Article Text

  1. J. Isaac1,
  2. R. J. DiGeronimo1,
  3. P. Dixon1,
  4. B. M. Henson2,
  5. S. B. Mustafa2
  1. 1Wilford Hall Medical Center, San Antonio, TX
  2. 2University of Texas Health Science Center


Mechanical stretch of the alveolar epithelium initiates the transduction of intracellular signals in alveolar epithelial cells that elicit numerous cellular responses. Alveolar epithelial Na+ channels (ENaC), in particular the α-subunit, are essential for the maintenance of fluid-free lungs. We studied the effect of mechanical stretch on the expression of α-ENaC in murine lung epithelial (MLE-12) cells. MLE-12 cells were cultured on flexible collagen-coated membranes and exposed to either cyclic (20% strain, 30 cycles/min) or static (10% strain) stretch for up to 24 h. A significant rise in α-ENaC mRNA expression over resting cells occurred 3 h after either cyclic or static stretch. This increase in α-ENaC mRNA expression was attenuated in the presence of actinomycin D. Cyclic stretch induced a 3.4-fold increase and static stretch a 3.2-fold increase in α-ENaC protein after 24 h compared to resting cells. This response was reduced when stretched in the presence of cycloheximide. Cell counts and 3[H] leucine incorporation increased over 24 h in all study groups and were comparable between resting and either static or cyclic stretched cells. In resting cells, low levels of α-ENaC protein were intracellularly localized as detected by immunofluorescence. After cyclic stretch for 24 h, α-ENaC protein was elevated and appeared interspersed at the cell membrane, whereas after static stretch for 24 h, increased α-ENaC protein appeared in a better-defined pattern at the cell membrane. Exposure of MLE-12 cells to either static or cyclic stretch for 2 h caused activation of the mitogen-activated protein kinase (MAPK) pathways; extracellular signal-regulated protein kinase (ERK1,2), p38 MAPK and the stress-activated protein kinase/c-Jun NH2-terminal kinase (SAPK/JNK). Inhibition of ERK1,2 by PD98059 did not inhibit the cyclic or static stretch-induced increases in α-ENaC protein, whereas inhibition of p38 MAPK by SB203580 and inhibition of SAPK/JNK by JNK inhibitor II significantly attenuated the increase in either cyclic or static stretch-induced α-ENaC protein after 24 h. From our study, we conclude that either cyclic or static stretch of MLE-12 cells for 24 h induces de novo synthesis of α-ENaC protein. This increase in α-ENaC protein is mediated at least in part by the MAPK pathways, specifically the p38 and JNK pathways.

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