Acquisition of Lubrol Insolubility, a Common Step for Growth Hormone and Prolactin in the Secretory Pathway of Neuroendocrine Cells

Rat prolactin in the dense cores of secretory granules of the pituitary gland is a Lubrol-insoluble aggregate. In GH4C1 cells, newly synthesized rat prolactin and growth hormone were soluble, but after 30 min about 40% converted to a Lubrol-insoluble form. Transport from the endoplasmic reticulum is necessary for conversion to Lubrol insolubility, since incubating cells with brefeldin A or at 15 °C reduced formation of insoluble rat 35S-prolactin. Formation of Lubrol-insoluble aggregates has protein and cell specificity; newly synthesized human growth hormone expressed in AtT20 cells underwent a 40% conversion to Lubrol insolubility with time, but albumin did not, and human growth hormone expressed in COS cells underwent less than 10% conversion to Lubrol insolubility. del32–46 growth hormone, a naturally occurring form of growth hormone, and P89L growth hormone underwent conversion, although they were secreted more slowly, indicating that there is some tolerance in structural requirements for aggregation. An intracellular compartment with an acidic pH is not necessary for conversion to Lubrol insolubility, because incubation with chloroquine or bafilomycin slowed, but did not prevent, the conversion. GH4C1 cells treated with estradiol, insulin, and epidermal growth factor accumulate more secretory granules and store more prolactin, but not more growth hormone, than untreated cells; Lubrol-insoluble aggregates of prolactin and growth hormone formed to the same extent in hormone-treated or untreated GH4C1 cells, but prolactin was retained longer in hormone-treated cells. These findings indicate that aggregation alone is not sufficient to cause retention of secretory granule proteins, and there is an additional selective process. Rat prolactin in the dense cores of secretory granules of the pituitary gland is a Lubrol-insoluble aggregate. In GH4C1 cells, newly synthesized rat prolactin and growth hormone were soluble, but after 30 min about 40% converted to a Lubrol-insoluble form. Transport from the endoplasmic reticulum is necessary for conversion to Lubrol insolubility, since incubating cells with brefeldin A or at 15 °C reduced formation of insoluble rat 35S-prolactin. Formation of Lubrol-insoluble aggregates has protein and cell specificity; newly synthesized human growth hormone expressed in AtT20 cells underwent a 40% conversion to Lubrol insolubility with time, but albumin did not, and human growth hormone expressed in COS cells underwent less than 10% conversion to Lubrol insolubility. del32–46 growth hormone, a naturally occurring form of growth hormone, and P89L growth hormone underwent conversion, although they were secreted more slowly, indicating that there is some tolerance in structural requirements for aggregation. An intracellular compartment with an acidic pH is not necessary for conversion to Lubrol insolubility, because incubation with chloroquine or bafilomycin slowed, but did not prevent, the conversion. GH4C1 cells treated with estradiol, insulin, and epidermal growth factor accumulate more secretory granules and store more prolactin, but not more growth hormone, than untreated cells; Lubrol-insoluble aggregates of prolactin and growth hormone formed to the same extent in hormone-treated or untreated GH4C1 cells, but prolactin was retained longer in hormone-treated cells. These findings indicate that aggregation alone is not sufficient to cause retention of secretory granule proteins, and there is an additional selective process. 4-morpholineethanesulfonic acid N-(-((2, 4-dinitrophenyl)amino)propyl)-N-(3-aminopropyl)methylamine growth hormone Secretory proteins are synthesized on polysomes attached to the endoplasmic reticulum and transported through its membranes into its lumen, where the proteins fold. Vesicular or tubular structures transport folded proteins to the cis region of the Golgi complex, and the proteins process through the stacks of the Golgi complex to the trans side, after which vesicles deliver secretory proteins to the cell surface. Neuroendocrine cells, exocrine cells, peptidergic neurons, mast cells, and some other secretory cells have a specialized pathway to the cell surface in which certain secreted proteins are stored in concentrated forms in vesicles called secretory granules, so that the proteins are rapidly available in large amounts when needed. Concentration of protein hormones in granules is extensive; prolactin, for example, is 200 times more concentrated in dense cores of secretory granules than in the lumen of the endoplasmic reticulum (1Farquhar M.G. Reid J.J. Daniell L.W. Endocrinology. 