A src-like kinase activates outwardly rectifying chloride channels in CFTR-defective lymphocytes

Defective activation of chloride channels is a hallmark of cystic fibrosis (CF). Recently we have described activation of a volume-sensitive, outwardly rectifying chloride conductance (I(OR)) through the src-like tyrosine kinase p56(lck). Here we show that p56(lck) activates I(OR) independently of CFTR. In lymphocytes from healthy donors, chloride channels could be opened by either intracellular cAMP, p56(lck) or osmotic swelling. In CF lymphocytes, p56(lck) and cell swelling but not cAMP could activate chloride channels. Regulation of I(OR) by p56(lck) thus represents an alternative pathway of stimulating membrane chloride conductance that is left intact in cystic fibrosis.

Tyrosine kinases open lymphocyte chloride channels

Osmotic swelling of lymphocytes opens outwardly rectifying Cl(-) channels (ORCC) through the src-like kinase p56(lck). The central role of this tyrosine protein kinase has been shown by genetic and pharmacologic manipulation of the enzyme. Furthermore, p56(lck) activates ORCC independently of cell volume increase. ORCC in lymphocytes and epithelial cells from cystic fibrosis (CF) patients are resistant to activation by cAMP. However, osmotic swelling as well as intracellular purified p56(lck) can activate ORCC in CF lymphocytes. In non-CF lymphocytes ORCC is opened by either, intracellular cAMP, p56(lck) or by osmotic swelling. Osmotic activation of ORCC can be blocked by the tyrosine kinase inhibitor lavendustin in both cell types. Regulation of ORCC by p56(lck) thus represents an alternative pathway of stimulating membrane chloride conductance that is left functional in cystic fibrosis. In addition to osmoregulation these mechanisms could play a major role when cells actively change their volume, i.e. during proliferation and apoptosis. Activation of the tyrosine kinase p56(lck) is an important regulatory step for opening of chloride channels in lymphocytes.

Stimulation of CD95 (Fas) blocks T lymphocyte calcium channels through sphingomyelinase and sphingolipids

Calcium influx through store-operated calcium release-activated calcium channels (CRAC) is required for T cell activation, cytokine synthesis, and proliferation. The CD95 (Apo-1/Fas) receptor plays a role in self-tolerance and tumor immune escape, and it mediates apoptosis in activated T cells. In this paper we show that CD95-stimulation blocks CRAC and Ca(2+) influx in lymphocytes through the activation of acidic sphingomyelinase (ASM) and ceramide release. The block of Ca(2+) entry is lacking in CD95-defective lpr lymphocytes as well as in ASM-defective cells and can be restored by retransfection of ASM. C2 ceramide, C6 ceramide, and sphingosine block CRAC reversibly, whereas the inactive dihydroceramide has no effect. CD95-stimulation or the addition of ceramide prevents store-operated Ca(2+) influx, activation of the transcriptional regulator NFAT, and IL-2 synthesis. The block of CRAC by sphingomyelinase metabolites adds a function to the repertoire of the CD95 receptor inhibiting T cell activation signals.