Regulated and constitutive protein targeting can be distinguished by secretory polarity in thyroid epithelial cells

Defective protein folding and intracellular retention of thyroglobulin-R19K mutant as a cause of human congenital goiter

It has been suggested that a thyroglobulin (Tg)-R19K missense mutation may be a newly identified cause of human congenital goiter, which is surprising for this seemingly conservative substitution. Here, we have examined the intracellular fate of recombinant mutant Tg expressed in COS-7 cells. Incorporation of the R19K mutation largely blocked Tg secretion, and this mutant was approximately 90% degraded intracellularly over a 24-h period after synthesis. Before its degradation, the Tg-R19K mutant exhibited abnormally increased association with molecular chaperones BiP, calnexin, and protein disulfide isomerase, and was unable to undergo anterograde advance from the endoplasmic reticulum (ER) through the Golgi complex. Inhibitors of proteasomal proteolysis and ER mannosidase-I both prevented ER-associated degradation of the Tg-R19K mutant and increased its association with ER molecular chaperones. ER quality control around Tg residue 19 is not dependent upon charge but upon side-chain packing, because Tg-R19Q was efficiently secreted. Whereas a Tg mutant truncated after residue 174 folds sufficiently well to escape ER quality control, introduction of the R19K point mutation blocked its secretion. The data indicate that the R19K mutation induces local misfolding in the amino-terminal domain of Tg that has global effects on Tg transport and thyroid hormonogenesis.

Protein sorting and secretion granule formation in regulated secretory cells

Formation of secretion granules in regulated secretory cells involves packaging a subject of proteins undergoing intracellular transport into specific vesicular carriers that function in stimulus-dependent exocytosis. Recent findings suggest that immature granules are a site of passive sorting, involving condensation of regulated secretory proteins. Proteins that are not condensed are stored to a lesser degree and are enriched in unstimulated, constitutive-like secretion. While these observations have helped to distinguish possible mechanisms of secretory protein sorting, there are only recent hints about the sorting processes that may be required to create the regulated secretory carrier membranes.

Polarized trafficking of thyrocyte proteins in MDCK cells

Recent studies suggest striking similarities between polarized protein sorting in thyrocytes and MDCK epithelial cells, including apical trafficking of thyroglobulin (Tg), thyroid peroxidase, and aminopeptidase N; as well as basolateral targeting of heparan sulfate proteoglycans, thrombospondin 1 (TSP1), type 1 5'-deiodinase, sodium-potassium ATPase, and the thyrotropin receptor. In this report, we have firstly expressed in stably transfected MDCK II cells a range of truncation mutants lacking up to 78% of the C-terminus of TSP1; these studies indicate that the N-terminal region containing the heparin binding domain is sufficient for basolateral targeting of TSP1. Secondly, we have stably transfected MDCK II cells with both Tg and sodium-iodide symporter (NIS) cDNAs, obtaining clones that simultaneously express both thyroid-specific proteins at the apical and basolateral cell surfaces, respectively. These studies represent promising early steps towards designing artificial thyrocytes by thyroid gene transfer into MDCK cells.

Thyroglobulin is selected as luminal protein cargo for apical transport via detergent-resistant membranes in epithelial cells

Thyroid hormone synthesis by thyrocytes depends upon apical secretion of thyroglobulin (Tg), the glycoprotein prohormone. In stably transfected MDCK cells, recombinant Tg is also secreted apically. All secreted Tg has undergone Golgi carbohydrate modification, whereas most intracellular Tg (which is slow to exit the endoplasmic reticulum) is sensitive to digestion with endoglycosidase H. However, in MDCK cells and PC Cl3 thyrocytes, a subpopulation of newly synthesized recombinant and endogenous Tg, respectively, is recovered in a Triton X-100 insoluble, glycosphingolipid/cholesterol-enriched (GEM/raft) fraction, and this small subpopulation is overwhelmingly endoglycosidase H resistant. Upon apical secretion, Tg solubility is restored. Apical secretion of Tg is inhibited by cellular cholesterol depletion. In FRT cells, recombinant Tg becomes Triton X-100 insoluble within 60 min after synthesis and a portion is actually endoglycosidase H-sensitive, suggesting early Tg entry into GEMs/rafts. Interestingly in FRT cells, Tg remains associated with the apical plasma membrane upon exocytosis, and all surface Tg is GEM/raft-associated. Thus, Tg is the first secretory protein demonstrated to enter Triton X-100 insoluble membranes en route to the apical surface of epithelial cells. The data imply that Tg utilizes a cargo-selective mechanism for apical sorting.