1978; 102: 296-311Crossref PubMed Scopus (116) Google Scholar). A possible mechanism for concentrating proteins is self-association or aggregation; the formation of insoluble aggregates of prolactin in secretory granules is evidence for such a mechanism. Giannattasio et al. (2Giannattasio G. Zanini A. Meldolesi J. J. Cell Biol. 1975; 64: 246-251Crossref PubMed Scopus (50) Google Scholar) isolated intact prolactin-containing secretory granules from rat pituitary glands and then removed the granule membranes with detergent. When the nonionic detergent Lubrol was used, the granule membranes dissolved, but the prolactin contents remained insoluble, and the dense cores of the granules retained their morphology (2Giannattasio G. Zanini A. Meldolesi J. J. Cell Biol. 1975; 64: 246-251Crossref PubMed Scopus (50) Google Scholar). Dense cores of secretory granules with no surrounding membrane appear occasionally in the pericapillary space of the pituitary gland, indicating that the cores dissolve relatively slowly after exocytosis in animals (3Farquhar M.G. Dellmann H.-D. Johnson J.A. Klachko D.M. Comparative Endocrinology of Prolactin. Plenum Press, New York1974Google Scholar, 4Zanini A. Giannattasio G. Nussdorfer G. Margolis R.K. Margolis R.U. Meldolesi J. J. Cell Biol. 1980; 86: 260-272Crossref PubMed Scopus (47) Google Scholar). Aggregates of prolactin resembling what are found in secretory granule cores are detectable by electron microscopy in the lumen of the trans-Golgi cisternae, so the process begins before the granules form (5Rambourg A. Clermont Y. Chretien M. Olivier L. Anat. Rec. 1992; 232: 169-179Crossref PubMed Scopus (42) Google Scholar). Prolactin in solution has a tendency to aggregate at mildly acidic pH (6Thompson M.E. Zimmer W.E. Haynes A.L. Valentine D.L. Forss-Petter S. Scammell J.G. Endocrinology. 1992; 131: 318-326Crossref PubMed Scopus (32) Google Scholar, 7Rindler M.J. J. Biol. Chem. 1998; 273: 31180-31185Abstract Full Text Full Text PDF PubMed Scopus (56) Google Scholar); the tendency to aggregate in solution at acidic pH is a property shared by many proteins that are stored in granules (8Song L. Fricker L.D. J. Biol. Chem. 1995; 270: 7963-7967Abstract Full Text Full Text PDF PubMed Scopus (55) Google Scholar, 9Colomer V. Kicska G.A. Rindler M.J. J. Biol. Chem. 1996; 271: 48-55Abstract Full Text Full Text PDF PubMed Scopus (133) Google Scholar, 10Gorr S.U. Shioi J. Cohn D.V. Am. J. Physiol. 1989; 257: E247-E254PubMed Google Scholar, 11Chanat E. Huttner W.B. J. Cell Biol. 1991; 115: 1505-1519Crossref PubMed Scopus (383) Google Scholar, 12Freedman S.D. Scheele G.A. Biochem. Cell Biol. Commun. 1993; 197: 992-999Google Scholar, 13Leblond F.A. Viau G. Laine J. Lebel D. Biochem. J. 1993; 291: 289-296Crossref PubMed Scopus (62) Google Scholar). Such aggregation in solution has led to one of the simplest models for concentrating secretory granule proteins, which is that secretory proteins are soluble in the secretory pathway until they reach the trans-Golgi region, where they aggregate, facilitated by the decrease in pH that occurs there. In this model, aggregation in the trans-Golgi cisternae may play a role in sorting proteins as well as concentrating secretory granule proteins (14Arvan P. Castle D. Biochem. J. 1998; 332: 593-610Crossref PubMed Scopus (440) Google Scholar, 15Dannies P.S. Endocr. Rev. 1999; 20: 3-21PubMed Google Scholar), since small vesicles carry off soluble proteins, leaving behind aggregated proteins, too large to remove, to form dense cores of secretory granules. Prolactin and growth hormone have several properties that make them useful for understanding the processes that lead to storage in secretory granules. They are small (22 kDa), monomeric proteins that are stored in granules and neither glycosylated nor proteolytically processed beyond the cleavage of the signal sequence necessary for transport into the endoplasmic reticulum. They are structurally similar proteins (16Nicoll C.S. Mayer G.L. Russell S.M. Endocr. Rev. 1986; 7: 169-203Crossref PubMed Scopus (324) Google Scholar). Despite these similarities, their storage is regulated in a pituitary cell are rat pituitary cells that make prolactin, growth hormone, and These cells accumulate secretory granules with dense cores of prolactin and store prolactin when with insulin, estradiol, and epidermal growth factor J.G. P.S. Endocrinology. 