Loss and restoration of glucagon receptors and responsiveness in a transformed kidney cell line

Canine kidney glucagon receptor: evidence for a structurally-different, tissue-specific variant of the glucagon receptor

125I-Glucagon was directly cross-linked to its receptor sites on the MDCK plasma membranes using a UV irradiation procedure. Analysis of the affinity labeled membranes by SDS-PAGE and autoradiography, demonstrated the presence of a single band at 74 kDa. The incorporation of radiolabeled glucagon into this band was abolished by the presence of excess unlabeled hormones, thus indicating a specificity of labeling. Also this band was observed in affinity labeled dog kidney plasma membranes. The size of the MDCK and the dog kidney glucagon receptors were consistently larger than that of the dog liver receptor as judged by electrophoretic mobility. Treatments with neuraminidase, endoglycosidase F, or N-glycanase failed to convert the renal form into the hepatic form of the receptor. Proteolytic mapping of the MDCK and the dog liver glucagon receptors revealed that major domains of both proteins are remarkably similar, yet transient variations in the size of the fragments could be detected after short duration digestions. Overall the data presents evidence that the dog renal receptor represents a structurally unique isoform of the glucagon receptor.

Evidence of a functional VIP receptor in cultured human retinal pigment epithelium

The effect of VIP on the intracellular cyclic AMP of human retinal pigment epithelium cultures has been studied. Functional VIP receptor has been demonstrated in cultures from eyes given by five normal donors (age 16-64) (N-HRPE). But it has been found to be absent from high passage number cultures obtained from a retinitis pigmentosa eye of an 84-year-old patient (RP-HRPE). After 3 min of reaction with 1 x 10(-6) M VIP, the intracellular cyclic AMP level has increased to 5-15-fold over the basal level. The maximal effect of VIP (20-fold over the basal level) has been observed at 1 x 10(-7) M VIP. The half maximal activity of VIP is 3-5 x 10(-8) M. The present study also demonstrates the inducibility of the VIP responsiveness in RP-HRPE cultures after they have been treated with butyrate. Curr. Eye Res. 14: 1009-1014, 1995.

VIP modulation of cultured glial cells of the rat retina

Cultured retinal glial cells from the rat are responsive to modulation by vasoactive intestinal peptide (VIP). VIP (1 X 10(-6) M) elevated the intracellular cyclic AMP concentration from the basal level of (4.4-11.1) p mole/mg protein to (354-440) p mole/mg protein in three minutes at 25 degrees C. The half-maximal concentration is 4.8 X 10(-8) M, which is similar to that observed in the cultured retinal glial cells from the chick embryo.

Induction of glucagon sensitivity in a transformed kidney cell line by prostaglandin E2 and its inhibition by epidermal growth factor

A model system using a transformed dog kidney cell line (Madin-Darby canine kidney), has been established for studying the process of differentiation. Glucagon responsiveness can be restored to these transformed cells by various differentiation inducers, including prostaglandin E2. Glucagon response was measured in terms of the ability of glucagon to stimulate cAMP production. Induction of glucagon sensitivity seems to be mediated by cAMP. The ability of various prostaglandin analogs to elevate the cAMP level correlates closely with their ability to induce glucagon sensitivity. In fact, 8-Br-cAMP is also a potent inducer. To define the nature of this cAMP-mediated process, we identified several inhibitors of this induction process. These differentiation inhibitors include serum, phorbol ester, and epidermal growth factor. These inhibitors do not have a direct effect on cAMP production by cells in the presence or absence of hormones. Furthermore, induction by 8-Br-cAMP is also inhibited by these agents. Therefore, the site of inhibition is located beyond the point of cAMP production. Possible interaction between cAMP- and epidermal growth factor-dependent phosphorylations is discussed.

Induction of glucagon sensitivity in a transformed kidney cell line by prostaglandin E2 and its inhibition by epidermal growth factor

A model system using a transformed dog kidney cell line (Madin-Darby canine kidney), has been established for studying the process of differentiation. Glucagon responsiveness can be restored to these transformed cells by various differentiation inducers, including prostaglandin E2. Glucagon response was measured in terms of the ability of glucagon to stimulate cAMP production. Induction of glucagon sensitivity seems to be mediated by cAMP. The ability of various prostaglandin analogs to elevate the cAMP level correlates closely with their ability to induce glucagon sensitivity. In fact, 8-Br-cAMP is also a potent inducer. To define the nature of this cAMP-mediated process, we identified several inhibitors of this induction process. These differentiation inhibitors include serum, phorbol ester, and epidermal growth factor. These inhibitors do not have a direct effect on cAMP production by cells in the presence or absence of hormones. Furthermore, induction by 8-Br-cAMP is also inhibited by these agents. Therefore, the site of inhibition is located beyond the point of cAMP production. Possible interaction between cAMP- and epidermal growth factor-dependent phosphorylations is discussed.