Basic mechanisms of secretion: sorting into the regulated secretory pathway

Targeting proteins to their correct cellular location is crucial for their biological function. In neuroendocrine cells, proteins can be secreted by either the constitutive or the regulated secretory pathways but the mechanism(s) whereby proteins are sorted into either pathway is unclear. In this review we discuss the possibility that sorting is either an active process occurring at the level of the trans-Golgi network, or that sorting occurs passively in the immature granules. The possible involvement of protein-lipid interactions in the sorting process is also raised.

Key words: lipid rafts, regulated secretory pathway, secretion, sorting receptors, sorting signals, trans-Golgi network.

Protein-disulfide isomerase (PDI) in FRTL5 cells. pH-dependent thyroglobulin/PDI interactions determine a novel PDI function in the post-endoplasmic reticulum of thyrocytes

Thyroglobulin (TG) is secreted by the thyrocytes into the follicular lumen of the thyroid. After maturation and hormone formation, TG is endocytosed and delivered to lysosomes. Quality control mechanisms may occur during this bidirectional traffic since 1) several molecular chaperones are cosecreted with TG in vivo, and 2) lysosomal targeting of immature TG is thought to be prevented via the interaction, in acidic conditions, between the Ser(789)-Met(1172) TG hormonogenic domain (BD) and an unidentified membrane receptor. We investigated the secretion and cell surface expression of PDI and other chaperones in the FRTL5 thyroid cell line, and then studied the characteristics of the interaction between TG and PDI. We demonstrated that PDI, but also other chaperones such as calnexin and KDEL-containing proteins are exposed at the cell surface. We observed on living cells or membrane preparations that PDI specifically binds TG in acidic conditions, and that only BD is involved in binding. Surface plasmon resonance analysis of TG/PDI interactions indicated: 1) that PDI bound TG but only in acidic conditions, and that it preferentially recognized immature molecules, and 2) BD is involved in binding even if cysteine-rich modules are deleted. The notion that PDI acts as an "escort" for immature TG in acidic post-endoplasmic reticulum compartments is discussed.

Polarized targeting of epithelial cell proteins in thyrocytes and MDCK cells

Polarized trafficking signals may be interpreted differently in different cell types. In this study, we have compared the polarized trafficking of different proteins expressed endogenously in primary porcine thyroid epithelial cells to similar proteins expressed in MDCK cells. As in MDCK cells, NH4Cl treatment of filter-grown thyrocytes caused mis-sorted soluble proteins to exhibit enhanced secretion to the apical medium. In independent studies, thrombospondin 1 (a thyroid basolaterally secreted protein) was secreted basolaterally from MDCK cells. Likewise, the 5′-deiodinase (a thyroid basolateral membrane protein) encoded by the DIO1 gene was also distributed basolaterally in transfected MDCK cells. Consistent with previous reports, when the secretion of human growth hormone (an unglycosylated regulated secretory protein) was examined from transfected MDCK cells, the release was nonpolarized. However, transfected thyrocytes secreted growth hormone apically in a manner dependent upon zinc addition. Moreover, two additional regulated secretory proteins expressed in thyrocytes, thyroglobulin (the major endogenous glycoprotein) and parathyroid hormone (an unglycosylated protein expressed transiently), were secreted apically even in the absence of zinc. We hypothesize that while cellular mechanisms for interpreting polarity signals are generally similar between thyrocytes and MDCK cells, thyrocytes allow for specialized packaging of regulated secretory proteins for apical delivery, which does not require glycosylation but may involve availability of certain ions as well as appropriate intracellular compartmentation.

