Secretory cargo composition affects polarized secretion in MDCK epithelial cells

Posttranslational processing of FGF23 in osteocytes during the osteoblast to osteocyte transition

FGF23 is an O-glycosylated circulating peptide hormone with a critical role in phosphate homeostasis; it is inactivated by cellular proprotein convertases in a pre-release degradative pathway. We have here examined the metabolism of FGF23 in a model bone cell line, IDG-SW3, prior to and following differentiation, as well as in regulated secretory cells. Labeling experiments showed that the majority of (35)S-labeled FGF23 was cleaved to smaller fragments which were constitutively secreted by all cell types. Intact FGF23 was much more efficiently stored in differentiated than in undifferentiated IDG-SW3 cells. The prohormone convertase PC2 has recently been implicated in FGF23 degradation; however, FGF23 was not targeted to forskolin-stimulatable secretory vesicles in a regulated cell line, suggesting that it lacks a targeting signal to PC2-containing compartments. In vitro, PC1/3 and PC2, but not furin, efficiently cleaved glycosylated FGF23; surprisingly, PC5/6 accomplished a small amount of conversion. FGF23 has recently been shown to be phosphorylated by the kinase FAM20C, a process which was shown to reduce FGF23 glycosylation and promote its cleavage; our in vitro data, however, show that phosphorylation does not directly impact cleavage, as both PC5/6 and furin were able to efficiently cleave unglycosylated, phosphorylated FGF23. Using qPCR, we found that the expression of FGF23 and PC5/6, but not PC2 or furin, increased substantially following osteoblast to osteocyte differentiation. Western blotting confirmed the large increase in PC5/6 expression upon differentiation. FGF23 has been linked to a variety of bone disorders ranging from autosomal dominant hypophosphatemic rickets to chronic kidney disease. A better understanding of the biosynthetic pathway of this hormone may lead to new treatments for these diseases.

The microenvironment patterns the pluripotent mouse epiblast through paracrine Furin and Pace4 proteolytic activities

The fate of pluripotent cells in early mouse embryos is controlled by graded Nodal signals that are activated by the endoproteases Furin and Pace4. Soluble forms of Furin and Pace4 cleave proNodal in vitro and after secretion in transfected cells, but direct evidence for paracrine activity in vivo is elusive. Here, we show that Furin and Pace4 are released by the extraembryonic microenvironment, and that they cleave a membrane-bound reporter substrate in adjacent epiblast cells and activate Nodal to maintain pluripotency. Secreted Pace4 and Furin also stimulated mesoderm formation, whereas endoderm was only induced by Pace4, correlating with a difference in the spatiotemporal distribution of these proteolytic activities. Our analysis of paracrine Furin and Pace4 activities and their in vivo functions significantly advances our understanding of how the epiblast is patterned by its microenvironment. Adding cell-cell communication to the pleiotropic portfolio of these proteases provides a new framework to study proprotein processing also in other relevant contexts.

Differential sorting of human parathyroid hormone after transduction of mouse and rat salivary glands

Gene transfer to salivary glands leads to abundant secretion of transgenic protein into either saliva or the bloodstream. This indicates significant clinical potential, depending on the route of sorting. The aim of this study was to probe the sorting characteristics of human parathyroid hormone (hPTH) in two animal models for salivary gland gene transfer. PTH is a key hormone regulating calcium levels in the blood. A recombinant serotype 5 adenoviral vector carrying the hPTH cDNA was administered to the submandibular glands of mice and rats. Two days after delivery, high levels of hPTH were found in the serum of mice, leading to elevated serum calcium levels. Only low amounts of hPTH were found in the saliva. Two days after vector infusion into rats, a massive secretion of hPTH was measured in saliva, with little secretion into serum. Confocal microscopy showed hPTH in the glands, localized basolaterally in mice and apically in rats. Submandibular gland transduction was effective and the produced hPTH was biologically active in vivo. Whereas hPTH sorted toward the basolateral side in mice, in rats hPTH was secreted mainly at the apical side. These results indicate that the interaction between hPTH and the cell sorting machinery is different between mouse and rat salivary glands. Detailed studies in these two species should result in a better understanding of cellular control of transgenic secretory protein sorting in this tissue.

Function of CSE1L/CAS in the secretion of HT-29 human colorectal cells and its expression in human colon

The secretion of colorectal epithelium is important for maintaining the physiological function of colorectal organ. Herein, we report that cellular apoptosis susceptibility (CAS) (or CSE1L) protein regulates the secretion of HT-29 human colorectal cells. Polarity is essential for directed secretion of substances produced by epithelial cells to the external (luminal) compartment; CAS overexpression induced polarization of HT-29 cells. CAS was punctate stained in the cytoplasm of HT-29 cells, and CAS overexpression increased the translocation of CAS-stained vesicles to the cytoplasm near cell membrane and cell protrusions. CAS overexpression increased the secretion of carcinoembryonic antigen (CEA) and cathepsin D. Immunohistochemistry showed CAS was positively stained in the goblet cells of colon mucosa and cells in the crypts of Lieberkühn of human colon as well as the glands in metastatic colorectal cancer tissue. Our results suggest that CAS regulates the secretion of colorectal cells and may regulate the metastasis of colorectal cancer.

Polarized secretion of leukemia inhibitory factor

Background

The 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.

Results

When 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.

Conclusion

The 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.