Introduction:  Signal Transducer and Activator of Transcription 3 (STAT3) is an intracellular mediator of trophoblast invasion. In other cells, Suppressor Of Cytokine Signalling 3 (SOCS3) is known to be a major antagonist of STAT3 activation. For auto-regulation and to avoid excessive signals, SOCS3 gene expression is partly induced by STAT3.

Methods:  The human choriocarcinoma cell line JEG-3 was used for all experiments. STAT3 expression and STAT3 tyrosine (705) phosphorylation were analyzed by polyacrylamid gele electrophoresis followed by Western blotting. Interleukin-6 (IL-6) or Leukemia Inhibitory Factor (LIF) were used for stimulation. SOCS3 expression was silenced by RNA interference. For this approach in a first step, small interfering RNA (siRNA) was used. Subsequently, Jeg-3 cells were transfected with genes for the respective, self-designed small hairpin RNA (shRNA), neomycin resistance and green fluorescent protein. Application of neomycin and cell sorting by green fluorescence were used for elimination of non-transfected cells. Non-genomic siRNA and shRNA were respectively used for controls. Colorimetric proliferation assays were performed.

Results:  In IL-6 stimulated cells, SOCS3 knockdown leads to increased STAT3 tyrosine phosphorylation. After LIF stimulation, proliferation in SOCS3 knockdown cells is significantly higher than in controls. STAT3 expression is not affected by SOCS3 knockdown.

Conclusion:  SOCS3 plays a major role in intracellular regulation of trophoblast proliferation and invasiveness. The balance of STAT3 and SOCS3 may tune the behavior of invading trophoblast cells.