Regulation of the three-dimensional organization of thyroid epithelial cells into follicle structures by the matricellular protein, thrombospondin-1

Thyroid epithelial cells in primary culture have the capacity to organize into thyroid-specific three-dimensional structures, the follicles, in response to TSH. We studied whether thrombospondin 1 (TSP1), which represents, besides thyroglobulin, the main protein secreted by thyroid cells, could play a role in the process of folliculogenesis. TSH promoted follicle formation and inhibited TSP1 production. On the contrary, the phorbol ester, 12-O-tetradecanoyl-phorbol 13-acetate (1-100 nM) prevented TSH-induced follicle formation and strongly increased the synthesis of TSP1. Activation of TSP1 synthesis was dependent upon messenger RNA synthesis. Transforming growth factor-beta, like 12-O-tetradecanoyl-phorbol 13-acetate, increased TSP1 synthesis and prevented TSH-induced follicle formation. Thus, signaling molecules that depressed or conversely activated TSP1 production, respectively promoted or prevented thyroid folliculogenesis. TSP1, purified from platelets, was devoid of effect on cell substratum attachment, but exerted a concentration-dependent inhibition of the TSH-activated reconstitution of thyroid follicles (half-inhibition at 40 microg/ml). TSP1 exhibited the same effect when added to thyroid cell aggregates representing primitive follicle structures. Our data suggest that the control of thyroid follicle formation may operate at least in part through regulation of the production of the matricellular protein TSP1, which acts as a negative modulator of the cell-cell adhesion process involved in thyroid follicle morphogenesis.

Enhanced binding to the molecular chaperone BiP slows thyroglobulin export from the endoplasmic reticulum

To examine how binding of BiP (a molecular chaperone of the hsp70 family that resides in the endoplasmic reticulum) influences the conformational maturation of thyroglobulin (Tg, the precursor for thyroid hormone synthesis), we have developed a system of recombinant Tg stably expressed in wild-type Chinese hamster ovary (CHO) cells and CHO-B cells genetically manipulated for selectively increased BiP expression. The elevation of immunoreactive BiP in CHO-B cells is comparable to that seen during the unfolded protein response in the thyrocytes of certain human patients and animals suffering from congenital hypothyroid goiter with defective Tg. However, in CHO-B cells, we expressed Tg containing no mutations that induce misfolding (i.e. no unfolded protein response), so that levels of all other endoplasmic reticulum chaperones were normal. Increased availability of BiP did not accelerate Tg secretion; rather, the export of newly synthesized Tg was delayed. Tg detained intracellularly was concentrated in the endoplasmic reticulum. By coimmunoprecipitation, BiP exhibited enhanced binding to Tg in CHO-B cells. Moreover, two-dimensional gel analysis showed that BiP associated especially well with intracellular Tg containing mispaired disulfide bonds, thought to represent early Tg folding intermediates. An endoplasmic reticulum chaperone of the hsp90 family, GRP94, was also associated in Tg-chaperone complexes. The results suggest that increased binding of BiP to Tg leads to its delayed conformational maturation in the endoplasmic reticulum.

