Ganglioside GQ1b-induced terminal differentiation in cultured mouse keratinocytes. Phosphoinositide turnover forms the onset signal

Modulation of phospholipase D-mediated phosphatidylglycerol formation by differentiating agents in primary mouse epidermal keratinocytes

The major component of the epidermis, keratinocytes, must continuously proliferate and differentiate to form the mechanical and water permeability barrier of the skin. Our previous data have suggested a potential role in these processes for phospholipase D (PLD), an enzyme that hydrolyzes phospholipids to generate phosphatidic acid. In the presence of primary alcohols, PLD also catalyzes a transphosphatidylation reaction to produce phosphatidylalcohols, and this characteristic has been exploited to monitor the activity of PLD in intact cells. In this report, PLD was demonstrated to utilize the physiological alcohol glycerol to form phosphatidylglycerol (PG) in vitro. In intact primary murine epidermal keratinocytes treated for 24 h with elevated extracellular Ca(2+) levels, but not 1,25-dihydroxyvitamin D(3), incubation with radioactive glycerol resulted in an increase in PLD-mediated radiolabeled PG production. This effect was dose-dependent and biphasic, with maximal PG formation detected after exposure to an intermediate (125 microM) Ca(2+) concentration. Furthermore, the biphasic nature of the response was due, in part, to a corresponding biphasic change in glycerol uptake. Finally, short-term treatment of keratinocytes with phorbol 12-myristate 13-acetate (PMA) failed to increase PG synthesis and inhibited glycerol uptake. Since (1) PMA is reported to activate PLD-1 to a greater extent than PLD-2, (2) 1,25-dihydroxyvitamin D(3) increases the expression/activity of PLD-1 in keratinocytes, and (3) PLD-2 is co-localized with a glycerol channel in keratinocyte membrane microdomains, we speculate that radiolabeled PG production from radioactive glycerol is a measure of PLD-2 activation in these cells. Our results also suggest that PLD-mediated PG synthesis may be regulated at the level of both PLD activity and alcohol substrate availability via changes in glycerol uptake.

8-Cl-adenosine induces growth arrest without differentiation of primary mouse epidermal keratinocytes

In some cell systems, the antiproliferative effects of 8-Cl-cAMP, a site-selective cAMP analog specific for the type I cAMP-dependent protein kinase, are mediated by its metabolite, 8-Cl-adenosine. These effects were once thought to be specific to transformed cells. We investigated the ability of 8-Cl-adenosine to regulate growth and differentiation in primary cultures of mouse epidermal keratinocytes. A 24 h exposure of keratinocytes to 8-Cl-adenosine inhibited [3H]thymidine incorporation in a dose-dependent manner with an apparent IC(50) of 7.5 microM, and these effects were completely reversible. To determine the ability of 8-Cl-adenosine to induce differentiation of primary keratinocytes, we measured keratin-1 expression and transglutaminase activity, markers of early and later stages of keratinocyte differentiation, respectively. Interestingly, exposure of keratinocytes to 25 microM 8-Cl-adenosine for 24 h had no effect on keratin-1 expression or transglutaminase activity. The 8-Cl-adenosine-induced growth arrest of keratinocytes required uptake of the compound and was accompanied by an increase in protein expression of the cyclin-dependent protein kinase inhibitor p21(WAF1/Cip1). These results demonstrate that 8-Cl-adenosine inhibits growth in a non-transformed/non-immortalized cell system, possibly through an elevation in p21(WAF1/Cip1) protein levels, without inducing differentiation.

1,25-Dihydroxyvitamin D(3), phospholipase D and protein kinase C in keratinocyte differentiation

1,25-Dihydroxyvitamin D(3), thought to be a physiological regulator of epidermal keratinocyte growth and differentiation, also elicits the complete differentiative program in vitro, with expression of various genes/proteins characteristic of both early and late differentiation. 1,25-Dihydroxyvitamin D(3) functions by interacting with an intracellular receptor that binds to DNA at vitamin D response elements (VDRE) thereby affecting transcription. 1,25-Dihydroxyvitamin D(3) has been demonstrated to alter the expression of several enzymes involved in signal transduction, and presumably this is the mechanism through which the hormone regulates differentiation. It has recently been shown that 1,25-dihydroxyvitamin D(3) specifically increases the expression/activity of phospholipase D-1, an enzyme that hydrolyzes phospholipids to generate lipid messengers, such as diacylglycerol (DAG). DAG, in turn, is known to activate several members of the protein kinase C (PKC) family. It has been proposed that this signaling pathway mediates late differentiation events in epidermal keratinocytes. In this article the data supporting a role for PKC and phospholipase D in keratinocyte differentiation, as well as in the pathogenesis of skin diseases, are reviewed and a model is proposed for the signaling pathways that regulate this process upon exposure to 1,25-dihydroxyvitamin D(3).