Epinephrine regulates cholinergic transmission mediated by rat retinal neurons in culture

The purpose of this study was to investigate the effects of epinephrine on neurotransmission mediated by cholinergic neurons derived from the rat retina. We used a culture system in which striated muscle cells served as postsynaptic targets for cholinergic neurons of the embryonic retina. This culture system permitted the physiological monitoring of acetylcholine released by retinal neurons. Here, we report that epinephrine facilitates evoked transmission across retina-muscle synapses. This facilitation of cholinergic transmission by epinephrine is reversible, can be mimicked by isoproterenol (a beta adrenoceptor agonist) and blocked by propranolol (a beta adrenoceptor antagonist). Neither the alpha-2 adrenoceptor blocker, yohimbine, nor the dopamine receptor antagonist, haloperidol, blocked this effect of epinephrine. Since epinephrine was found not to influence the membrane potential of muscle cells nor the responses of myotubes to acetylcholine, epinephrine appeared to have mediated its facilitatory effect on cholinergic transmission by affecting retinal cells. Because previous findings indicated that adenosine 3',5'-cyclic monophosphate may be involved in the modulation of transmission at retina-muscle synapses, the effect of epinephrine on adenosine 3',5'-cyclic monophosphate levels was investigated. Our biochemical studies demonstrated that epinephrine could increase adenosine 3',5'-cyclic monophosphate levels markedly in cultured retinal cells. The accumulation of adenosine 3',5'-cyclic monophosphate induced by epinephrine could be blocked by propranolol, but not by yohimbine nor haloperidol. Taken together, the results indicate that the facilitatory effect of epinephrine is mediated via a beta adrenoceptor and may involve an increase in adenosine 3',5'-cyclic monophosphate levels. Our findings are in agreement with the hypothesis that epinephrine may be a modulatory neurotransmitter in the rat retina.

Different molecular mechanisms for cAMP regulation of gene expression during Dictyostelium development

Alterations in cAMP concentrations have been implicated in developmentally regulated gene expression in Dictyostelium. Using a variety of culture conditions to control the metabolism of cAMP during cytodifferentiation, I have examined the role of the cyclic nucleotide in development. Conditions which allow intracellular synthesis of cAMP promote the normal developmental repression of gene M4-1 by a mechanism which is completely independent of the formation of multicellular aggregates. If, however, cells are inhibited in their ability to activate adenylate cyclase and, thus, intracellular cAMP signaling, they prove unable to repress M4-1, even in the presence of exogenous cAMP. In contrast, expression of genes which exhibit maximal activity after aggregate formation depends upon accumulation of extracellular cAMP. Inhibition of intracellular cAMP signaling does not prevent the expression of these genes if cultures are simultaneously exposed to high levels of exogenously added extracellular cAMP. These results indicate that there are at least two independent mechanisms involved in the developmental regulation of gene expression by cAMP in Dictyostelium. I discuss plausible molecular mechanisms through which cAMP might alter gene expression.

The effect of viral transformation on prostaglandin production depends on cell type

The role of prostaglandins in cellular differentiation and transformation has been widely studied. We have found previously that prostaglandin E2 production was greatly diminished in dog kidney cells (MDCK) after transformation by Harvey murine sarcoma virus. In the present study, we have shown that viral transformation can have differing effects in the ability to modify the production of prostaglandin in cultured cells. For example, the prostaglandin E2 production in rat kidney cells (NRK) is decreased after transformation by Rous sarcoma virus, while production in 3T3 cells is increased markedly after transformation by the same virus. Similarly, SV40 transformation increases prostaglandin E2 production of 3T3 cells and decreases the production in rat thyroid cells (FRTL). These results indicate that the biosynthetic pathway for prostaglandin production has varying susceptibility following viral transformation and the effect of transformation depends more on the type of cell than virus. Taking advantage of the well-defined transforming proteins encoded by polyomavirus, we have further studied the relationship between prostaglandin production in cells and the expression of T antigens in transformed cells. We showed that the expression of middle T antigen, which is associated with a protein kinase and is responsible for phenotype of transformed cells, is required for the change in prostaglandin production in cells. How these changes of prostaglandin production relate to the progression of viral transformation remains to be explored.