Thyroglobulin transport along the secretory pathway. Investigation of the role of molecular chaperone, GRP94, in protein export from the endoplasmic reticulum

GRP94 serves as a molecular chaperone in the endoplasmic reticulum (ER). In normal thyrocytes, GRP94 interacts transiently with thyroglobulin (Tg), and in thyrocytes of animals suffering from congenital hypothyroid goiter with defective thyroglobulin, GRP94 and thyroglobulin associate in a protracted fashion. In order explore possible consequences of GRP94 binding, we have studied recombinant nonmutant thyroglobulin expressed in control Chinese hamster ovary (CHO) cells in comparison to that produced in CHO cells genetically manipulated for selectively increased GRP94 expression. Levels of ER chaperones other than GRP94 did not detectably differ, and thyroglobulin achieved transport competence in both kinds of CHO cells. However, increased availability of GRP94 caused the residence time of Tg in the ER to be remarkably prolonged. This was accompanied by a major increase in Tg directly associated with GRP94 and an increase in the ER pool size of Tg. Importantly, co-immunoprecipitation analysis revealed disulfide-linked Tg complexes (previously reported as an early Tg-folding intermediate) especially associated with GRP94. Indeed, non-native Tg, GRP94, and a 78-kDa protein likely to be BiP, appeared in ternary complexes. Under these conditions, GRP94 association appears directly involved in prolongation of Tg folding and export, consistent with a role in quality control in the ER.

Differential sorting of lysosomal enzymes out of the regulated secretory pathway in pancreatic beta-cells

In cells specialized for secretory granule exocytosis, lysosomal hydrolases may enter the regulated secretory pathway. Using mouse pancreatic islets and the INS-1 beta-cell line as models, we have compared the itineraries of procathepsins L and B, two closely related members of the papain superfamily known to exhibit low and high affinity for mannose-6-phosphate receptors (MPRs), respectively. Interestingly, shortly after pulse labeling INS cells, a substantial fraction of both proenzymes exhibit regulated exocytosis. After several hours, much procathepsin L remains as precursor in a compartment that persists in its ability to undergo regulated exocytosis in parallel with insulin, while procathepsin B is efficiently converted to the mature form and can no longer be secreted. However, in islets from transgenic mice devoid of cation-dependent MPRs, the modest fraction of procathepsin B normally remaining within mature secretory granules is increased approximately fourfold. In normal mouse islets, immunoelectron microscopy established that both cathepsins are present in immature beta-granules, while immunolabeling for cathepsin L, but not B, persists in mature beta-granules. By contrast, in islets from normal male Sprague-Dawley rats, much of the proenzyme sorting appears to occur earlier, significantly diminishing the stimulus-dependent release of procathepsin B. Evidently, in the context of different systems, MPR-mediated sorting of lysosomal proenzymes occurs to a variable extent within the trans-Golgi network and is continued, as needed, within immature secretory granules. Lysosomal proenzymes that fail to be sorted at both sites remain as residents of mature secretory granules.

Intracellular protein transport to the thyrocyte plasma membrane: potential implications for thyroid physiology

We present a snapshot of developments in epithelial biology that may prove helpful in understanding cellular aspects of the machinery designed for the synthesis of thyroid hormones on the thyroglobulin precursor. The functional unit of the thyroid gland is the follicle, delimited by a monolayer of thyrocytes. Like the cells of most simple epithelia, thyrocytes exhibit specialization of the cell surface that confronts two different extracellular environments-apical and basolateral, which are separated by tight junctions. Specifically, the basolateral domain faces the interstitium/bloodstream, while the apical domain is in contact with the lumen that is the primary target for newly synthesized thyroglobulin secretion and also serves as a storage depot for previously secreted protein. Thyrocytes use their polarity in several important ways, such as for maintaining basolaterally located iodide uptake and T4 deiodination, as well apically located iodide efflux and iodination machinery. The mechanisms by which this organization is established, fall in large part under the more general cell biological problem of intracellular sorting and trafficking of different proteins en route to the cell surface. Nearly all exportable proteins begin their biological life after synthesis in an intracellular compartment known as the endoplasmic reticulum (ER), upon which different degrees of difficulty may be encountered during nascent polypeptide folding and initial export to the Golgi complex. In these initial stages, ER molecular chaperones can assist in monitoring protein folding and export while themselves remaining as resident proteins of the thyroid ER. After export from the ER, most subsequent sorting for protein delivery to apical or basolateral surfaces of thyrocytes occurs within another specialized intracellular compartment known as the trans-Golgi network. Targeting information encoded in secretory proteins and plasma membrane proteins can be exposed or buried at different stages along the export pathway, which is likely to account for sorting and specific delivery of different newly-synthesized proteins. Defects in either burying or exposing these structural signals, and consequent abnormalities in protein transport, may contribute to different thyroid pathologies.