1986; PubMed Scopus Google Scholar). of prolactin from GH4C1 cells in these and prolactin from of are similar when is by several D. P.S. PubMed Scopus Google Scholar), indicating that the regulated pathway of secretory granules in these cells as in When are in the of estradiol, insulin, and epidermal growth there are dense secretory granules in the cells, and prolactin storage is reduced J.G. P.S. Endocrinology. 1986; PubMed Scopus Google Scholar). not storage of other proteins in secretory granules in these cells, growth hormone, its to prolactin J.G. P.S. Endocrinology. 1986; PubMed Scopus Google Scholar, P.S. PubMed Scopus Google Scholar, L.D. P.S. PubMed Scopus Google Scholar, S.U. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar). have hormone the of cells to store prolactin by their to aggregate prolactin were in a of and with or hormone estradiol, insulin, and epidermal growth as and after of cells were with 200 of in and with and the cells were with and with or estradiol, insulin, and epidermal growth at and cells were and cells were in pH and and and then at for at When the cell was a before was in and by was as M. P.S. Endocrinology. 1998; PubMed Scopus Google Scholar). in the and was after a AtT20 and COS cells were in 10% at and no more than at a stored in to was after to human growth hormone was from the and and A. and to human albumin was from a of human albumin from the was the which by after the sequence into by the and the growth hormone in growth hormone was a from PubMed Scopus Google Scholar) and and into by and and as and del32–46 as J. P.S. Endocrinology. PubMed Scopus Google Scholar). P89L growth hormone was and by the of et al. 1989; PubMed Scopus Google Scholar). were by the M. and to GH4C1 cells were with N-(-((2, 4-dinitrophenyl)amino)propyl)-N-(3-aminopropyl)methylamine in the or of 30 chloroquine for 30 min and then in and pH was in cells microscopy was on a at the for Cell AtT20 cells were in and pH and for human growth hormone from the and and A. and to as a properties of newly synthesized proteins were after a incubation with and a incubation with to as proteins through the secretory the of a with is in the endoplasmic reticulum (1Farquhar M.G. Reid J.J. Daniell L.W. Endocrinology. 1978; 102: 296-311Crossref PubMed Scopus (116) Google Scholar). In GH4C1 cells treated with of newly synthesized was soluble when cell membranes were with longer times after some of the intracellular insoluble in Lubrol a property of prolactin aggregates in the dense cores of secretory granules (2Giannattasio G. Zanini A. Meldolesi J. J. Cell Biol. 1975; 64: 246-251Crossref PubMed Scopus (50) Google Scholar). of the was insoluble 30 min after the of the incubation with in the in the of that insoluble at 30 min from to of the in of prolactin aggregates in dense cores of secretory granules Lubrol to the membranes (2Giannattasio G. Zanini A. Meldolesi J. J. Cell Biol. 1975; 64: 246-251Crossref PubMed Scopus (50) Google Scholar). In Lubrol to the found no in the of that was insoluble of Lubrol from to Lubrol is a more detergent for aggregates of prolactin in the dense cores of secretory granules than or (2Giannattasio G. Zanini A. Meldolesi J. J. Cell Biol. 1975; 64: 246-251Crossref PubMed Scopus (50) Google Scholar), and this was in GH4C1 cells. was less after a when cells were with or than when they were by prolactin at the of a in but aggregates of prolactin in the dense cores of secretory granules through by a (2Giannattasio G. Zanini A. Meldolesi J. J. Cell Biol. 1975; 64: 246-251Crossref PubMed Scopus (50) Google Scholar). is shared by of the Lubrol-insoluble form in GH4C1 cells. of in GH4C1 cells that the and to through with after In detergent to cells to and of but in one the cells with a and removed the with a before were similar to when cells were the of that was Lubrol-insoluble after a was and through indicating that the of did not to the of prolactin in cells were for min and then for 30 min after the were were in the and insoluble was isolated as in to are the of intracellular that is in a properties of of of insoluble that cells were with and cells were at the times after the of that insoluble is the of intracellular that forms a in after for times did not the rapidly is the of that forms to a through after for in a GH4C1 cells were for min and then for 30 min after the were were in the and insoluble was isolated as in to are the of intracellular that is GH4C1 cells were with and cells were at the times after the of that insoluble is the of intracellular that forms a in after for times did not the rapidly is the of that forms to a through after for dense cores of prolactin secretory granules dissolve slowly so that they are cells of the pituitary gland after exocytosis (3Farquhar M.G. Dellmann H.