The skin of atopic dermatitis patients contains a novel enzyme, glucosylceramide sphingomyelin deacylase, which cleaves the N-acyl linkage of sphingomyelin and glucosylceramide

We have demonstrated previously that there is an abnormal expression of sphingomyelin (SM) deacylase in the epidermis of patients with atopic dermatitis (ADe). In the present study, we have prepared N-[palmitic acid-1-(14)C]SM and N-[palmitic acid-1-(14)C]glucosylceramide (GCer) to use as substrates and have quantified SM deacylase activity by detecting the release of [(14)C]palmitic acid in extracts of the stratum corneum or the epidermis of ADe patients. In studies using [palmitic acid-1-(14)C]SM as a substrate, a pH dependency of catalytic activity with a peak at pH 5.0 was found. Preparative SDS/PAGE using an extract of ADe epidermis revealed that the molecular mass of SM deacylase is 40000 Da, which is consistent with its apparent molecular mass of 42000 Da estimated by gel-filtration analysis of stratum corneum extracts. Analytical isoelectric focusing (IEF) chromatography demonstrated that the pI values of SM deacylase, beta-glucocerebrosidase (GlcCDase), sphingomyelinase (SMase) and acid ceramidase were 4.2, 7.4, 7.0 and 5.7, respectively. In enzymic analysis using pI-4.2 SM deacylase partially purified by IEF, which had no detectable contamination with acid ceramidase, GlcCDase or SMase, radio-TLC analysis revealed that radiolabelled sphingosylphosphocholine or [1-(14)C]palmitic acid was enzymically liberated from [choline-methyl-(14)C]SM or N-[palmitoyl-1-(14)C]GCer, respectively, used as substrates. Further the pI-4.2 protein purified from extracts of the stratum corneum of ADe patients was able to hydrolyse N-[palmitoyl-1-(14)C]SM and GCer, but not N-[palmitoyl-1-(14)C]ceramide. These results indicate that a hitherto undiscovered epidermal enzyme, termed here glucosylceramide sphingomyelin deacylase, is expressed in the skin of ADe patients, which plays an important role in ceramide deficiency (including acylceramides) in the stratum corneum.

Functional roles of glycosphingolipids in signal transduction via lipid rafts

The formation of glycosphingolipid (GSL)-cholesterol microdomains in cell membranes has been proposed to function as platforms for the attachment of lipid-modified proteins, such as glycosylphosphatidylinositol (GPI)-anchored proteins and src-family tyrosine kinases. The microdomains are postulated to be involved in GPI-anchored protein signaling via src-family kinase. Here, the functional roles of GSLs in signal transduction mediated by the microdomains are discussed. Antibodies against GSLs co-precipitate GPI-anchored proteins, src-family kinases and several components of the microdomains. Antibody-mediated crosslinking of GSLs, as well as that of GPI-anchored proteins, induces a rapid activation of src-family kinases and a transient increase in the tyrosine phosphorylation of several substrates. Enzymatic degradation of GSLs reduces the activation of src-family kinase and tyrosine phosphorylation by antibody-mediated crosslinking of GPI-anchored protein. Furthermore, GSLs can also modulate signal transduction of immunoreceptors and growth factor receptors in the microdomains. Thus, GSLs have important roles in signal transduction mediated by the microdomains.

Ganglioside GQ1b enhances anti-double-stranded DNA antibody and IgG production of PBMCs from patients with systemic lupus erythematosus

BACKGROUND

Previously, we reported that ganglioside GQ1b greatly enhanced spontaneous immunoglobulin production in vitro by PBMCs from normal human subjects.

OBJECTIVE

We examined in vitro effects of GQ1b on anti-double-stranded DNA (anti-dsDNA) antibody production by PBMCs from patients with systemic lupus erythematosus (SLE).