A gene expressed in undifferentiated vegetative Dictyostelium is repressed by developmental pulses of cAMP and reinduced during dedifferentiation

We describe the gene M4-1, whose unique pattern of developmental expression will allow us to study the molecular mechanisms controlling expression in undifferentiated cells in addition to repression in response to cAMP during development and reinduction during dedifferentiation. M4-1 is a Dictyostelium gene expressed in the undifferentiated cell. We have shown that M4-1 continues to be expressed very early during the developmental cycle but is repressed at a later stage of development, at a time coincident with the establishment of oscillations in the cAMP pool. Studies on the expression of the M4-1 gene in shaking culture, under conditions that mimic early development, have established that pulsatile stimulation of cells with cAMP is sufficient to repress M4-1 expression. Consistent with this, cells that are exposed to high levels of cAMP are unable to respond normally to cAMP oscillations and continue to express M4-1 at vegetative levels. These data indicate that low-level oscillations of cAMP are required for the repression of M4-1 expression rather than the continuous high levels of cAMP responsible for the regulation of a different class of Dictyostelium genes. We suggest that cAMP may mediate developmental expression of the Dictyostelium genome by different mechanisms. We also show that cell-cell interaction, a developmental event that occurs subsequent to the cAMP pulse, does not normally influence the regulation of M4-1. Finally, we have shown that when cAMP-pulsed cells are induced to dedifferentiate, M4-1 RNA sequences rapidly reappear in nuclei and cytoplasm, suggesting that regulation of M4-1 expression is primarily mediated at the level of transcription.

Revertants of Ha-MuSV-transformed MDCK cells express reduced levels of p21 and possess a more normal phenotype

Four subclones of the originally cloned Harvey murine sarcoma virus-transformed Madin Darby canine kidney (MDCK) cells have been isolated. These subclones fall into two general classes. Two subclones have a fibroblastic morphology, have lost the growth requirement for prostaglandin E1 (PGE1), do not respond to glucagon or vasopressin, and, in general, appear transformed. Two other subclones have epithelioid morphologies, are growth-stimulated by PGE1, respond to vasopressin with an increase in intracellular cAMP. We propose that these cells represent revertants to a more non-transformed phenotype. Unlike normal cells, however, these revertants grow under anchorage-independent conditions, express detectable but reduced amounts of the transforming gene product, p21, and grow in nude mice. The appearance of such revertants may be one cause of the observed heterogeneity of tumor cells.

Transport and metabolic functions in cultured renal tubule cells

The study tool of cultured tubule epithelia has been applied to new areas in nephron cell biology, such as the evolution of epithelial membrane asymmetry. Studies utilizing monoclonal antibodies against plasma membrane glycoproteins in MDCK revealed that the development of surface cell polarity is a continuous process requiring intact tight junctions and their electrical resistor function [101]. The role of the junctional complex to establish and maintain distinct membrane protein domains had been suggested earlier from work utilizing the apical aminopeptidase [102] and fluorescent membrane probes [103]. Cultured tubule epithelia lend themselves for the evaluation of cell-specific membrane protein synthesis [104] and antigenic determinants [105]. Human renal epithelia, from normal [106, 107] and defined abnormal kidney [108], have been maintained functional in primary and passage culture [106]. Pathophysiological mechanisms may be examined in cultured tubule epithelia, as shown first [109] by studies on the recovery from ischemic failure, where anoxia and substrate deprivation resulted in cell swelling which was prevented in culture by an oncotic agent. This article has not attempted to give an exhaustive account of the studies in which cultured tubule cells have served as a tool. Instead, the investigations quoted herein represent some principal lines of study, as seen from renal physiology, which may disclose details in culture of complex in vivo phenomena. It was Bernard [110] who, in 1865, suggested that "physiological events must be isolated outside the organism . . . to better understand the deepest associations of the phenomena."

Expression of glucagon sensitivity by transformed MDCK cells normally unresponsive to glucagon: early commitment to differentiation

A cloned line of canine kidney cells (MDCK) transformed with Harvey murine sarcoma virus, in contrast to the parental, untransformed line, expressed glucagon sensitivity only under controlled culture conditions. The glucagon sensitivity of transformed MDCK cells appeared after 10 days of culture if plated at less than 100,000 cells/dish or after 3 days if cells were plated at greater than 300,000 cells/dish. As there was no effect of conditioned medium from glucagon-sensitive cells on insensitive cells, media components seemed not to be involved in this phenomenon. Glucagon sensitivity appeared more readily in defined as opposed to serum-containing medium. In fact, as little as 2% fetal bovine serum inhibited the expression of glucagon sensitivity when included in defined medium over the course of the experiment. Furthermore, when transformed MDCK cells were exposed to serum for only the first 24 hr of culture, glucagon sensitivity on day 11 was identical to that of cells exposed to serum throughout the entire experiment. In contrast, exposure to serum later in culture (days 4-8) had no inhibitory effect on the expression of glucagon sensitivity on day 11. The data suggest that differentiation, or glucagon sensitivity, occurs when transformed, glucagon-insensitive cells achieve a critical high density and that differentiation is sensitive to inhibition by serum only during the first 24 hr of culture.

