Retinoids antagonize the induction of ornithine decarboxylase activity by phorbol esters and phospholipase C in rat tracheal epithelial cells

Potentiation of oncogenic N-ras-induced neurite outgrowth and ornithine decarboxylase activity by phorbol dibutyrate and protein kinase inhibitor H-8

A recombinant N-ras oncogene, under the transcriptional control of a corticosteroid-inducible mouse mammary tumor virus (MMTV) promoter, has been stably transfected into a PC12 rat pheochromocytoma subline. This cell line, designated UR61, undergoes N-ras-induced neurite outgrowth and cessation of division when treated with dexamethasone (Guerrero et al.: Biochemical and Biophysical Research Communications 150:1185-1192, 1988). We have employed the UR61 cell line as a model for ras oncogene-induced neuronal differentiation. In UR61 cells, dexamethasone-induced expression of the recombinant N-ras gene resulted in time-dependent expression of ornithine decarboxylase enzyme (ODC) activity. Prompted by recent reports of possible functional (Lacal et al.: Molecular and Cellular Biology 7:4146-4149, 1987; Wolfman and Macara: Nature 325: 359-361, 1987) and direct (Jeng et al.: Biochemical and Biophysical Research Communications 145:782-788, 1987) interactions between oncogene ras-coded p21 and protein kinase C (PK-C; Ca++/phospholipid-dependent protein kinase), we employed the protein kinase inhibitor H-8 (N-[2-(methylamino)ethyl]-5-isoquinoline sulfonamide dihydrochloride) and phorbol 12,13-dibutyrate (PDBu) to investigate this putative interaction in the UR61 cells, where ODC activity and neurite outgrowth were used as indicators of oncogenic N-ras action. Treatment of UR61 cells with PDBu depleted cells of PK-C and failed to promote neurite outgrowth but enhanced N-ras-induced neurite outgrowth and ODC activity. H-8, which suppressed ODC induction by forskolin and phorbol myristate acetate, enhanced both N-ras-induced ODC activity and neurite outgrowth. Inhibition of ODC activity by difluoromethylornithine (DFMO) did not suppress oncogenic ras-induced neurite outgrowth, suggesting that these two ras-triggered events are mechanistically independent. These findings suggest that certain actions of N-ras can occur in cells depleted of PK-C, and thus, the role of PK-C in ras-induced differentiation differs from its role in ras-induced mitogenesis and transformation.

Mechanism of action of parathyroid hormone-induced proteoglycan synthesis in the growth plate chondrocyte

In the growth plate chondrocyte, parathyroid hormone (PTH) stimulates phosphoinositol 4,5 bisphosphate (PIP2) degradation, which results in the rapid production of inositol (1,4,5) triphosphate (IP3). IP3 induced the release of calcium from an intracellular store, which caused a rapid increase in the cytosolic ionized calcium concentration. Parathyroid hormone also induced a 30-50% increase in proteoglycan synthesis. Phorbol esters, which pharmacologically activate protein kinase C, resulted in a 70-80% increase in proteoglycan synthesis. Treatment of the chondrocytes with retinoic acid (0.2 microM) inhibited the parathyroid hormone and phorbol ester-induced increase in intracellular ionized calcium and the increase in proteoglycan synthesis. From this data we postulate that the stimulation of proteoglycan synthesis in growth plate chondrocytes by PTH is mediated by the breakdown of membrane phosphoinositides, which results in the production of IP3 and an increase in ionized intracellular calcium. It is suggested that the degradation of membrane phosphoinositides also results in production of diacylglycerol and, thereby, an activation of protein kinase C, which has a large stimulatory effect on proteoglycan synthesis. The increase in cytosolic calcium most likely acts synergetically with diacylglycerol to activate protein kinase C. Retinoic acid blocks the effect of PTH and phorbol ester-induced proteoglycan synthesis and may act through the inhibition of protein kinase C. The overall effect of PTH on the growth plate chondrocyte appears to be a stimulation of proteoglycan synthesis that is mediated by the degradation products of membrane phosphoinositides.

