragraft injections of Id1. Using the same RA ST SCID mouse chimera system, we found that intragraft injections of RA SF immunodepleted of Id1 resulted in a 50% reduction in EPC recruitment compared to mice injected similarly with sham depleted RA SF (*p<0.05). Cell signaling experiments to Id1 showed Jnk was upregulated in both HMVECs and EPCs, and P38 only in EPCs. We also show that Id1 induces in vivo blood vessel formation, and that inhibiting HMVEC associated Jnk with silencing RNA reverses Id1 induced HMVEC vessel formation in Matrigel plugs in vivo. Lastly, we show that monocytes and especially fibroblasts, but not HMVECs and EPCs, actively secrete Id1, indicating that mature fibroblasts may be a primary source of Id1 in RA SF.

Conclusion: Our data indicates that Id1 induces EPC recruitment in the RA ST SCID mouse chimera, and that by depleting RA SF of soluble Id1, we can attenuate EPC migration by 50%. Id1 was also found to be potently angiogenic in the mouse Matrigel angiogenesis assay, and activates Jnk signaling pathways in HMVECs and EPCs. Lastly, we found that fibroblasts actively secrete Id1 and may be a primary source for its expression in SF. These studies identify Id1 as both a regulatory transcription factor and a unique angiogenic target within the RA synovium.

---

« Back to 2013 ACR/ARHP Annual Meeting

ACR Meeting Abstracts - https://acrabstracts.org/abstract/inhibitor-of-dna-binding-1-mediates-angiogenesis-in-rheumatoid-arthritis-by-recruitment-of-endothelial-progenitor-cells/