Rheumatoid arthritis affects about 1% of the world's population, including 2.1 million Americans. It is a complex, systemic, little-understood autoimmune disease characterized by inflamed, painful joints and is mediated by an intricate network of cytokines, including pro-inflammatory tumor necrosis factor alpha (TNFA) and interleukins-1, 4 and 10 (IL1, IL4, and IL10, respectively) (Joosten et al. 1999). A critical and previously unknown component of this network was identified in 2011. A research team led by Dr. Chuan-ju Liu of the Department of Orthopaedic Surgery, New York University School of Medicine and NYU Hospital for Joint Diseases, discovered that progranulin, a growth factor that plays a role in many physiological processes, is a ligand of tumor necrosis factor receptors 1 and 2 (TNFR1 and TNFR2) and an antagonist of TNFA signaling, and plays a critical role in the pathology of inflammatory arthritis in mice (Tang et al. 2011). The findings may usher in new therapeutic possibilities for rheumatoid arthritis and other TNFA-mediated disorders.
Progranulin's receptors identified
Progranulin (PGRN) had been known as a growth factor expressed in rapidly cycling epithelial cells, leukocytes, neurons and chondrocytes. Although progranulin was known to play a role in many physiological processes, including cancer, embryogenesis, inflammation and cartilage development/degradation, its receptors were unknown. Liu and his team aimed to identify them.
By screening a yeast cDNA library and performing a series of in vitro experiments, they determined that progranulin binds to TNFR1 and TNFR2. Further, they demonstrated that its affinity for these two receptors, especially TNFR2, surpasses that of their better known ligand, TNFA. They then found that collagen-induced arthritis (CIA) in PGRN-deficient mice - B6(Cg)-Grntm1.1Aidi/J (013175) - develops earlier, is more severe and occurs at a higher incidence than it does in wild-type controls. Administering recombinant human PGRN (rhPGRN) to these mice completely heals their arthritis. Lastly, Liu and his team found that deleting progranulin from transgenic mice that overexpress human TNFA and that develop arthritis spontaneously exacerbates the animals’ disease. Administering rhPGRN to these mice eliminates all signs of their arthritis. Together, these results convinced the Liu team that progranulin suppresses arthritis by interfering with TNF/TNFR signaling.
Atsttrin
Liu and his team identified three key domain regions necessary for progranulin to bind to TNFRs. With this information, they engineered a molecule that is antagonistic to TNF/TNFR signaling and called it Atsttrin (Antagonist of TNF/TNFR Signaling via Targeting to TNF Receptors). They found that Atsttrin selectively interacts with TNFR1 and TNFR2, has a lower affinity for TNFR1 but a higher affinity for TNFR2 than TNFA, inhibits several intracellular TNFA signaling pathways (including osteoclastogenesis), has a substantially longer half-life than progranulin (~120 vs. 40 hours), and has no cytotoxic effects.
Liu and his team wanted to know how Atsttrin's ability to interfere with TNFA-mediated arthritis compares to that of rhPGRN. They found that both substances either markedly mitigate or reverse collagen antibody-induced arthritis (CAIA) in both BALB/cJ (000651) and DBA/1J (000670) mice, Atsttrin more effectively than rhPGRN. In the CAIA model, it is even more effective than etanercept, a TNFA blocker used to treat inflammation. In the CIA model, both substances stop osteoclast activity. CIA-treated mice exhibit significantly lower levels of pro-inflammatory cytokines (IL1B and IL6) and significantly higher levels of anti-inflammatory cytokines (IL10 and IL13) than controls.
Liu and his team determined the pharmacokinetics of Atsttrin and found that Atsttrin mitigates or reverses CIA in DBA/1J mice in a dose-dependent manner. It is similarly effective in the arthritic transgenic mice that overexpress human TNFA. To determine whether TNFR1 or TNFR2 plays a greater role in Atsttrin's efficacy, Liu and his team analyzed whether Atsttrin mitigates CIA more effectively in Tnfr1-deficient B6.129-Tnfrsf1atm1Mak/J (002818) mice or Tnfr2-deficient B6.129S2-Tnfrsf1btm1Mwm/J (002620) mice. They found that Atsttrin mitigates CIA in a dose-dependent manner in Tnfr1-deficient mice, but that only the highest dose of Atsttrin mitigates CIA in Tnfr2-deficient mice. The fact that the Tnfr2-deficient mice are less sensitive to Atsttrin may reflect Atsttrin's greater affinity to TNFR2 than to TNFR1 and/or to the different distributions and functions of TNFR1 and TNFR2 in T cells (TNFR1 is expressed ubiquitously; TNFR2 is expressed mostly in hematopoietic cells).
In summary, Liu and his team solved a significant part of the progranulin mystery. They found that, in mice, it binds to TNFRs and mitigates arthritic inflammation at least in part by preventing TNFA from binding to them. Additionally they engineered Atsttrin, a progranulin-derived protein whose anti-inflammatory effects are superior to those of progranulin and which may help scientists develop novel therapies for rheumatoid arthritis and other TNFA-mediated diseases.
References
Joosten LA, Lubberts E, Helsen MM, Saxne T, Coenen-de Roo CJ, Heinegard D, van den Berg WB. 1999. Protection against cartilage and bone destruction by systemic interleukin-4 treatment in established murine type II collagen-induced arthritis. Arthritis Res 1:81-91.
Tang W, Lu Y, Tian QY, Zhang Y, Guo FJ, Liu GY, Syed NM, Lai Y, Lin EA, Kong L, Su J, Yin F, Ding AH, Zanin-Zhorov A, Dustin ML, Tao J, Craft J, Yin Z, Feng JQ, Abramson SB, Yu XP, Liu CJ. 2011. The growth factor progranulin binds to TNF receptors and Is therapeutic against inflammatory arthritis in mice. Science. 2011 Mar 10. [Epub ahead of print].
Yin F, Banerjee R, Thomas B, Zhou P, Qian L, Jia T, Ma X, Ma Y, Iadecola C, Beal MF, Nathan C, Ding A. 2010. Exaggerated inflammation, impaired host defense, and neuropathology in progranulin-deficient mice. J Exp Med 207:117-28.