Cyclic GMP metabolism in relation to the regulation of cell growth in BALB/c3T3 cells

Moving and stationary actin filaments are involved in spreading of postmitotic PtK2 cells

We have investigated spreading of postmitotic PtK2 cells and the behavior of actin filaments in this system by time-lapse microscopy and photoactivation of fluorescence. During mitosis PtK2 cells round up and at cytokinesis the daughter cells spread back to regain their interphase morphology. Normal spreading edges are quite homogenous and are not comprised of two distinct areas (lamellae and lamellipodia) as found in moving edges of interphase motile cells. Spreading edges are connected to a network of long, thin, actin-rich fibers called retraction fibers. A role for retraction fibers in spreading was tested by mechanical disruption of fibers ahead of a spreading edge. Spreading is inhibited over the region of disruption, but not over neighboring intact fibers. Using photoactivation of fluorescence to mark actin filaments, we have determined that the majority of actin filaments move forward in spreading edges at the same rate as the edge. As far as we are aware, this is the first time that forward movement of a cell edge has been correlated with forward movement of actin filaments. In contrast, actin filaments in retraction fibers remain stationary with respect to the substrate. Thus there are at least two dynamic populations of actin polymer in spreading postmitotic cells. This is supported by the observation that actin filaments in some spreading edges not only move forward, but also separate into two fractions or broaden with time. A small fraction of postmitotic cells have a spreading edge with a distinct lamellipodium. In these edges, marked actin polymer fluxes backward with respect to substrate. We suggest that forward movement of actin filaments may participate in generating force for spreading in postmitotic cells and perhaps more generally for cell locomotion.

Ultracytochemical localization of guanylate cyclase in the oviduct of estrogen-stimulated rat

Many reports have indicated that estrogens increase uterine guanosine 3',5'-cyclic monophosphate (cGMP) levels via increasing the activity of guanylate cyclase. In the present study, guanylate cyclase activity was studied cytochemically in the oviduct of immature rats 24 hours after one or two doses (20 micrograms/kg, subcutaneously) of diethylstilbestrol (DES), one dose per day. The reaction product indicating guanylate cyclase was localized in the plasma membrane of epithelial cells, endothelial cells, and smooth muscle cells of all DES-treated animals, but was absent from those of the vehicle control immature rats. The Golgi saccules of secretory cells and the periciliary membrane of ciliated cells were stained for the enzyme 24 hours after the first and second dose of DES, but the activity seemed diminished 24 hours after the second dose. Likewise, reaction product indicating guanylate cyclase was more prominent in the plasma membrane of epithelial cells of animals treated with one dose of DES than those animals treated with two doses of DES. However, in the endothelial and smooth muscle cells, guanylate cyclase activity increased after two doses of the estrogen. Further, pinocytotic vesicles or caveolae in these cells were also strongly stained for the enzyme after one and two doses of DES. These findings confirm the suggestion that guanylate cyclase plays a significant role in the growth, differentiation, and function (secretion and transport) of the oviduct.

Effect of Yersinia enterocolitica ST on cyclic guanosine 3',5'-monophosphate levels in mouse intestines and cultured cells

We determined the effect of heat-stable enterotoxin produced by Yersinia enterocolitica (Y. enterocolitica ST) on cyclic nucleotide levels in the intestines of 6-day-old mice and in cultured cell line cells. The concentration of cyclic guanosine 3',5'-monophosphate (cyclic GMP) in homogenates of the intestines increased four- to fivefold by 3 min after intragastric administration of 10 units of purified Y. enterocolitica ST. This increase continued for 60 min, and then the concentration of cyclic GMP fell toward the levels of the controls. On the other hand, fluid accumulation in the intestines was not evident until 60 min after administration of the toxin. Thus, the increase in intestinal cyclic GMP concentration preceded measurable fluid accumulation. The effect on both cyclic GMP levels and fluid accumulation was abolished by treatment of the ST with either alkali solution (pH 10.7) or 2-mercaptoethanol. Likewise, cyclic GMP levels in cultured cells (CCL-6, HeLa, L, and Mm-1 cells) increased dose-dependently by 10 min after incubation of the cells with the ST. Cyclic adenosine 3',5'-monophosphate levels in both intestines and cultured cells were not affected by the toxin.

Changes in cyclic nucleotide levels and phosphodiesterase and protein kinase activities in mitogenically stimulated 3T3 fibroblasts

Activities of cyclic-nucleotide-hydrolysing enzymes cAMP-, cCMP- and cGMP phosphodiesterase, the intracellular concentrations of cAMP, cCMP, and cGMP, and the activity of the cAMP-dependent protein kinase were studied in serum-starved 3T3 cells stimulated to proliferate by serum. Within 1 and 2 min after stimulation the activities of cAMP- and cGMP phosphodiesterase were unaffected while the concentration of cGMP was raised and that of cAMP lowered, suggesting increased synthesis of cGMP and simultaneously reduced synthesis of cAMP. 48 h after stimulation, when the cells multiplied rapidly, both the cAMP phosphodiesterase and the cCMP phosphodiesterase were reduced. Evidence was also obtained that cAMP-dependent protein kinase is important for expressing the cAMP effect in the 3T3 cells.

Fibroblast growth factor causes an early increase in phosphorylation of a membrane protein in quiescent 3T3 cells

THE regulatory systems controlling cell division have not been identified, but it has been shown that growth factors such as epidermal growth factor, fibroblast growth factor (FGF), and serum initiate rapid changes in cellular metabolism, probably involving post-transcriptional control mechanisms(1-5). As phosphorylation has been shown to be an important regulatory mechanism in several metabolic pathways, we initiated experiments to determine whether factors which stimulate DNA synthesis also stimulate endogenous phosphorylation. We find mat, within 5 min of addition of FGF or serum to (32)P-labelled Swiss 3T3 cells, there is a specific increase in the phosphorylation of a membrane protein with an apparent molecular weight of 33,000. Experiments with isolated cell fractions demonstrate that the phosphorylation of this protein is stimulated by cyclic AMP. This rapid and specific response to mitogens raises the possibility that this phosphorylation might be part of the initial, cellular signal for DNA synthesis.