METHODS

PBMCs from patients with SLE were cultured with GQ1b. IgG anti-dsDNA antibody, total IgG, and cytokine amounts in the culture supernatants and protein kinase C (PKC) activity of T cells were measured by using ELISA.

RESULTS

GQ1b enhanced both anti-dsDNA and total IgG production of PBMCs from patients with SLE who were seropositive for anti-dsDNA. Among the seropositive patients, the active patients were more responsive to GQ1b in anti-dsDNA production than the inactive patients. GQ1b also enhanced total IgG production of PBMCs from patients with SLE who were seronegative for anti-dsDNA but did not induce their anti-dsDNA production. In contrast to PBMCs, GQ1b did not affect the antibody production either of purified CD5(+) or of CD5(-) B cells. Anti-IL-6 or anti-IL-10 antibody each partially blocked the GQ1b-induced enhancement of antibody production in PBMCs, and the addition of both antibodies completely blocked the enhancement. GQ1b increased IL-6 and IL-10 production of T cells. The supernatant from GQ1b-treated T cells enhanced antibody production both of CD5(+) and of CD5(-) B cells to a greater extent than that from medium-treated T cells. Exogenous IL-6 and IL-10 additively increased the antibody production both of CD5(+) and CD5(-) B cells. GQ1b-induced increases in IL-6 and IL-10 production of T cells were both blocked by PKC inhibitors, calphostin C and staurosporine. GQ1b enhanced PKC activity of T cells.

CONCLUSION

These results suggest that GQ1b may polyclonally increase the production of IgG, including IgG anti-dsDNA antibody, in PBMCs from patients with SLE by promoting IL-6 and IL-10 production of T cells through the enhancement of their PKC activity.

Possible roles of glycosphingolipids in lipid rafts

Recent studies have suggested that glycosphingolipid (GSL)-cholesterol microdomains in cell membranes may function as platforms for the attachment of lipid-modified proteins, such as glycosylphosphatidylinositol (GPI)-anchored proteins and src-family tyrosine kinases. The microdomains are proposed to be involved in membrane trafficking of GPI-anchored proteins and in signal transduction via src-family kinases. Here, the possible roles of GSLs in the physical properties of these microdomains, as well as in membrane trafficking and signal transduction, are discussed. Sphingolipid depletion inhibits the intracellular transport of GPI-anchored proteins in biosynthetic traffic and endocytosis via GPI-anchored proteins. Antibodies against GSLs as well as GPI-anchored proteins co-precipitate src-family kinases. Antibody-mediated cross-linking of GSLs, as well as that of GPI-anchored proteins, induces a transient increase in the tyrosine phosphorylation of several substrates. Thus, GSLs have important roles in lipid rafts.

Combinatorial PCR approach to homology-based cloning: cloning and expression of mouse and human GM3-synthase

GM3-synthase, also known as sialyltransferase I (ST-I), catalyzes the transfer of a sialic acid residue from CMP-sialic acid onto lactosylceramide to form ganglioside GM3. In order to clone this enzyme, as well as other sialyltransferases, we developed an approach that we termed combinatorial PCR. In this approach, degenerate primers were designed on the basis of conserved sequence motifs of the ST3 family of sialyltransferases (STs). The nucleotide sequence of the primers was varied to cover all amino acid variations occurring in each motif. In addition, in some primers the sequence was varied to cover possible homologous substitutions that are absent in the available motifs. A panel of cDNA from 12 mouse and 8 human tissues was used to enable cloning of tissue- and stage-specific sialyltransferases. Using this approach, the fragments of 11 new putative sialyltransferases were isolated and sequenced so far. Analysis of the expression pattern of a particular sialyltransferase across the panel of cDNA from the different tissues provided information about the tissue specificity of ST expression. We chose two new ubiquitously expressed human and mouse STs to clone full-length copies and to assay for GM3-synthase activity. One of the STs, which exhibited the highest homology to ST3 Gal III, showed activity toward lactosylceramide (LacCer) and was termed ST3 Gal V according to the suggested nomenclature [1]. The other ubiquitously expressed sialyltransferase was termed ST3Gal VI. All isolated sialyltransferases were screened for alternatively spliced forms (ASF). Such forms were found for both human ST3Gal V and ST3Gal VI in human fetal brain cDNA library. The detailed cloning strategy, functional assay, and full length cDNA and protein sequences of GM3 synthase (ST3Gal V, or ST-I) are presented.

