Oxygen sensing by primary cardiac fibroblasts -: A key role of p21Waf1/Cip1/Sdi1

In mammalian organs under normoxic conditions, O-2 concentration ranges from 12% to <0.5%, with O-2 ≈14% in arterial blood and <10% in the myocardium. During mild hypoxia, myocardial O-2 drops to approximate to1% to 3% or lower. In response to chronic moderate hypoxia, cells adjust their normoxia set point such that reoxygenation-dependent relative elevation of PO2 results in perceived hyperoxia. We hypothesized that O-2, even in marginal relative excess of the PO2 to which cardiac cells are adjusted, results in activation of specific signal transduction pathways that alter the phenotype and function of these cells. To test this hypothesis, cardiac fibroblasts (CFs) isolated from adult murine ventricle were cultured in 10% or 21% O-2 (hyperoxia relative to the PO2 to which cells are adjusted in vivo) and were compared with those cultured in 3% O-2 (mild hypoxia). Compared with cells cultured in 3% O-2, cells that were cultured in 10% or 21% O-2 demonstrated remarkable reversible G(2)/M arrest and a phenotype indicative of differentiation to myofibroblasts. These effects were independent of NADPH oxidase function. CFs exposed to high O-2 exhibited higher levels of reactive oxygen species production. The molecular signature response to perceived hyperoxia included (1) induction of p21, cyclin D1, cyclin D2, cyclin G1, Fos-related antigen-2, and transforming growth factor-beta1, (2) lowered telomerase activity, and (3) activation of transforming growth factor-beta1 and p38 mitogen-activated protein kinase. CFs deficient in p21 were resistant to such O-2 sensitivity. This study raises the vital broad-based issue of controlling ambient O-2 during the culture of primary cells isolated from organs.