Characterization of hormone-sensitive Madin-Darby canine kidney cells

Intercellular junctions: downstream and upstream of Ras?

Most human tumors are of epithelial origin, and these tumors gradually lose their epithelial character in a process termed the epithelial-mesenchymal transition. Approximately 40% of human tumors have activating mutations in one of the three RAS genes. Given these statistics, it is critically important to understand the role of Ras signaling in the epithelial-mesenchymal transition. This review considers the mechanisms and effectors through which Ras may regulate intercellular junction formation in epithelial cells. Conversely, intercellular junction proteins themselves may play a role in regulating Ras activation and signaling.

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.

Corticotropin-releasing factor increases [Ca2+]i via receptor-mediated Ca2+ channels in human epidermoid A-431 cells

Corticotropin-releasing factor (CRF) has been shown to attenuate vascular leakage in injured skin, mucous membrane, muscle, and brain. Calcium is thought to play an important role in many of the physiological responses to CRF, but there has been little characterization of how calcium is involved in process by which CRF protects damaged tissues. The goal of this study was to characterize changes in cytosolic free calcium concentrations ([Ca2+]i) in human epidermoid A-431 cells exposed to human/rat-CRF and to investigate the mechanisms by which these changes occur. The resting [Ca2+]i in normal cells at 37 degrees C was 66 +/- 4 nM (n = 32). When cells were treated with CRF, [Ca2+]i increased immediately. The increase depended on CRF concentration, with a median effective concentration of 11 pM. This increase in [Ca2+]i depended on external Ca2+ but not Na+, Mg2+, or K+. La3+ (10 microM) and Co2+ (10 microM) inhibited the CRF-induced [Ca2+]i increase, whereas verapamil and nifedipine tested at concentrations up to 1 mM did not. alpha-Helical CRF-(9-41), a synthetic CRF receptor antagonist, and pertussis toxin blocked the increase in [Ca2+]i induced by CRF, which suggests that the entry of extracellular Ca2+ is mediated by receptor-operated Ca2+ channels coupled with pertussis toxin-sensitive G proteins. Although 420 pM CRF stimulated an immediate increase in [Ca2+]i, inositol trisphosphate and cellular cAMP levels did not change within 1 min either in the presence or absence of external Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Heat treatment induces an increase in intracellular cyclic AMP content in human epidermoid A-431 cells

The basal level of intracellular cyclic AMP (cAMPi) in A-431 cells incubated at 37 degrees C in Na(+)-containing Hanks solution is 2086 +/- 139 fmol/10(6) cells. When cells are exposed to 45 degrees C for 10 min, cAMPi increases by 40 +/- 4%, and then returns to basal levels within 30 min. Incubating cells in Ca(2+)-free or Mg(2+)-free Hanks solution has no effect on the heat-induced increase in cAMPi, but the increase is inhibited by acid-loading cells to intracellular pH 7.0 or 6.8. In unheated cells, cAMPi increases by 16 +/- 8%, 53 +/- 7%, or 39 +/- 8%, when incubated with isobutyl-1-methylxanthine (1 mM), Ro 20-1724 (0.5 mM), or theophylline (1 mM) respectively. However, heat treatment further elevates cAMPi in cells treated with phosphodiesterase inhibitors, indicating that heat treatment and phosphodiesterase inhibitors elevate cAMPi by a different pathway(s). Heat treatment increases adenylate cyclase activity 2.5-fold. When forskolin (150 microM), an adenylate cyclase stimulator, is applied to cells, the basal cAMPi increases 28 +/- 6-fold compared with controls. Subsequent heating of these cells lowers cAMPi levels to 7.0 +/- 0.5 times that in control cells. This decrease is prevented by pretreatment with pertussis toxin (30 ng/ml, 24 h), suggesting that G-proteins are involved in the process of heat-induced cAMPi increase. 2-Deoxy-D-glucose (10 mM), NaN3 (10 mM) and 2,4-dinitrophenol (1 mM) also increase cAMPi in A-431 cells. However, application of these metabolic inhibitors to cells before heat treatment does not result in cAMPi levels greater than that observed in cells with heat alone. Similar observations are obtained in heat-treated cells previously exposed to adenosine, but not to AMP or ADP. These data are the first to suggest that thermally induced increase in cAMPi is due to a combination of activation of adenylate cyclase and G-proteins, and an increase in adenosine owing to ATP breakdown caused by hyperthermia.