Congenital hypothyroid goiter with deficient thyroglobulin. Identification of an endoplasmic reticulum storage disease with induction of molecular chaperones

Recent advances in understanding the molecular pathogenesis of congenital hypothyroid goiter in cog/cog mice, have raised important questions concerning the maturation of thyroglobulin (the thyroid prohormone) in certain human kindreds with congenital goiter. We have now examined affected siblings from two unrelated families that synthesize an apparently normally glycosylated, > 300 kD immunoreactive thyroglobulin, yet have a reduced quantity of intraglandular thyroglobulin and that secreted into the circulation. From thyroid tissues of the four patients, light microscopic approaches demonstrated presence of intracellular thyroglobulin despite its absence in thyroid follicle lumina, while electron microscopy indicated abnormal distention of the endoplasmic reticulum (ER). We have confirmed biochemically that most intrathyroidal thyroglobulin fails to reach the (Golgi) compartment where complex carbohydrate modification takes place. Moreover, the disease in the affected patients is associated with massive induction of specific ER molecular chaperones including the hsp90 homolog, GRP94, and the hsp70 homolog, BiP. The data suggest that these patients synthesize a mutant thyroglobulin which is defective for folding/assembly, leading to a markedly reduced ability to export the protein from the ER. Thus, these kindreds suffer from a thyroid ER storage disease, a cell biological defect phenotypically indistinguishable from that found in cog/cog mice.

Endocytotic functions of ameloblasts and odontoblasts: immunocytochemical and tracer studies on the uptake of plasma proteins

BACKGROUND

Biochemical, (immuno)cytochemical, and radioautographic data accumulated over several years have lead to the view that ameloblasts carry out both secretory and degradative functions throughout amelogenesis. Whereas it has been assumed that maturation stage ameloblasts endocytose aged enamel proteins from the enamel layer, the origin of the newly formed ones detected in the endosomal/lysosomal compartment of ameloblasts from all stages remains to be elucidated. One possible source is from secretory products released ectopically along basolateral surfaces.

METHODS

To test this hypothesis, we have investigated, using colloidal gold immunocytochemistry, whether plasma proteins (albumin and alpha 2HS-glycoprotein) found in the interstitial fluid are endocytosed by rat incisor ameloblasts and other cells from hard and soft tissues. Rat albumin, tagged with dinitrophenol, was injected intravenously to trace the movement of this protein.

RESULTS

Plasma proteins were immunodetected along the baso-lateral surfaces and in multivesicular bodies of ameloblasts where enamel proteins were also found. By 2 hours following intravenous administration of dinitrophenylated albumin, the tracer had left the blood and diffused into the enamel organ and between odontoblasts and osteoblasts. The tracer was also found in multivesicular bodies of all cells examined.

CONCLUSIONS

The uptake of albumin by many different cell types suggests that this process is not restricted to ameloblasts and likely occurs in a nonselective manner. Hence, baso-lateral uptake in ameloblasts may play a role not only in the continuous removal of plasma proteins leaking from the blood, but also of enamel proteins 'dumped' laterally between these cells. Likewise, odontoblasts may use the same mechanism to internalize some of the plasma proteins and any enamel protein that diffuse toward them.