-D. Johnson J.A. Klachko D.M. Comparative Endocrinology of Prolactin. Plenum Press, New York1974Google Scholar, 4Zanini A. Giannattasio G. Nussdorfer G. Margolis R.K. Margolis R.U. Meldolesi J. J. Cell Biol. 1980; 86: 260-272Crossref PubMed Scopus (47) Google Scholar), the cores Lubrol-insoluble prolactin in GH4C1 cells insoluble for after the membranes are when at °C but soluble rapidly when at with the properties of prolactin in of on of cells were with for and cells were 30 min after the of that cell were for at the before are the of that is in a GH4C1 cells were with for and cells were 30 min after the of that cell were for at the before are the of that is GH4C1 cells in the of estradiol, insulin, and epidermal growth factor secretory granules and store prolactin to what they make J.G. P.S. Endocrinology. 1986; PubMed Scopus Google Scholar). synthesized the same conversion to a Lubrol-insoluble form with in GH4C1 cells not treated with hormones after the incubation with of the was this time, but was secreted by after the intracellular Lubrol-insoluble form as was secreted from the cells, indicating that this and not soluble prolactin, was was secreted from cells that not treated with hormones than from hormone-treated cells of the Lubrol-insoluble form of 30 min after through a solution indicating that the insoluble form of prolactin has the same properties in GH4C1 cells that not store prolactin as in cells that of newly synthesized prolactin and growth hormone from of rat hormone after a as a of the in the is of is cells were in with no and treated cells were in estradiol, insulin, and epidermal growth in a of rat hormone after a as a of the in the is of is cells were in with no and treated cells were in estradiol, insulin, and epidermal growth hormone is structurally similar to prolactin and is stored in a concentrated form in secretory granules in the pituitary gland (16Nicoll C.S. Mayer G.L. Russell S.M. Endocr. Rev. 1986; 7: 169-203Crossref PubMed Scopus (324) Google Scholar). GH4C1 cells prolactin and growth hormone, but they not store growth hormone and its storage is not by the hormone that prolactin storage J.G. P.S. Endocrinology. 1986; PubMed Scopus Google Scholar). In GH4C1 cells, intracellular rat hormone as prolactin Lubrol-insoluble 30 min after the and the of the insoluble form as growth hormone was secreted from the cells GH4C1 cells secreted hormone after its cells were treated with hormones or not and conversion to an insoluble form to transport the secretory the of GH4C1 cells with brefeldin which transport from the endoplasmic reticulum J.G. J. J. Cell Biol. 1992; PubMed Scopus Google Scholar), or incubating them at 15 which transport from the endoplasmic reticulum J. J. Cell 1991; PubMed Google Scholar), reduced formation of the Lubrol-insoluble form of rat prolactin in GH4C1 cells at which transport from the trans-Golgi cisternae G. S. J. Cell Biol. PubMed Scopus Google Scholar), reduced formation of the Lubrol-insoluble form of rat prolactin but not as as the at 15 conversion did not an acidic compartment in the secretory of 30 chloroquine at that acidic in cells D. J. Biol. Chem. 1989; Full Text PDF PubMed Google Scholar, V. J. Biol. Chem. Full Text PDF PubMed Google Scholar) the conversion of to the insoluble form in GH4C1 cells but did not the conversion of but not chloroquine similar and the conversion of rat hormone was in a similar to in GH4C1 cells and acidic in cells A. S. A. PubMed Scopus Google Scholar) and a similar to chloroquine but did not conversion to the insoluble form. that acidic in the cells are by for which in acidic R.K. Full Text PDF PubMed Scopus Google Scholar). with chloroquine the of in the cells of in the or of chloroquine in GH4C1 cells treated with insulin, and epidermal growth were with in the or of 30 and the of was by conversion of growth hormone to a Lubrol-insoluble form occurs in cell a pituitary cell that and in secretory granules. hormone, expressed in AtT20 cells, as rat growth hormone in GH4C1 cells; was soluble and about 40% Lubrol-insoluble after 30 min albumin is secreted from the storage and the same when expressed in AtT20 cells L. Fricker L.D. Biochem. J. PubMed