Elevated maternal soluble Gp130 and IL-6 levels and reduced Gp130 and SOCS-3 expressions in women complicated with preeclampsia

Abstract

Increased inflammatory response plays a significant role in the vascular pathophysiology in preeclampsia. However, the mechanism for increased inflammatory response in preeclampsia is largely unknown. Interleukin (IL)-6 levels are elevated in women with preeclampsia. IL-6 and its receptors, IL-6R and glycoprotein (gp)130, play a critical role in mediating antiinflammatory response via induction of SOCS-3 (suppressor of cytokine signaling-3). However, IL-6 receptor levels and expressions have not been studied in preeclampsia. In this study, we measured IL-6 and its 2 soluble receptors, soluble IL-6R and soluble gp130, in maternal plasma from normal and preeclamptic pregnant women and found that not only IL-6 but also soluble gp130 levels were significantly higher in preeclamptic women than in normotensive pregnant controls. We further examined IL-6R, gp130, and SOCS-3 expressions in maternal vessels and leukocytes and found that gp130 and SOCS-3 expressions were downregulated in both vessel endothelium and leukocytes from preeclampsia. Different patterns for IL-6R and gp130 expressions were found. IL-6R expression was also downregulated in leukocytes from preeclampsia. Our results suggest that increased plasma soluble gp130/soluble IL-6R/IL-6 ratio and reduced membrane transsignaling gp130 expression could contribute to decreased SOCS-3 expression and subsequent reduction in SOCS-3 antiinflammatory activity in women with preeclampsia. Thus, reduced gp130 and SOCS-3 expressions may offer, at least in part, a plausible explanation of reduced antiinflammatory protection in the maternal vascular system in preeclampsia.

Figure 1

IL-6R and gp130 expressions in maternal vessels and leukocytes from normal and preeclamptic pregnant women. A, Immunostaining of IL-6R and gp130 expressions in maternal vessels. a and b, IL-6R. c through f, Gp130. Normal: a, c, and e; preeclampsia (PE): b, d, and f. c and d are arteries; e and f, veins. Gp130, but not IL-6R, immunostaining is detectable in maternal vessels. Gp130 immunostaining is markedly reduced in endothelium of vessels from PE compared to normal pregnant controls. Bar=50 µm for a, b, e, and f; bar=100 µm for c and d. B, Top, Western blot of IL-6R expression in leukocytes. Bottom, Scatter graphs of relative IL-6R expression after normalization with β-actin expression. Four bands are shown with molecular masses of 80, 55, 50, and 45 kDa. Total expression includes all bands; the upper band represents the 80-kDa band, and the lower bands include the 45- to 55-kDa bands, respectively. The densities of total IL-6R and 80-kDa IL-6R were significantly reduced in PE leukocyte samples. *P<0.05. C, Western blot of gp130 expression in leukocytes. Two bands are shown with molecular masses of 90 and 50 kDa. Bottom, Scatter graphs of relative gp130 expression after normalization with β-actin expression. Total expression includes both 90- and 50-kDa bands; the upper band represents the 90-kDa band, and the lower band represents the 50-kDa band. The total gp130 expression and the 90-kDa gp130 expression were significantly reduced in leukocyte samples from PE compared to normal control samples, *P<0.05.

Figure 2

SOCS-3 expression in maternal vessels and leukocytes from normal and preeclamptic pregnant women. A, Immunostaining of SOCS-3 in maternal vessels. Normal: a and c; preeclampsia (PE): b and d. a and b are arteries; c and d are veins. SOCS-3 immunostaining is markedly reduced in endothelium and underlying smooth muscle cells in vessels from PE compared to those from normal pregnant women. Bar=50 µm. B, SOCS-3 expression in leukocytes. Scatter graph shows relative SOCS-3 expression after normalization with β-actin expression. SOCS-3 expression was significantly downregulated in leukocytes from PE (n=5) compared to those from normal (n=6) pregnant controls. *P<0.05, respectively.

Figure 3

Proposed mechanisms of IL-6 and its receptor signaling pathway in SOCS-3 induction in cells with or without IL-6R. A, Cells have both IL-6R and gp130 receptors such as in leukocytes. IL-6 binds to IL-6R on the cell membrane and forms a receptor complex with gp130. The binding process induces receptor dimerization and Janus kinase (JAK) phosphorylation of the signal transducing subunit gp130. Phosphorylated JAK further induces STAT3 phosphorylation, and dimeric STAT3 migrates to the nucleus, where it recognizes specific elements in the promoter of SOCS-3 gene and induces SOCS-3 expression. Once induced, SOCS-3 acts back on the JAK/STAT pathway to inhibit signal transduction, attenuate cytokine-activated signal transduction pathway, and suppress cytokine generation and cytokine-induced inflammatory responses. B, Cells have gp130, but not IL-6R, such as in endothelial cells. In the extracellular compartment such as in the blood circulation, IL-6 cannot directly bind to gp130 on the cell membrane. Instead, IL-6 binds sIL-6R and forms receptor/ligand complex, sIL-6R/IL-6. The complex could then bind to gp130 on the cell membrane and induce gp130 phosphorylation and SOCS-3 induction. Therefore, sIL-6R is considered as an agonist to IL-6 and plays a critical role in IL-6–induced gp130 activation, and the integrity of the cell membrane gp130 receptor is critical for signal transduction and SOCS-3 induction. C, Proposed mechanism of reduced antiinflammatory activity in vascular endothelial cells in preeclampsia (PE): a condition of increased sgp130 levels, increased ratio of sgp130/sIL-6R/IL-6, and decreased membrane gp130 receptor expression in cells without IL-6R. In this condition, sgp130 binds to sIL-6R/IL-6 and forms a double-receptor complex, sgp130/sIL-6R/IL-6, which prevents sIL-6R/IL-6 binding to membrane receptor gp130. Reduced membrane gp130 receptor expression then leads to less receptor binding activity. As a result, the JAK/STAT pathway signaling and SOCS-3 induction would be significantly impaired. Consequently, endogenous antiinflammatory SOCS-3 activity would be reduced, which possibly is the case of vascular endothelial cells in preeclampsia.