An endoplasmic reticulum storage disease causing congenital goiter with hypothyroidism

In humans, deficient thyroglobulin (Tg, the thyroid prohormone) is an important cause of congenital hypothyroid goiter; further, homozygous mice expressing two cog/cog alleles (linked to the Tg locus) exhibit the same phenotype. Tg mutations might affect multiple different steps in thyroid hormone synthesis; however, the microscopic and biochemical phenotype tends to involve enlargement of the thyroid ER and accumulation of protein bands of M(r) < 100. To explore further the cell biology of this autosomal recessive illness, we have examined the folding and intracellular transport of newly synthesized Tg in cog/cog thyroid tissue. We find that mutant mice synthesize a full-length Tg, which appears to undergo normal N-linked glycosylation and glucose trimming. Nevertheless, in the mutant, Tg is deficient in the folding that leads to homodimerization, and there is a deficiency in the quantity of intracellular Tg transported to the distal portion of the secretory pathway. Indeed, we find that the underlying disorder in cog/cog mice is a thyroid ER storage disease, in which a temperature-sensitive Tg folding defect, in conjunction with normal ER quality control mechanisms, leads to defective Tg export. In relation to quality control, we find that the physiological response in this illness includes the specific induction of five molecular chaperones in the thyroid ER. Based on the pattern of chaperone binding, different potential roles for individual chaperones are suggested in glycoprotein folding, retention, and degradation in this ER storage disease.

Peptidergic transmission: from morphological correlates to functional implications

Like non-peptidergic transmitters, neuropeptides and their receptors display a wide distribution in specific cell types of the nervous system. The peptides are synthesized, typically as part of a larger precursor molecule, on the rough endoplasmic reticulum in the cell body. In the trans-Golgi network, they are sorted to the regulated secretory pathway, packaged into so-called large dense-core vesicles, and concentrated. Large dense-core vesicles are preferentially located at sites distant from active zones of synapses. Exocytosis may occur not only at synaptic specializations in axonal terminals but frequently also at nonsynaptic release sites throughout the neuron. Large dense-core vesicles are distinguished from small, clear synaptic vesicles, which contain "classical' transmitters, by their morphological appearance and, partially, their biochemical composition, the mode of stimulation required for release, the type of calcium channels involved in the exocytotic process, and the time course of recovery after stimulation. The frequently observed "diffuse' release of neuropeptides and their occurrence also in areas distant to release sites is paralleled by the existence of pronounced peptide-peptide receptor mismatches found at the light microscopic and ultrastructural level. Coexistence of neuropeptides with other peptidergic and non-peptidergic substances within the same neuron or even within the same vesicle has been established for numerous neuronal systems. In addition to exerting excitatory and inhibitory transmitter-like effects and modulating the release of other neuroactive substances in the nervous system, several neuropeptides are involved in the regulation of neuronal development.

Polarized distribution and delivery of plasma membrane proteins in thyroid follicular epithelial cells

Thyroid follicular cells coordinate several oppositely located surface enzyme activities. Recent studies have raised questions about the basic mechanisms used to achieve thyroid surface polarity. We investigated these mechanisms in primary thyroid epithelial monolayers cultured on porous filters. In the steady state, most Na+/K(+)-ATpase and aminopeptidase N were available for surface biotinylation, and these proteins exhibited physiological distributions (basolateral and apical, respectively). Glycosylphosphatidylinositol-anchored proteins were also apically distributed. By pulse-chase, newly synthesized transmembrane proteins exhibited polarized surface delivery that was oriented similarly to that observed at steady state. Little time elapsed between acquisition of Golgi-specific processing and cell surface arrival. Interestingly, when either newly synthesized or steady state-labeled thyroid peroxidase was similarly analyzed, only approximately 30% of the enzyme was ever detected at the cell surface. Of this, the majority was localized apically. The data suggest that most thyroid peroxidase remains intracellular in these monolayers, consistent with the possibility of intracellular iodination activity in addition to apical extracellular iodination. Nevertheless, in filter-polarized thyrocytes, most newly synthesized plasma membrane proteins appear to be sorted in the Golgi complex for direct delivery to apical and basolateral domains.

