Oxygen levels that optimize TSC culture are identified by maximizing growth rates and minimizing stress

Accumulating data suggest that 20% O2 causes human and mouse placental trophoblast stem cell (TSC) differentiation and suppresses proliferation. We tested the hypotheses that phosphorylated stress-activated protein kinase (pSAPK) levels report the optimal O2 level for TSC culture, and that pSAPK responds to contradictory signals. We tested the dose range of 0–20% O2 (0, 0.5, 2, and 20%) on five effects in cultured TSC. The results showed 1) TSC accumulation rates were highest at 2% O2, lower at 20% and lowest at 0–0.5%; 2) pSAPK protein levels were lowest at 2% O2, higher at 20%, and highest at 0–0.5%; 3) Cleaved caspase 3, an apoptosis marker, increased at 0.5% O2, and was highest at 0% O2; 4) Three markers for multipotency were highest at 2 and 20% and significantly decreased at 0.5%–0%; 5) In contrast three differentiation markers were lowest at 2% and highest at 0.5%–0%. Thus, 2% O2 is the optimum as defined by lowest pSAPK and differentiation markers and highest growth rate and multipotency markers, without appreciable apoptosis. In addition, two lines of evidence suggest that fibroblast growth factor (FGF)4 does not directly activate SAPK. SAPK activity increases transiently with FGF4 removal at 2% O2, but SAPK activity decreases when O2 is switched from 20% to 2% with FGF4 present. Thus, SAPK is activated by contradictory signals, but activity decreases when either signal is removed. Taken together, the findings suggest that pSAPK senses suboptimal signals during TSC culture and probably in vivo.