Sustained phospholipase D activation is associated with keratinocyte differentiation

Our previous results and data in the literature have suggested a potential role for phospholipase D (PLD) in the regulation of epidermal keratinocyte growth and differentiation. Therefore, we investigated the effect of agents reported to modulate keratinocyte growth and differentiation on PLD activation. The purported protein kinase C (PKC) 'inhibitor', staurosporine (Stsp), has been reported to activate PKC in keratinocytes, eliciting many of the same effects as active tumor promoters such as 12-O-tetradecanoylphorbol-13-acetate (TPA). Stsp also induces a programmed pattern of differentiation similar to that seen in keratinocytes in vivo; TPA, on the other hand, appears to preferentially elicit markers consistent with late (granular) differentiation. In contrast, bradykinin is reported to stimulate keratinocyte proliferation. We found that these three agents had different effects on PLD activation in primary mouse epidermal keratinocytes. TPA increased PLD activity acutely and in a sustained fashion. In contrast, Stsp did not acutely activate PLD and inhibited acute TPA-induced activation of PLD. However, treatment of keratinocytes with Stsp for longer time periods (3-5 h) induced sustained PLD activation and this long-term effect was additive with that of TPA. Bradykinin activated PLD acutely but transiently. Both TPA and Stsp increased transglutaminase activity, a marker of late differentiation, whereas bradykinin had little or no effect on either cell proliferation or transglutaminase activity. These results suggest that a sustained activation of PLD is associated with the induction of keratinocyte differentiation. We hypothesize that PLD activity mediates late keratinocyte differentiation through generation of diacylglycerol and activation of specific PKC isoforms. Furthermore, we propose that the profound and immediate TPA-induced stimulation of PLD activity 'drives' the keratinocytes to late differentiation steps. However, the less efficacious (and more gradual) sustained activation of PLD by Stsp may allow a patterned differentiation more like that observed in skin.

1,25-dihydroxyvitamin D3 induces phospholipase D-1 expression in primary mouse epidermal keratinocytes

The hormone 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) elicits the programmed pattern of differentiation in epidermal keratinocytes. Based on data indicating a potential role of phospholipase D (PLD) in mediating keratinocyte differentiation, we investigated the effect of 1,25(OH)2D3 on PLD expression. A 24-h exposure to 1, 25(OH)2D3 stimulated PLD-1, but not PLD-2, mRNA expression. This 1, 25(OH)2D3-enhanced expression was accompanied by increased total PLD and PLD-1 activity. Time course studies indicated that 1,25(OH)2D3 induced PLD-1 expression by 8 h, with a maximal increase at 20-24 h. Exposure to 1,25(OH)2D3 inhibited proliferation over the same time period with similar kinetics. Expression of the early (spinous) differentiation marker keratin 1 decreased in response to 1, 25(OH)2D3 over 12-24 h. Treatment with 1,25(OH)2D3 enhanced the activity of transglutaminase, a late (granular) differentiation marker, by 12 h with a maximal increase after 24 h. In situ hybridization studies demonstrated that the highest levels of PLD-1 expression are in the more differentiated (spinous and granular) layers of the epidermis, with little expression in basal keratinocytes. Our results suggest a role for PLD expression/activity during keratinocyte differentiation.

A potential role for ceramide in the regulation of mouse epidermal keratinocyte proliferation and differentiation

We have previously determined that sustained phospholipase D (PLD) activation is associated with differentiation induction in primary mouse epidermal keratinocytes. We therefore investigated the effect of two bacterial PLD on keratinocyte proliferation and differentiation. We found that Streptomyces sp. PLD was much less potent at inhibiting proliferation than S. chromofuscus PLD, with a half-maximal inhibitory concentration of 0.05 versus less than 0.001 IU per ml for S. chromofuscus PLD. Similarly, S. chromofuscus PLD stimulated transglutaminase activity more effectively and potently than S. sp. PLD. When we examined the formation of products by the two PLD, we found that the S. sp. PLD showed higher activity at all concentrations. Whereas the PLD from S. sp. is relatively inactive on sphingomyelin, S. chromofuscus PLD is known to hydrolyze both glycerophospholipids and sphingomyelin. Based on recent data indicating a role for ceramide in regulating cell growth and differentiation, we hypothesized that the ability of S. chromofuscus PLD to hydrolyze sphingomyelin might underlie its greater potency. Therefore, we examined the effect of exogenous sphingomyelinase and synthetic ceramides on DNA synthesis. We found that sphingomyelinase exhibited a potent concentration-dependent effect on [3H]thymidine incorporation, much like S. chromofuscus PLD. Synthetic cell-permeable ceramides (C6- and C2-ceramide) also concentration dependently inhibited DNA synthesis, with a half-maximal inhibitory concentration of approximately 12 microM. Finally, we obtained evidence suggesting that ceramide is generated in response to a physiologically relevant agent, because tumor necrosis factor-alpha, a known effector of sphingomyelin turnover in other systems and a cytokine that is produced and released by keratinocytes, increased ceramide levels in primary epidermal keratinocytes.

