TNF-α increases ceramide without inducing apoptosis in alveolar type II epithelial cells

Abstract

Ceramide is a bioactive lipid mediator that has been observed to induce apoptosis in vitro. The purpose of this study was to determine whether endogenous ceramide, generated in response to in vivo administration of tumor necrosis factor-α (TNF-α), increases apoptosis in primary rat alveolar type II epithelial cells. Intratracheal instillation of TNF-α (5 μg) produced a decrease in sphingomyelin and activation of a neutral sphingomyelinase. These changes were associated with a significant increase in lung ceramide content. TNF-α concomitantly activated the p42/44 extracellular signal-related kinases and induced nuclear factor-κB activation in the lung. Hypodiploid nuclei studies revealed that intratracheal TNF-α did not increase type II cell apoptosis compared with that in control cells after isolation. A novel observation from separate in vitro studies demonstrated that type II cells undergo a gradual increase in apoptosis after time in culture, a process that was accelerated by exposure of cells to ultraviolet light. However, culture of cells with a cell-permeable ceramide, TNF-α, or a related ligand, anti-CD95, did not increase apoptosis above the control level. The results suggest that ceramide resulting from TNF-α activation of sphingomyelin hydrolysis might activate the mitogen-activated protein kinase and nuclear factor-κB pathways without increasing programmed cell death in type II cells.

Tumor Necrosis Factor-α Prevents Desensitization of Gαs-Coupled Receptors by Regulating GRK2 Association with the Plasma Membrane

“…Although inhibition of PKA and PKC might partly affect phosphorylation and membrane translocation of GRK2 in control cells, the inability of inhibitors of these kinases to alter the effect of TNF-␣ on the ligand-induced association of GRK2 with the plasma membrane suggests that other mechanisms are responsible for ligand-induced desensitization of adenosine A 2A receptors. Moreover, our results indicate involvement of the TNF-␣-activated, sphingomyelinase-dependent pathway (signaling through the protein kinase JNK) (Dbaibo et al, 1993;Muller et al, 1998;Mallampalli et al, 1999) in the TNF-␣-mediated blockade of A 2A receptor desensitization. It is still unclear, however, how ceramide and/or JNK, activated by TNF-␣, modulate GRK2 subcellular localization during GPCR occupancy.…”

“…2D), we examined the role of other signaling pathways for TNF-␣ in the prevention of desensitization of A 2A Rs. TNF-␣ has previously been shown to signal for cellular activation in monocytes and macrophage-like cells (Wiegmann et al, 1992;Dbaibo et al, 1993;MacKichan and DeFranco, 1999;Mallampalli et al, 1999) via activation of sphingomyelinases (SMases), which generate lipid-derived signaling elements such as C 2 -ceramide. Pretreatment with 20 M C 2 -ceramide decreased GRK2 levels in the plasma membrane after CGS 21680 stimulation to 84 Ϯ 5% of basal level (Fig.…”

Pulmonary phosphatidic acid phosphatase and lipid phosphate phosphohydrolase

“…Although type II cells are differentiated and highly specialized, they retain the ability to proliferate during lung injury resulting from a variety of insults, including ARDS, radiation, and exposure to high O 2 . Postmitotic type II cells transdifferentiate to replace the more vulnerable type I cells lost during injury (1,34,48,65,147). Numerous factors, including fibroblast growth factor (FGF), heparin-binding growth factor, transforming growth factor-␤, keratinocyte growth factor, PDGF, and EGF have been implicated in type II cell proliferation (134), but growth promotion in vivo must still be further defined.…”

“…Numerous factors, including fibroblast growth factor (FGF), heparin-binding growth factor, transforming growth factor-␤, keratinocyte growth factor, PDGF, and EGF have been implicated in type II cell proliferation (134), but growth promotion in vivo must still be further defined. In addition, it has become evident that apoptosis plays a central role in cellular remodeling of the developing lung (48) and during recovery from lung injury (48,65,147). It may be recalled that LPP1 was cloned using nucleotide sequence from HIC53, an H 2 O 2 -inducible gene product (57).…”

LASS5 is the predominant ceramide synthase isoform involved in de novo sphingolipid synthesis in lung epithelia

“…In separate studies, cells were pulsed with l-[3-3 H]serine (1 Ci/dish for the last 3 h) in medium alone or in combination with FB1 as described above, and incorporation of radioactivity into ceramide was then determined (right) as in Fig. 1 molecule inhibitory for surfactant phospholipid synthesis (6,8,(29)(30)(31)(32). Thus, the primary aim of this study was to ascertain whether ceramide, generated via a de novo pathway, coordinately inhibits PtdCho production in lung epithelia.…”