Abstract
BackgroundThe direction of cytokine secretion from polarized cells determines the cytokine's cellular targets. Leukemia inhibitory factor (LIF) belongs to the interleukin-6 (IL-6) family of cytokines and signals through LIFR/gp130. Three factors which may regulate the direction of LIF secretion were studied: the site of stimulation, signal peptides, and expression levels. Stimulation with IL-1β is known to promote IL-6 secretion from the stimulated membrane (apical or basolateral) in the human intestinal epithelial cell line Caco-2. Since LIF is related to IL-6, LIF secretion was also tested in Caco-2 following IL-1β stimulation. Signal peptides may influence the trafficking of LIF. Two isoforms of murine LIF, LIF-M and LIF-D, encode different signal peptides which have been associated with different locations of the mature protein in fibroblasts. To determine the effect of the signal peptides on LIF secretion, secretion levels were compared in Madin-Darby canine kidney (MDCK) clones which expressed murine LIF-M or LIF-D or human LIF under the control of an inducible promoter. Low and high levels of LIF expression were also compared since saturation of the apical or basolateral route would reveal specific transporters for LIF.ResultsWhen Caco-2 was grown on permeable supports, LIF was secreted constitutively with around 40% secreted into the apical chamber. Stimulation with IL-1β increased LIF production. After treating the apical surface with IL-1β, the percentage secreted apically remained similar to the untreated, whereas, when the cells were stimulated at the basolateral surface only 20% was secreted apically. In MDCK cells, an endogenous LIF-like protein was detected entirely in the apical compartment. The two mLIF isoforms showed no difference in their secretion patterns in MDCK. Interestingly, about 70% of murine and human LIF was secreted apically from MDCK over a 400-fold range of expression levels within clones and a 200,000-fold range across clones.ConclusionThe site of stimulation affected the polarity of LIF secretion, while, signal peptides and expression levels did not. Exogenous LIF is transported in MDCK without readily saturated steps.
Highlights
The direction of cytokine secretion from polarized cells determines the cytokine's cellular targets
Our results suggest that exogenous Leukemia inhibitory factor (LIF) is predominantly secreted in an apical direction in Madin-Darby canine kidney (MDCK), regardless of signal peptide or expression level, indicating a lack of specific transporters for LIF
Conditioned media was collected over a 48 hour period and assayed for LIF-like activity using a BA/F3mLIFR-mgp130 assay
Summary
The direction of cytokine secretion from polarized cells determines the cytokine's cellular targets. Three factors which may regulate the direction of LIF secretion were studied: the site of stimulation, signal peptides, and expression levels. Stimulation with IL-1β is known to promote IL-6 secretion from the stimulated membrane (apical or basolateral) in the human intestinal epithelial cell line Caco-2. The spatial regulation of cytokine secretion is important since it determines which neighbouring cells respond, but little is known about how cytokine secretion in the constitutive pathway is regulated. With their prominent apical and basolateral domains, epithelial cells have been studied extensively for (page number not for citation purposes). Lectins may cooperate with lipid rafts in the transport routes [5]
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