Multivalent ganglioside and sphingosine conjugates modulate myelin protein kinases

Gangliosides, added exogenously at concentrations of 10-100 microM, inhibit intrinsic protein kinase activities in purified rat brain myelin. Multivalent neoganglioproteins--gangliosides covalently attached, via their lipid moieties, to bovine serum albumin--were much more potent, inhibiting myelin protein phosphorylation half-maximally at a concentration of 100 nM. Different ganglioside conjugates varied 10-fold in inhibitory potency; GT1b-conjugates being the most potent and GM3-conjugates being the least. Conjugates of ganglioside oligosaccharides, lacking the lipid moiety, did not inhibit myelin protein phosphorylation, whereas conjugates of sphingosine inhibited nearly as potently as GT1b conjugates. Conjugate-mediated inhibition of myelin protein phosphorylation was due to inhibition of a protein serine kinase activity rather than activation of a phosphatase activity. We conclude that (i) clustered gangliosides or sphingosine are potent myelin protein kinase inhibitors, and (ii) sphingolipid metabolism is not required for myelin protein kinase inhibition. In contrast to their effects on myelin protein phosphorylation, ganglioside conjugates stimulated phosphorylation of a presumptive axon membrane protein. The data support the conclusion that gangliosides and other sphingolipids, when appropriately clustered, are potent modulators of central nervous system protein phosphorylation.

Molecular cloning and expression of human alpha2,8-sialyltransferase (hST8Sia V)

The cDNA encoding human alpha2,8-sialyltransferase (hST8Sia V) which exhibits activity toward gangliosides, GM1b, GD1a, GT1b, and GD3, was isolated by screening of human brain cDNA library with a DNA probe generated from the cDNA sequence of mouse ST8Sia V (mST8Sia V) and by 5'-RACE of mRNA from human brain tissue. Comparative analysis of this cDNA with mST8Sia V showed that each sequence of the predicted coding region contains 84% identity in both nucleotide and amino acid. Northern analysis of this cDNA indicated that, in contrast to mST8Sia V, two different sizes of transcripts corresponding to 11 and 2.5 kb were expressed in both human fetal and adult brain, while the transcript of 2.5 kb was detected only in adult heart and skeletal muscle. The enzyme expressed in COS cells showed a substrate specificity very similar to that of mST8Sia V.

Changes in IP3 metabolism during skeletal muscle development in vivo and in vitro

We have investigated whether IP3 metabolism presents particular changes during critical stages of muscle development. With this aim, we have measured IP3 formation through phospholipase C activity, IP3 removal through IP3 5-phosphatase and IP3 3-kinase activities, as well as IP3 mass, during myogenesis in vivo and in vitro. In developing rat skeletal muscle, both IP3 3-kinase and 5-phosphatase activities were relatively constant from embryonary day 15, the earliest age studied to postnatal day 10; 5-phosphatase decreased upon further development. A transient, major increase in phospholipase C activity was evident at embryonary day 18 while a non-significant increase in IP3 mass was detected at this embrionary age. In rat skeletal muscle in primary culture, all enzyme activities as well as the mass of IP3 increased significantly in myotubes compared to myoblasts. Myotubes incubated with calcitonin gene-related peptide, responded with a transient increase in IP3 mass after 2 to 10 sec; the CGRP-induced increase being completely blocked by U-73122, a phospholipase C inhibitor. Furthermore, IP3 mass increased within 1 hr after exposure to differentiating agents of both RCMH cells, a line derived from normal human skeletal muscle, and C2C12 cells. These results indicate that changes in IP3 metabolism can be correlated to critical stages of muscle development and differentiation, suggesting a possible role for IP3 in these processes.