Retinoic acid inhibits phospholipid turnover and protein kinase C activity in RA-sensitive but not in RA-resistant cells

Treatment with 10(-5) M retinoic acid causes loss of anchorage-independent growth in src-transformed RR1022 cells but not in ras-transformed KNRK cells. In an effort to elucidate the mechanisms underlying this difference, we investigated the effect of RA on phospholipid turnover and PKC activity in these two cell lines. 10(-5) M RA treatment caused a drastic inhibition of 32P incorporation into PI and PA and a large increase in 32P incorporation into PC in RR1022 cells. Similar treatment of KNRK cells yielded no change in PC or PA labelling and a much smaller decrease in PI labelling. Furthermore, 10(-5) M RA treatment causes a large decrease in PKC activity in RR1022 cells (35% of control) but only a small decrease in KNRK cells (78% of control). We suggest that these effects are part of an altered signal transduction pathway which mediates the differential effects of RA on anchorage-independent growth in these two cell lines.

Epidermal growth factor potentiates the induction of ornithine decarboxylase activity by prostaglandins in embryonic palate mesenchymal cells: effects on cell proliferation and glycosaminoglycan synthesis

Epidermal growth factor (EGF) and prostaglandins (PGs) have been implicated in the regulation of a number of developmental processes in the mammalian embryonic palate. Normal palatal ontogenesis is dependent on the presence and quite possibly on the interaction of various hormones and growth factors. The interaction between EGF and PGs in regulation of murine embryonic palate mesenchymal (MEPM) cell growth and differentiation was therefore investigated by monitoring the activity of ornithine decarboxylase (ODC), the principle and rate limiting enzyme of polyamine biosynthesis. ODC activity is tightly coupled to the proliferative and differentiative state of eukaryotic cells and therefore serves as a reliable indicator of such cellular functions. Treatment of confluent cultures of MEPM cells with EGF (1-50 ng/ml) resulted in a dose-related increase in ODC activity, while similar treatment with either PGE2 or PGF2 alpha (at concentrations up to 1 microM) did not elicit a dose-dependent increase in enzyme activity. Concurrent treatment of MEPM cells with EGF (20 ng/ml) and either PGE2 or PGF2 alpha (0.1-10000 nM) resulted in a marked prostaglandin dose-dependent induction of ODC activity, suggesting a strong cooperative interaction between these factors. ODC activity was maximal by 4 to 8 hr and could be completely inhibited by preincubation of the cells with actinomycin D or cycloheximide, indicating that de novo synthesis of RNA and protein is necessary for enzyme induction. Stimulation of ODC activity by EGF and PGE2 in these cells was not positively correlated with the level of cellular DNA synthesis but did result in a ninefold increase in the synthesis of extracellular glycosaminoglycans (GAGs), a key macromolecular family implicated in palatal morphogenesis. Stimulation of GAG synthesis was significantly inhibited by the administration of 5 mM DFMO (an irreversible inhibitor of ODC), indicating that the marked increase in GAG production was dependent, in part, on the induction of ODC activity by EGF and PGE2. Qualitative analysis of the palatal GAGs indicated that synthesis of several major classes of GAGs was stimulated. Collectively these data demonstrate a cooperative interaction between EGF and PGs in the induction of ODC activity. Such activity may serve to regulate the synthesis of GAGs, which are instrumental in mammalian palatal ontogenesis.