Differential expression of thrombospondin, collagen, and thyroglobulin by thyroid-stimulating hormone and tumor-promoting phorbol ester in cultured porcine thyroid cells

In the present study, we have investigated the potential regulation of thyroglobulin (Tg) and extracellular matrix components synthesis by thyroid-stimulating hormone (TSH) and tetradecanoyl phorbol-13-acetate (TPA) on thyroid cells. Porcine thyroid cells isolated by trypsin-EGTA digestion of thyroid glands were maintained in serum containing medium on poly (L-lysine)-coated dishes. Cells differentiated into follicular or vesicular-like structures were distinguished by their ability to organify Na[125I] and to respond to TSH stimulation. After an incubation of the cells with radiolabeled proline or methionine, two major proteins were identified, p450-480 and p290 (so named because of their molecular masses). Tg (p290) synthesis was demonstrated by the synthesis of [131I]-labeled polypeptides with electrophoretic properties identical to those of authentic Tg molecules. P450-480 resolved to M(r) 190,000 under reducing sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) conditions. It was identified as thrombospondin by its reactivity with a monoclonal anti-human thrombospondin and by peptide sequencing of some of its tryptic fragments that displayed identity to thrombospondin I. Collagen synthesis was demonstrated by the formation of radioactive hydroxyproline and by the synthesis of pepsin-resistant polypeptides ranging from M(rs) 120,000 to 200,000. When the cells were cultured in the presence of 100 nM TPA, the culture medium contents of thrombospondin and collagen were increased by 2.7 and 1.6-fold, respectively, whereas Tg content was decreased by a factor 3.9. In contrast, the acute treatment of control cells with TPA induced a decrease in both Tg and collagen content by factors 3.0 and 1.5, respectively, and an increase in thrombospondin content by a factor 2.5. In the presence of 100 nM TPA, TSH (1 mU/ml) did not counteract the stimulating effect of TPA on extracellular matrix components synthesis. In contrast, when cells were cultured in the presence of TSH alone at concentrations higher than 0.1 mU/ml, collagen and thrombospondin in the medium were decreased by a factor 2.0 and 1.9, respectively, and TSH preferentially activated Tg synthesis. However, no acute response to TSH was observed in cells incubated for 2 days without effectors (control cells). On TSH differentiated cells, TPA decreased both collagen and Tg accumulation by factors 1.2 and 1.8, respectively, whereas it increased the one of thrombospondin by a factor 2. These results, together with the stimulating effect of TPA on TSH mediated cell proliferation, argue for a role of thrombospondin in cell adhesion and migration events within the thyroid epithelium.

Phasic secretion of newly synthesized atrial natriuretic factor from unstimulated atrial myocytes in culture

We have examined kinetics and composition of newly synthesized proteins secreted from cultured atrial myocytes from adult rats. Under unstimulated conditions, noncontracting cultured atrial myocytes, which were pulse-labeled for 10 minutes with [35S]methionine, rapidly released a considerable portion of newly synthesized atrial natriuretic factor (ANF) in a phasic secretion with a peak at 40 to 80 minutes of chase time. The phasic secretion almost ceased after 80 minutes of chase, after which relatively slow release of the hormone was observed. The ability to stimulate the phasic secretion with secretagogues and a marked resemblance of the radiochemical composition of released proteins in the unstimulated phasic secretion to that in stimulated secretion suggest that the proteins discharged from the cells during the phasic secretion might be derived from secretory granules. Examination of the quantitative change of intracellular ANF showed that approximately 60% of newly synthesized labeled ANF was still retained in the cells after the termination of the phasic secretion, indicating that the termination of the phasic secretion was not due to depletion of the labeled protein in the cells. These results suggest that a proportion of newly synthesized ANF was rapidly released from the unstimulated atrial myocytes via a secretory route that shares certain features with both the regulated and the constitutive secretory pathway and that a part of newly synthesized ANF is processed for rapid release while the remainder is destined for slow release or storage within the cells.