Acceptor substrate specificity of a cloned GD3 synthase that catalyzes the biosynthesis of both GD3 and GD1c/GT1a/GQ1b

To address the role of alpha2,8-sialyltransferase (GD3 synthase) in the biosynthesis of gangliosides, we examined the substrate specificity of the enzyme. In the ganglioside synthesis pathway, it has been generally accepted that sialyltransferase II (SAT II) catalyzes the production of GD3 from GM3, and sialyltransferase V (SAT V) catalyzes the production of GD1c/GT1a/GQ1b from GM1h/GD1a/GT1b. However, acceptor specificity of the clones GD3 synthase that was isolated from human melanoma cells [Nara, K., Watanabe, Y., Maruyama, K., Kasahara, K., Nagai. Y. & Sanai, Y. (1994) Proc. Natl Acad. Sci. USA 91, 7952-7956] has revealed that this enzyme utilized the gangliosides containing the terminal Sia(alpha2-3)Gas structure of the carbohydrate moiety, which includes GM3, GM1b, GD1a and GT1B as exogenous substrates. Kinetic data also showed that the enzyme was able to utilize both GM3 and GM1b/GD1a/GT1b as acceptor substrates. These data indicate that the enzyme catalyzes the formation of not only GD3 but also GD1c, GT1a, and GQ1B in vitro. Furthermore, by transfection of the cloned human alpha2,8-sialyltransferase cDNA, transient and stable expression of GT1a and GQ1b wa also observed in COS-7 cells and Swiss 3T3 cells that originally lacked SAT II and SAT V activities. These observations indicate that the enzyme has both SAT II and SAT V activities in vivo.

Purification and biochemical characterization of membrane-bound epidermal ceramidases from guinea pig skin

Ceramidase (CDase) catalyzes the hydrolysis of ceramides to yield sphingosine and fatty acid. In this paper, two forms of membrane-bound alkaline ceramidase, have been, for the first time, purified from guinea pig epidermis by chromatography on DEAE-cellulose, phenyl-Superose, HCA-hyroxyapatite, isoelectric focusing, Mono Q, and TSK-3000SW column. One species (CDase-I) migrated upon SDS-polyacrylamide gel electrophoresis as a single band with an apparent molecular mass of 60 kDa; the other (CDase-II) was only partially purified with apparent M(r) of about 148,000 estimated by gel filtration. The specific activities of the two species increased by 1.130- (for CDase-I) and 400-fold (for CDase-II) over the original tissue extract. The activity of both enzymes for ceramide species decreased in the order of linoleoyl > oleoyl > palmitoylsphingosine. The optimal pH for enzyme activity was approximately 7.0-9.0 for CDase-I and 7.5-8.5 for CDase-II. Interestingly, both enzymes were inhibited by the reaction product sphingosine with a concentration for half-maximal inhibition (ID50) of 100-130 microM, compared to the apparent kinetic parameters with CDase-I (Km = 90 microM, Vmax = 0.62 unit) and CDase-II (Km = 140 microM, Vmax = 0.50 units). Some lipids, such as phosphatidylcholine and sphingomyelin, are also inhibitory with IC50 values of 50-250 microM, suggesting well controlled CDase activity by sphingolipid metabolites. These studies begin to elucidate a regulatory mechanism for the balance of the ratio of ceramide/sphingosine which can serve as an intracellular effector molecule in epidermis.

Involvement of phospholipase D in ganglioside GQ1b-induced biphasic diacylglycerol production in human keratinocytes