Differential response to retinoic acid of Syrian hamster embryo fibroblasts expressing v-src or v-Ha-ras oncogenes

It has been shown that treatment of many but not all tumor cell lines with retinoids affects cell proliferation and expression of the transformed phenotype. To determine whether the response of the tumor cell to retinoids is influenced by specific oncogenes activated in the cell, we studied the action of these agents in the immortal, nontumorigenic Syrian hamster embryo cell lines DES-4 and 10W transfected with either v-Ha-ras or v-src oncogenes. In this paper we show that in transformed DES-4 cells expressing v-src, retinoic acid inhibited anchorage-independent growth, reduced saturation density, and inhibited the induction of ornithine decarboxylase by the phorbol ester 12-O-tetradecanoylphorbol-13-acetate. In contrast, retinoic acid enhances the expression of the transformed phenotype in DES-4-derived cells that express v-Ha-ras. In these cells retinoic acid increases the number and the average size of colonies formed in soft agar. Moreover, retinoic acid enhances ornithine decarboxylase activity and acts in a synergistic fashion with 12-O-tetradecanoylphorbol-13-acetate. These results indicate that oncogenes activated in cells can indeed influence the response of cells to retinoids. Retinoic acid does not appear to alter the levels of pp60src or p21ras proteins in these cells, suggesting that retinoic acid does not affect the synthesis of these oncogene products. Furthermore, retinoic acid does not affect the protein kinase activity of pp60src. Transformed cell lines derived from 10W cells responded differently, indicating that the presence of a specific oncogene is not the only factor determining the response to retinoids. Possible mechanisms by which retinoic acid may interfere with the expression of the oncogene products are discussed.

Differential response to retinoic acid of Syrian hamster embryo fibroblasts expressing v-src or v-Ha-ras oncogenes

It has been shown that treatment of many but not all tumor cell lines with retinoids affects cell proliferation and expression of the transformed phenotype. To determine whether the response of the tumor cell to retinoids is influenced by specific oncogenes activated in the cell, we studied the action of these agents in the immortal, nontumorigenic Syrian hamster embryo cell lines DES-4 and 10W transfected with either v-Ha-ras or v-src oncogenes. In this paper we show that in transformed DES-4 cells expressing v-src, retinoic acid inhibited anchorage-independent growth, reduced saturation density, and inhibited the induction of ornithine decarboxylase by the phorbol ester 12-O-tetradecanoylphorbol-13-acetate. In contrast, retinoic acid enhances the expression of the transformed phenotype in DES-4-derived cells that express v-Ha-ras. In these cells retinoic acid increases the number and the average size of colonies formed in soft agar. Moreover, retinoic acid enhances ornithine decarboxylase activity and acts in a synergistic fashion with 12-O-tetradecanoylphorbol-13-acetate. These results indicate that oncogenes activated in cells can indeed influence the response of cells to retinoids. Retinoic acid does not appear to alter the levels of pp60src or p21ras proteins in these cells, suggesting that retinoic acid does not affect the synthesis of these oncogene products. Furthermore, retinoic acid does not affect the protein kinase activity of pp60src. Transformed cell lines derived from 10W cells responded differently, indicating that the presence of a specific oncogene is not the only factor determining the response to retinoids. Possible mechanisms by which retinoic acid may interfere with the expression of the oncogene products are discussed.

Action of retinoic acid on the diacylglycerol-induced ornithine decarboxylase activity, reduction in EGF binding and protein kinase C activation in rat tracheal epithelial 2C5 cells

We have shown previously that in rat tracheal epithelial 2C5 cells the induction of ornithine decarboxylase (ODC) activity and the reduction in the binding of epidermal growth factor (EGF) by diacylglycerol is related to the activation of protein kinase C. In this paper we analyse the action of retinoic acid (RA) on these two parameters in order to determine whether RA acts on the level of protein kinase C. RA inhibits the induction of ODC activity by diacylglycerol (sn-1,2-dioctanoylglycerol) in a dose- and time-dependent manner. A biologically inactive analog of RA has no effect on this induction. RA does not affect the activation of protein kinase C by diacylglycerol in an in vitro assay. In contrast to the effect on ODC induction, RA does not counteract the reduction in EGF binding induced by diacylglycerol. These results are consistent with the concept that RA does not act at the level of protein kinase C and inhibits ODC induction during a stage following protein kinase C activation.