Scopus Google Scholar). expressed in AtT20 cells, was soluble after and no in with conversion of growth hormone to a Lubrol-insoluble form was less in COS cells than was in cells. than 10% of the hormone insoluble in COS cells to Lubrol insolubility in cells to the structural requirements are for the process. Prolactin and growth hormone from a have a in the in human growth is not in to growth hormone but may because of its structural that a in the so this the other M. 1992; PubMed Scopus Google Scholar). of a for this is to the such a form of growth hormone has found with growth hormone and S. New Scholar). hormone has a acid that to A naturally occurring form of human growth hormone that from of the has G. Endocr. Rev. 1991; PubMed Scopus Google Scholar). del32–46 is of M. M. S. E. Y. M. 1998; PubMed Scopus Google Scholar), but the has less in the and a small found in M. 1992; PubMed Scopus Google Scholar). P89L and del32–46 Lubrol insolubility when expressed in AtT20 cells to the same extent as hormone proteins are secreted more slowly than of these in AtT20 cells is the same as that of with in a region the with in the Golgi complex, and in the with in secretory granules that growth hormone and the is a in the at which of Lubrol insolubility and than a of growth hormone in AtT20 cells with or growth growth P89L growth del32–46 growth Rat prolactin in the dense cores of secretory granules is Lubrol-insoluble (2Giannattasio G. Zanini A. Meldolesi J. J. Cell Biol. 1975; 64: 246-251Crossref PubMed Scopus (50) Google Scholar), and rat prolactin in GH4C1 cells is soluble after and Lubrol insolubility with Lubrol-insoluble form in GH4C1 cells is to the same form of prolactin found in dense cores of secretory granules for several Lubrol-insoluble form in GH4C1 cells has detergent and the to through that is similar to prolactin aggregates isolated from secretory granules. Lubrol-insoluble form in GH4C1 cells soluble at with what in to soluble at °C that the proteins have not to form an aggregate of or proteins, such as occurs with Lubrol has to secretory granule membranes (2Giannattasio G. Zanini A. Meldolesi J. J. Cell Biol. 1975; 64: 246-251Crossref PubMed Scopus (50) Google Scholar). Lubrol-insoluble forms of prolactin and growth hormone are to by the to membranes of other in the secretory pathway because albumin not insoluble its transport through the secretory pathway in AtT20 cells, but human growth hormone insoluble forms of secretory proteins are to by to large not by because prolactin and growth hormone are soluble after and because growth hormone insoluble when growth hormone is expressed in COS cells, so the to form Lubrol-insoluble aggregates is a the of Lubrol insolubility a to the aggregation that occurs when prolactin and growth hormone are concentrated into secretory granules in cells. of prolactin not to extent in the endoplasmic reticulum. after a is in the endoplasmic reticulum (1Farquhar M.G. Reid J.J. Daniell L.W. Endocrinology. 1978; 102: 296-311Crossref PubMed Scopus (116) Google Scholar), and prolactin is synthesized prolactin to the Golgi complex a (1Farquhar M.G. Reid J.J. Daniell L.W. Endocrinology. 1978; 102: 296-311Crossref PubMed Scopus (116) Google Scholar), and insoluble is formed by that protein transport from the endoplasmic reticulum is by brefeldin A or by incubating cells at 15 formation of the insoluble form is Lubrol-insoluble forms of prolactin and growth hormone are by 15 min after the of In the insolubility after Lubrol at for min to with that were for the of the of dense cores of prolactin from secretory granules (2Giannattasio G. Zanini A. Meldolesi J. J. Cell Biol. 1975; 64: 246-251Crossref PubMed Scopus (50) Google Scholar). same of after a was in the after at for min in as with the more insoluble aggregates appear to large when formed or after process to make the insoluble form may relatively or there may a formation of a aggregate that then rapidly to a Lubrol-insoluble form. In the by of the to through as the cores from secretory granules prolactin is a process that occurs more and from 15 to with the for granules to (1Farquhar M.G. Reid J.J. Daniell L.W. Endocrinology. 