Ganglioside IV3 (NeuAc)2, II3 (NeuAc)2-GgOse4Cer (GQ1b), which induces terminal differentiation in keratinocytes, was previously found to enhance the mass content of inositol 1,4,5-trisphosphate and intracellular calcium concentration ([Ca++]i), peaking at 30 seconds. In the present study, the biphasic accumulation of 1,2 diacylglycerol, i.e., the first transient and the second sustained phase, was observed in cultured human keratinocytes stimulated by GQ1b. On the other hand, II3 NeuAc-LacCer (GM3), which inhibits keratinocyte proliferation without inducing differentiation, did not cause diacylglycerol formation. Phosphatidylethanol, produced by transphosphatidylation and a potential marker for phospholipase D activity, was produced by the exposure to GQ1b in the presence of ethanol. The second sustained phase of diacylglycerol was repressed by ethanol, indicating that the diacylglycerol-formation pathway via phospholipase D followed by phosphatidic acid phosphohydrolase would in part account for the second diacylglycerol phase. Furthermore, this second phase of GQ1b-induced diacylglycerol generation was reduced by pretreatment with propranolol, an inhibitor of phosphatidic acid phosphohydrolase. In addition, the levels of [3H]choline, a direct metabolite of the phospholipase D pathway, were elevated within 1 min after GQ1b addition and then sustained for at least 20 min. Taken together, the results suggest that the phospholipase D pathway may contribute to the second phase of diacylglycerol formation, which might be involved in differentiation.

Ganglioside-induced terminal differentiation of human keratinocytes: early biochemical events in signal transduction

Previous studies have indicated that GQ1b, a tetrasialoganglioside containing two disialosyl residues, may be an important regulator of cellular differentiation in murine keratinocytes. In the present study, we examined the effect of gangliosides on the differentiation of human keratinocytes. Current evidence indicates that GQ1b induces cornified envelope formation and enhancement of transglutaminase (TGase) activity, which are characteristic parameters of terminal differentiation in human cultured keratinocytes, while the other gangliosides, GT1b and GM1, are much less effective. The mass contents of inositol 1,4,5-trisphosphate (1,4,5-IP3) and the intracellular calcium concentration ([Ca2+]i) were also measured in keratinocytes exposed to gangliosides. A rapid increase in 1,4,5-IP3 occurred at 30 s following stimulation, but no significant difference at the maximum level was observed among the three gangliosides in contrast to the finding in murine keratinocytes. In addition, [Ca2+] increases occurred concurrently with the 1,4,5-IP3 generation by the three gangliosides. On the other hand, [Ca2+] transients were unaffected by chelating extracellular Ca2+ with EGTA. It is thus considered that the mobilization by 1,4,5-IP3 from internal stores plays a crucial role. These [Ca2+]i profiles were also indistinguishable between the gangliosides. Taken together, in human keratinocytes, gangliosides differentially affect some other as yet unidentified site(s) in the post-calcium transmission pathway(s) which leads to TGase activation.

Activation of protein kinase C inhibits human keratinocyte migration

The involvement of protein kinase C (PKC) in epidermal growth factor (EGF)-induced human keratinocyte migration was studied with the phagokinetic assay. It was concluded that PKC activation does not mediate, but rather inhibits, EGF-induced keratinocyte migration. The following experimental observations support these conclusions: 1) The PKC inhibitor H-7 did not inhibit EGF-induced migration but instead led to a modest enhancement. 2) PKC activators such as phorbol-12-myristate-13-acetate (PMA), phorbol-12,13-dibutyrate (PDBu), and 1,2-dioctanoly-sn-glycerol inhibited migration, but biologically inactive 4 alpha-PMA had no effect. 3) PMA did not inhibit keratinocyte attachment and spreading but blocked migration almost immediately after addition. 4) Migration of PKC-depleted cells, which were produced by prolonged treatment with PDBu, was enhanced similarly to normal cells by EGF. 5) PKC-depleted cells were not susceptible to the inhibitory effects of phorbol esters on migration. Additional experiments, in which cells were preactivated with EGF, suggested that PKC inhibits the EGF effect at a post-receptor level. The inhibitory effect of PKC on keratinocyte migration was not restricted to EGF-induced migration; PKC activation also inhibited keratinocyte migration induced by bovine pituitary extract, insulin, insulin-like growth factor-1, and keratinocyte growth factor.

Diacylglycerol: formation and function in phospholipid-mediated signal transduction

1. Properties, distribution and multiplicity of phosphoinositidases (phospholipase C, PLC) are investigated. 2. Generation of diacylglycerol (DAG) by a variety of enzymes such as phosphoinositide and phosphatidylcholine specific PLC, by a combination of phospholipase D and phosphatidic hydrolase, and by triglyceride lipase is examined. 3. Ca2+ and phospholipid-dependent protein kinase C act as the target of DAG messenger action. 4. There are differences in the formation of DAG in normal and transformal cell.