Pro-opiomelanocortin (POMC) is the precursor to several pituitary hormones including adrenocorticotropic hormone and beta-endorphin (beta-END). POMC is also expressed in the brain, predominantly in discrete neuronal cell populations of the hypothalamus. In the pituitary and brain, POMC undergoes tissue-specific proteolysis to release different bioactive peptides. POMC processing in neuronal cell lines was studied after infection of PC12 and Neuro2A cells with a recombinant retrovirus carrying the porcine POMC cDNA. Our results indicate that both cell lines synthesize and target POMC to the regulated secretory pathway. Only the Neuro2A cells, however, can achieve proteolytic processing of POMC. Chromatographic and immunological characterization of the POMC-related material showed that beta-lipotropin (beta-LPH) and nonacetylated beta-END(1-31) are major maturation products of POMC in these cells. Release of both beta-LPH and beta-END(1-31) from infected Neuro2A cells can be stimulated by secretagogues in a calcium-dependent manner. Taken together, our results suggest that the cellular machinery of Neuro2A cells can recognize a foreign prohormone, target it to neurosecretory vesicles, process it into biologically active peptides, and secrete it in a manner characteristic to peptidergic neurons.

To facilitate the analysis of the cell division control apparatus in Escherichia coli, we studied extragenic suppressor mutatiots of a previously characterized temperature-sensitive division mutation, ftsMl. Cells of strain GiD40 which harbor this mutation were spread on agar plates and incubated at 42°C, and the surviving cells were anijlyzed for the presence of a suppressor muitation. One group of suppressed mutants had acquired a new mutation which, by conjugation, was found to be located in the 30-to 40-min region of the E. coli genetic map. The other group comprised revertants carrying a suppressor which appeared to map between thr and leu. This suppressor gene, called sftA, was cloned with a mini-Mu-derived in vivo cloning system Jy selection for suppression of temperature sensitivity in GD40 cells. Subsequent subcloning of a fragment of the chromosomal DNA from the mini-Mu plasmid into pBR325 resulted in the delineation of the suppressor gene on a 1.8-kilobase XhoI-PvuI fragment. A strain, CV514, which does not express the temperature sensitivity phenotype of theftsM1 mutation, was found to harbor a natural suppressor of this mutation. UV sensitivity, another known phenotype of theftsMl mutation, was also corrected by the presence of the sftA suppressor in the cell. Thus, the characterization of extragenic suppressors may allow the identification of new genes involved in the control of cell division.

Compared to corticotropes in the adult rat anterior pituitary, neonatal corticotropes exhibit a significantly lower extent of conversion of precursor molecules into ACTH and a substantially greater extent of ACTH cleavage into smaller product peptides similar in size to alpha-melanotropin and corticotropin-like intermediate lobe peptide (CLIP). In the present study we examined pro-ACTH/endorphin (PAE; also called POMC) processing in corticotropes at different times during postnatal development to determine when these cells cease cleaving ACTH into smaller peptides and when they cease accumulating large amounts of intact precursor in vivo. The pattern of processing observed in the newborn is maintained through the second week after birth. A dramatic decrease in ACTH cleavage occurs between the second and third postnatal weeks. The extent of precursor cleavage increases toward the adult pattern by the fifth postnatal week. Explants of newborn anterior pituitary were previously shown to exhibit increased cleavage of ACTH into ACTH-(1-13)NH2 and CLIP, a process that was suppressed by glucocorticoids. To determine whether corticotropes from older animals maintained this plasticity and what intercellular interactions might be required, dissociated primary cultures were maintained in complete serum-free medium with or without added glucocorticoids. After 7 days in complete serum-free medium, the cellular content of both intact precursor and peptides the size of CLIP was increased compared to the corresponding in vivo pattern for animals from birth through adulthood. Although corticotropes from younger animals exhibited more pronounced changes when placed in culture, even cultures from adult animals exhibited some ACTH cleavage. For corticotrope cultures prepared from animals up to postnatal day 15, chronic treatment with dexamethasone did not suppress ACTH cleavage activity, although glucocorticoids did suppress ACTH cleavage in cultures from animals older than postnatal day 22. Biosynthetic labeling studies with cultures from 4- to 5-week-old rats demonstrated that the powerful suppressive effect of dexamethasone on the cleavage of newly synthesized ACTH-(1-39) was only evident 24 h after addition of the glucocorticoid to the culture medium. In contrast, removal of dexamethasone allowed cleavage of ACTH to commence within a few hours.

The order of secretion of newly synthesized and older bioactive peptides was investigated using primary rat intermediate pituitary melanotropes, which synthesize, store, and secrete peptides derived from pro-ACTH/endorphin (PAE; also POMC). PAE-derived peptides produced by the cells were biosynthetically labeled by incubating the cells with radioactive amino acids at various times preceding the period during which secretion was examined; secreted and cellular peptides were characterized and quantitated by immunoprecipitation, using affinity-purified antibodies to selected regions of PAE, followed by polyacrylamide gel electrophoretic analysis. Release in the absence of secretagogues (basal or constitutive release) was compared to release in the presence of maximally effective levels of 8-bromo-cAMP and BaCl2 (stimulated or regulated release). Both cell types showed short-lived preferential basal release of newly synthesized and not fully mature peptides (less than 2-3 h old). Conversely, the cells showed preferential stimulated secretion of older peptides. A process of maturation occurred, taking 2-4 h, after which the secretion of newly synthesized and older peptides in response to secretagogues was nearly indistinguishable for the smallest product peptides. The data support a model of gradual processing of peptides from precursors into smaller products and maturation from molecules only available for basal release into peptides available for stimulated secretion as well as for basal release. Basal secretion was found to include mature peptides as well as intermediates and precursor molecules. The data do not support the existence of any preferential regulated secretion of newly synthesized peptides.