Induction of differentiation in v-Ha-ras-transformed MDCK cells by prostaglandin E2 and 8-bromo-cyclic AMP is associated with a decrease in steady-state level of inositol 1,4,5-trisphosphate

We used Ha-ras-transformed Madin-Darby canine kidney (MDCK) cells as a model to study possible signal transduction mechanisms underlying the induction of glucagon responsiveness by the differentiation inducers prostaglandin E2 (PGE2) and 8-bromo-cyclic (8-Br-cAMP) AMP and the inhibition of induction by phorbol ester or a serum factor. The steady-state level of inositol 1,4,5-trisphosphate (IP3) was higher in Ha-ras-transformed MDCK cells than in parental MDCK cells. In contrast, the steady-state level of intracellular cAMP of transformed cells was similar to that of normal cells. PGE2 and 8-Br-cAMP increased cAMP content but decreased IP3 levels in a concentration-dependent fashion after 5 days of treatment. We examined the time course for effects of PGE2 and 8-Br-cAMP and found that there was a lag period of 8 to 16 h between elevation of cAMP after the addition of 8-Br-cAMP or PGE2 and the decrease of IP3 levels. Another lag period of 2 days existed before the induction of differentiation. Both the reduction of IP3 levels and the induction of glucagon responsiveness were blocked by phorbol-12-myristate-13-acetate or serum, suggesting that a decrease in the IP3 level might be causally involved in induction of differentiation in transformed MDCK cells. However, induction of differentiation was not due to changes in the expression or guanine nucleotide-binding properties of p21 protein. It is likely that cAMP has a direct regulatory effect on the phospholipid signaling pathway. We conclude that perturbation of the inositol phosphate signaling pathway may be responsible for the induction of differentiation by PGE2 and 8-Br-cAMP in transformed MDCK cells.

Induction of differentiation in v-Ha-ras-transformed MDCK cells by prostaglandin E2 and 8-bromo-cyclic AMP is associated with a decrease in steady-state level of inositol 1,4,5-trisphosphate

We used Ha-ras-transformed Madin-Darby canine kidney (MDCK) cells as a model to study possible signal transduction mechanisms underlying the induction of glucagon responsiveness by the differentiation inducers prostaglandin E2 (PGE2) and 8-bromo-cyclic (8-Br-cAMP) AMP and the inhibition of induction by phorbol ester or a serum factor. The steady-state level of inositol 1,4,5-trisphosphate (IP3) was higher in Ha-ras-transformed MDCK cells than in parental MDCK cells. In contrast, the steady-state level of intracellular cAMP of transformed cells was similar to that of normal cells. PGE2 and 8-Br-cAMP increased cAMP content but decreased IP3 levels in a concentration-dependent fashion after 5 days of treatment. We examined the time course for effects of PGE2 and 8-Br-cAMP and found that there was a lag period of 8 to 16 h between elevation of cAMP after the addition of 8-Br-cAMP or PGE2 and the decrease of IP3 levels. Another lag period of 2 days existed before the induction of differentiation. Both the reduction of IP3 levels and the induction of glucagon responsiveness were blocked by phorbol-12-myristate-13-acetate or serum, suggesting that a decrease in the IP3 level might be causally involved in induction of differentiation in transformed MDCK cells. However, induction of differentiation was not due to changes in the expression or guanine nucleotide-binding properties of p21 protein. It is likely that cAMP has a direct regulatory effect on the phospholipid signaling pathway. We conclude that perturbation of the inositol phosphate signaling pathway may be responsible for the induction of differentiation by PGE2 and 8-Br-cAMP in transformed MDCK cells.

Characterization of subclones of Madin-Darby canine kidney renal epithelial cell line

Heterogeneity in Madin-Darby canine kidney (MDCK) epithelial cells has been reported, however, its details have not been well described. In the present study, we show that subclones obtained from a MDCK cell line could be divided into two morphologically and biochemically distinct cell types with different hormonal responsiveness. Clones of the first type, motile clones, which had extended and flattened cytoplasm, were devoid of carbonic anhydrase activity. Clones of the second type, nonmotile clones, formed colonies of cuboidal cells and showed carbonic anhydrase activity. Motile clones synthesized cAMP in response to arginine vasopressin, prostaglandin E1, and isoproterenol but not glucagon. In contrast, nonmotile clones responded to all of these hormones. These findings suggest MDCK cells have multiple cellular origins. The motile clones have characteristics similar to the principal cells of the collecting system, whereas the nonmotile clones may be derived from the thick ascending limb or the intercalated cell. Our studies also demonstrate a significant influence of culture condition on MDCK cellular behavior (carbonic anhydrase activity, Na+/K+-ATPase activity and vasopressin responsiveness). Therefore, physiologic and biochemical experiments with MDCK cells must be interpreted with reservations about cellular heterogeneity as well as differences induced by culture conditions.

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.

Inhibition of adenylate cyclase by IL 2 in human T lymphocytes is mediated by protein kinase C

Interleukin 2 (IL 2) inhibited basal as well as PGE2, isoproterenol and forskolin stimulated cAMP production in human T lymphocytes. Although the stimulation of adenylate cyclase by activators of the enzyme was evident in lymphocyte membrane preparations, the inhibitory effect of IL 2 was observed only if cells were pretreated with IL 2 and the membranes activated with Ca++ and ATP. Additionally, when purified protein kinase C was reconstituted into untreated membranes and activated with Ca++ and ATP, both receptor and non-receptor stimulated adenylate cyclase was inhibited. These results suggest that the inhibition of adenylate cyclase in human T lymphocytes by IL 2 is mediated by protein kinase C.

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.

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.