Agonist-mediated regulation of alpha 1- and beta 2-adrenergic receptors in cloned MDCK cells

Chronic exposure of cells to neurotransmitters and hormones can result in a decrease in receptor number (down-regulation). We asked whether two different types of receptors on the same cell that can both respond to a particular hormone are identically regulated. For this study we used MDCK-D-1, a cloned renal epithelial cell line that coexpresses alpha 1- and beta 2-adrenergic receptors, as a model system in which to examine the effects of an adrenergic agonist on the expression and function of these receptors. MDCK-D-1 cells retain differentiated features of renal tubular epithelium with respect to morphology and hormonal responsiveness. When MDCK-D-1 cells were incubated for 21 h with 100 microM epinephrine, alpha 1- and beta 2-receptor numbers decreased by 81 and 90%, respectively. The down-regulation of beta 2-receptors proceeded more rapidly than that of alpha 1-receptors. Down-regulated cells showed a greater than 80% loss of both alpha 1- and beta 2-adrenergic responses (alpha 1: stimulation of prostaglandin E2 release; beta 2: elevation of adenosine 3',5'-cyclic monophosphate levels). We conclude that in renal epithelial cells chronic exposure to a catecholamine agonist can result in a profound decrease in the number of alpha 1- and beta 2-receptors, this down-regulation is accompanied by a loss of adrenergic response, and the kinetics of receptor loss are different for alpha 1- and beta 2-receptors.

Cellular responses elicited by insulin mimickers in cells lacking detectable plasma membrane insulin receptors

Madin-Darby canine kidney (MDCK) cells were previously shown to have few or no plasma membrane insulin binding sites (Hofmann et al: J Biol Chem 258:11774, 1983]. Accordingly, neither insulin-stimulated incorporation of [14C]glucose into glycogen, nor insulin-induced uptake of radiolabeled alpha-aminoisobutyrate ([3H]AIB) could be demonstrated. To probe for receptors, MDCK cultures were surface-labeled with Na125I or were labeled with [35S]methionine. When solubilized cells were immunoprecipitated with sera containing antibodies to the insulin receptor, and immunoprecipitates were analyzed on SDS-gel electrophoresis, no evidence for insulin receptor components was found. Also, when intact MDCK cells wee incubated first with serum containing antibodies to the insulin receptor and then with 125I-protein A, no radiolabeling of insulin receptors occurred. Various agents reported to have insulin-like activity were tested on MDCK cells. The insulinomimetic lectins concanavalin A and wheat germ agglutinin as well as hydrogen peroxide enhanced incorporation of [14C]glucose into glycogen and induced stimulated [3H]AIB uptake, whereas trypsin, vanadate, and serum containing antibodies to the insulin receptor were without effects. Altogether, these results showed that MDCK cells had few or no insulin receptors and were correspondingly insulin-insensitive. However, since insulin-associated responses could be elicited by some insulin mimickers, the post-receptor limb of response in MDCK cells was apparently intact.

Elevation of intracellular cyclic AMP and stimulation of adenylate cyclase activity by vasoactive intestinal peptide and glucagon in the retinal pigment epithelium

Both vasoactive intestinal peptide (VIP) and glucagon rapidly elevated cyclic AMP levels in embryonic chick retinal pigment epithelium (RPE), in culture as well as in freshly dissected tissue. In cultured cells, the half-maximal activities of VIP and glucagon were 5 X 10(-8) M and 3 X 10(-8) M, respectively. After 3 min of reaction, VIP elevated intracellular cyclic AMP by 100-fold; elevation with glucagon was up to 10-fold. Both neuropeptides stimulated adenylate cyclase activity in RPE membranes. Glucagon showed a half-maximal activity of 1 X 10(-8) M. VIP remained more effective than glucagon in stimulating adenylate cyclase activity, but the dose-response curve was shifted to a higher concentration range when compared to that of the VIP-elevated intracellular cyclic AMP.