Coexpression of FGF-5 and bFGF by the retinal pigment epithelium in vitro

Members of the fibroblast growth factor (FGF) gene family have been proposed to play critical roles in the biology of the outer retina. In this study, in vitro cultures of human retinal pigment epithelial (RPE) cells were surveyed for the expression of FGF gene family members. Northern analysis provided evidence for the expression of the previously reported 7.0 and 3.7 kb basic FGF (bFGF) transcripts and for the 4.0 and 2.1 kb FGF-5 transcripts. Western analysis demonstrated the presence of three bFGF proteins ranging in size from 18 to 26 kDa and three FGF-5 proteins of molecular weights 28.5, 34, and 36 kDa. We were particularly interested in cellular mechanisms which might regulate the steady-state mRNA levels of these genes. It was determined that bFGF message levels decreased with increasing culture density, increased upon serum stimulation and when placed in contact with matrix components found in the extracellular matrix of RPE cells in vivo. In a similar fashion, the steady state mRNA levels for FGF-5 decreased with increasing culture density, increased upon serum stimulation, but appeared to be unaffected by matrix contact.

Constitutive protein secretion by guinea-pig seminal vesicle epithelial cells

1. Secretion of pulse-labelled protein by the isolated epithelium of guinea-pig seminal vesicle epithelium was rapid, unaffected by cholinergic and adrenergic drugs, cyclic nucleotides or changes in the sodium, potassium and calcium concentrations of the "chase" medium. 2. Low temperature, NH4Cl, hyper- and hypo-osmolarity and membrane-stabilizing agents inhibited secretion which was also dependent on aerobic metabolism. 3. Monensin reduced secretion of the six labelled, relatively low molecular weight proteins recovered from the medium in a concentration-dependent, apparently non-specific manner.

Proteins are secreted by both constitutive and regulated secretory pathways in lactating mouse mammary epithelial cells

Lactating mammary epithelial cells secrete high levels of caseins and other milk proteins. The extent to which protein secretion from these cells occurs in a regulated fashion was examined in experiments on secretory acini isolated from the mammary glands of lactating mice at 10 d postpartum. Protein synthesis and secretion were assayed by following the incorporation or release, respectively, of [35S]methionine-labeled TCA-precipitable protein. The isolated cells incorporated [35S]methionine into protein linearly for at least 5 h with no discernible lag period. In contrast, protein secretion was only detectable after a lag of approximately 1 h, consistent with exocytotic secretion of proteins immediately after passage through the secretory pathway and package into secretory vesicles. The extent of protein secretion was unaffected by the phorbol ester PMA, 8-bromo-cAMP, or 8-bromo-cGMP but was doubled by the Ca2+ ionophore ionomycin. In a pulse-label protocol in which proteins were prelabeled for 1 h before a chase period, constitutive secretion was unaffected by depletion of cytosolic Ca2+ but ionomycin was found to give a twofold stimulation of the secretion of presynthesized protein in a Ca(2+)-dependent manner. Ionomycin was still able to stimulate protein secretion after constitutive secretion had terminated. These results suggest that lactating mammary cells possess both a Ca(2+)-independent constitutive pathway and a Ca(2+)-activated regulatory pathway for protein secretion. The same proteins were secreted by both pathways. No ultrastructural evidence for apocrine secretion was seen in response to ionomycin and so it appears that regulated casein release involves exocytosis. Ionomycin was unlikely to be acting by disassembling the cortical actin network since cytochalasin D did not mimic its effects on secretion. The regulated pathway may be controlled by Ca2+ acting at a late step such as exocytotic membrane fusion.

Secretory granule and synaptic vesicle formation

Sorting of secreted proteins into dense-core secretory granules may involve selective aggregation of regulated secretory proteins, rather than a conventional sortase. Synaptic vesicles, which mediate paracrine communication between adjacent cells, appear to arise by a modification of the early endosome pathway. Targeting to the cell surface involves the actin-based cytoskeleton and small GTP-binding proteins.