1978; 102: 296-311Crossref PubMed Scopus (116) Google Scholar). A was that chloroquine and bafilomycin did not the conversion of prolactin and growth hormone to a Lubrol-insoluble form. has a that an acidic is for aggregation of proteins that are stored in secretory granules for is that the of the trans-Golgi lumen in cells acidic L. M. M. A. J. Cell Biol. 1986; PubMed Scopus Google Scholar), and is in this that hormone aggregates are by electron microscopy (5Rambourg A. Clermont Y. Chretien M. Olivier L. Anat. Rec. 1992; 232: 169-179Crossref PubMed Scopus (42) Google Scholar). is that proteins that are stored in secretory granules aggregate in mildly acidic pH (6Thompson M.E. Zimmer W.E. Haynes A.L. Valentine D.L. Forss-Petter S. Scammell J.G. Endocrinology. 1992; 131: 318-326Crossref PubMed Scopus (32) Google Scholar, 7Rindler M.J. J. Biol. Chem. 1998; 273: 31180-31185Abstract Full Text Full Text PDF PubMed Scopus (56) Google Scholar, L. Fricker L.D. J. Biol. Chem. 1995; 270: 7963-7967Abstract Full Text Full Text PDF PubMed Scopus (55) Google Scholar, 9Colomer V. Kicska G.A. Rindler M.J. J. Biol. Chem. 1996; 271: 48-55Abstract Full Text Full Text PDF PubMed Scopus (133) Google Scholar, 10Gorr S.U. Shioi J. Cohn D.V. Am. J. Physiol. 1989; 257: E247-E254PubMed Google Scholar, 11Chanat E. Huttner W.B. J. Cell Biol. 1991; 115: 1505-1519Crossref PubMed Scopus (383) Google Scholar, 12Freedman S.D. Scheele G.A. Biochem. Cell Biol. Commun. 1993; 197: 992-999Google Scholar, 13Leblond F.A. Viau G. Laine J. Lebel D. Biochem. J. 1993; 291: 289-296Crossref PubMed Scopus (62) Google Scholar). is not the factor that self-association of secretory granule proteins in prolactin, for example, has to aggregate at in the of (6Thompson M.E. Zimmer W.E. Haynes A.L. Valentine D.L. Forss-Petter S. Scammell J.G. Endocrinology. 1992; 131: 318-326Crossref PubMed Scopus (32) Google Scholar, P.S. P.S. 1996; Google Scholar, M.G. J.A. 1991; PubMed Scopus Google Scholar). An acidic pH is necessary in cells for the of and D. J. Biol. Chem. 1989; Full Text PDF PubMed Google Scholar, V. J. Biol. Chem. Full Text PDF PubMed Google Scholar), and may more for than for of secretory granule An acidic pH may aggregation of proteins for secretory granules, since prolactin aggregation in the of chloroquine was than although of the may by of An acidic not appear necessary for aggregation of prolactin or growth hormone in the secretory pathway of GH4C1 cells; additional for hormone aggregation after cells are transported form the endoplasmic reticulum. Such an is with the of aggregation of human growth hormone in COS cells, although aggregation occurs in AtT20 cells. to form a Lubrol-insoluble form in COS cells is to by of of the trans-Golgi lumen, since this region is acidic in cells that have R.K. Full Text PDF PubMed Scopus Google Scholar, R.K. Full Text PDF PubMed Scopus Google Scholar, S. S. J. Biol. Chem. 1998; 273: Full Text Full Text PDF PubMed Scopus Google Scholar, J. J. Biol. Chem. 1995; 270: Full Text Full Text PDF PubMed Scopus Google Scholar). of the structures for aggregation in cells is because that structures are to and the of the protein to the endoplasmic reticulum. P89L and del32–46 are transported through the secretory pathway and in a that is similar to growth the in aggregation and of these is by an in the that to the have that structures in the growth hormone on the of the not in to the M. 1992; PubMed Scopus Google Scholar). of the to aggregate that there is at some in the structures for into storage in GH4C1 cells has by the of intracellular to secreted hormone and the cells found to store growth hormone with or hormone to store more prolactin with and to store well with or J.G. P.S. Endocrinology. 1986; PubMed Scopus Google Scholar, S.U. J. Biol. 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J. 1998; 332: 593-610Crossref PubMed Scopus (440) Google Scholar), but these indicate that is retention of the aggregate that is this retention occurs for prolactin and growth hormone, may a sorting from aggregation. retention of aggregates may by retention of secretory granules. Secretory granule of proteins as well as the and the of proteins that and the of the granules may on the for such a is found in cells of In the trans-Golgi lumen of these cells, hormone is and of granules form that aggregates of the or aggregates of the of hormone granules are transported to of the cell A. S. A. PubMed Scopus Google Scholar, J. Cell Biol. PubMed Scopus Google Scholar). An is that sorting occurs before aggregation; sorting to of the cell more of granules that are retained after they are has for store than J. 1999; PubMed Google Scholar). that growth hormone and prolactin not in the same of the secretory pathway in a GH4C1 cell is with this A. J. P.S. Endocrinology. PubMed Scopus Google Scholar). and for the human growth hormone and for the AtT20 cells.