Interaction of neuropeptides and cultured glial (Müller) cells of the chick retina: elevation of intracellular cyclic AMP by vasoactive intestinal peptide and glucagon

Vasoactive intestinal peptide (VIP) and, to a lesser extent, glucagon were found to increase intracellular cyclic AMP rapidly in cultured glial (Müller) cells of the chick embryo retina. Although VIP elicited higher cyclic AMP accumulation than glucagon at each concentration tested, the half-maximal concentrations were similar, i.e., 6 X 10(-8) M for VIP and 8 X 10(-8) M for glucagon. Secretin had a minimal effect on cyclic AMP accumulation even at a very high (5 X 10(-6) M) concentration. Several other peptide and nonpeptide putative agonists also had little effect on cyclic AMP accumulation. The cultured Müller cell may thus be a useful model for examining VIP and glucagon effects on glial elements of the CNS.

Attachment culture of human retinoblastoma cells: long-term culture conditions and effects of dibutyryl cyclic AMP

Cells from the Y-79 human retinoblastoma line were successfully grown as long-term monolayer cultures in serum-supplemented or serum-free, defined medium after pretreatment of the substrate with poly-D-lysine and fibronectin. A significant proportion of the cells differentiated morphologically in serum-free, defined medium as well as in serum-containing medium after treatment with dibutyryl cyclic AMP (dbcAMP). In the defined medium, approximately 30% of the cells showed development of long, ramifying processes, consistent with putative neuronal differentiation. Addition of dbcAMP in the defined medium resulted in an increasing number of differentiating cells (up to 50%); scattered flat cells were observed as well. Treatment of serum-supported cultures with dbcAMP resulted in a larger proportion (10%) of flat cells. Attempts to differentiate the cells with retinoic acid were unsuccessful. These results show that Y-79 cells can grow in attachment culture and differentiate into at least two distinct morphological cell types as well as offering an interesting system for studying the factors controlling growth and differentiation in human tumor cells in vitro.

Modulators of cyclic AMP in monolayer cultures of Y-79 retinoblastoma cells: partial characterization of the response with VIP and glucagon

VIP markedly stimulates intracellular cAMP accumulation in the human retinoblastoma Y-79 cell line. cAMP increased about 5-fold above the basal level with 10(-8)M VIP and reached a maximum level (about 70-fold increase) with 2 X 10(-6)M VIP. Glucagon at 6 X 10(-8)M significantly increased cAMP accumulation with a maximal response at 4 X 10(-7)M. Secretin was only effective at micromolar concentrations. Glucagon at 2 X 10(-6)M had a synergistic effect with VIP at 2 X 10(-8)M. Of other substances tested, L-isoproterenol (25-fold increase) and PGE1 (4-fold increase) were most effective. These results demonstrate that VIP and glucagon modulate cAMP accumulation in Y-79 cells and provide a model for studying the effect of these substances on function of neuronal and on malignant cells in vitro.

Retinal pigment epithelium in culture demonstrates a distinct beta-adrenergic receptor

Retinal pigment epithelial (RPE) cells in culture, as well as freshly dissected embryonic RPE, exhibit a beta-adrenergic receptor. Agonist potency is: L-isoproterenol greater than (-)epinephrine greater than or equal to (-)norepinephrine much greater than dopamine. Propranolol blocks this effect. Hydroxybenzylpindolol (HYP), a beta-antagonist, also blocks the increase in cAMP elicited by L-isoproterenol. Binding of [125I]-hydroxybenzylpindolol, shows a half saturation concentration of 1.1 X 10(-9) M and 2.7 X 10(5) HYP binding sites per cell. The presence of such distinct beta-adrenergic responsiveness indicates a potentially important role for this receptor in RPE cell physiology.

Agonist effects on the intracellular cyclic AMP concentration of retinal pigment epithelial cells in culture

Accumulation of cyclic AMP in intact cultured pigment epithelial cells was rapidly enhanced by several agonists. These included vasoactive intestinal peptide (100-fold), glucagon (fivefold), thyroid-stimulating hormone (threefold), prostaglandin E1 (24-fold), L-isoproterenol (27-fold), and histamine (fourfold). The rapidity and magnitude of these effects suggest that these agonists may regulate important retinal pigment epithelial cell functions.

Chick retinal pigment epithelium. A new culture system for studying H2-histamine receptors

Histamine elevates the intracellular cyclic AMP levels in cultured embryonic chick retinal pigment epithelium. The half-maximal activity is 6 X 10(-6) M. The effect of histamine is mediated by H2 receptors, i.e., inhibited by the H2 antagonist cimetidine and not affected by the H1 antagonists diphenhydramine and pyrilamine. The inhibition constant (KI) of cimetidine is 1.3 X 10(-8) M. Thus, the present system offers the opportunity of studying the nature of coupling between histamine receptors and adenylate cyclase under controlled conditions in a homogeneous cell type.