Biomedical subjects

J F Riou

Publications and source records attributed to J F Riou.

At least 73 records · Page 4Linked to original sources

PubMed1994

Synthesis and biological properties of new benz[h]isoquinoline derivatives.

In order to determine the importance of the pyrrolic nitrogen atom in a series of suitably substituted gamma-carbolines derivatives for their cytotoxic and antitumor properties, a series of structurally related benz[h]isoquinolines were synthesized. When compared to the "parent" gamma-carbolines, these new compounds showed greatly decreased effects on topoisomerases I and II. Whereas the 8-hydroxylated derivatives retained a significant cytotoxicity, all the new compounds were devoid of antitumor effect in the P388 murine ip-ip system.

Animals↗

PubMed1994

Tenascin expression in developing, adult and regenerating caudal spinal cord in the urodele amphibians.

Tenascin (Tn) protein and transcripts were analyzed in developing, adult and regenerating caudal spinal cord (SC) of Pleurodeles waltl. A polyclonal antibody (PAb) against Xenopus Tn and a newt Tn cDNA probe were used. In Western blots, anti-Tn PAb recognized Tn polypeptides of 200-220 kDa in tail regenerate extracts, but also the homolog of Tn/Cytotactin/J1 in brain and SC of adult newt. Immunofluorescence studies showed some reactivity around ependymoglial cells and strong labeling in the nervous tracts, in the developing as well as in the regenerating SC or adult SC. Immunogold electron microscopy revealed the presence of Tn throughout the ependymoglial cells, particularly near and along the plasma membrane of radial processes surrounding axons, especially growth cones. Tn could be more precisely found within rough endoplasmic reticulum and Golgi structures, or again in the surrounding extracellular space. This suggested that Tn was at least produced by radial glial profiles forming axonal compartments in which axons grew. Using the DNA probe for Tn, expression of Tn mRNA was also examined by Northern blot and RNAase protection analyses and by in situ hybridization, respectively. The levels of transcripts, barely detectable in adult tail, increased in regenerates from 3 days through 4-8 weeks post-amputation. In situ Tn mRNA were mainly localized in the mesenchyme, especially at the epithelial-mesenchymal interface, and in the developing cartilage, at the early regeneration stages, whereas high amounts of transcripts were seen not only at these stages, but also later, in the regenerating SC. Our main results supported the view that, in the caudal SC of newts, Tn, synthesized by radial ependymoglial cells, was similarly expressed during regeneration as well as larval development, and exhibited a sustained high accumulation level in the adult SC. On the basis of the multifunctional properties of Tn, the putative roles played by Tn as a substrate for neuronal pathfinding and boundary shaping were discussed.

Animals↗

PubMed1994

[Pleiotropic resistance associated with topoisomerases].

There are at least two well-characterized mechanism of resistance to Topo I and II inhibitors: modifications of intracellular accumulation and reduced formation of cleavable complexes. Limited drug accumulation is usually due to P-glycoproteinMDR or to Multidrug Resistance associated Protein (MRP). Reduction of Topo I (or II) cleavable complexes not related to drug transport can either be due to decreased enzyme levels or enzyme mutations. For Topo II inhibitors, differential expression of the Topo II isoforms alpha and beta and changes in Topo II phosphorylation may also contribute to resistance. For dual Topo I and II inhibitors, resistance mechanisms are more complex to analyze but may also involve dual Topo I and II alterations. Finally, in a given cell line, several mechanisms are commonly associated in pleiotropic resistance to Topo inhibitors.

ATP Binding Cassette Transporter, Subfamily B, Mem↗

PubMed1993

Intoplicine (RP 60475) and its derivatives, a new class of antitumor agents inhibiting both topoisomerase I and II activities.

Intoplicine (RP 60475, NSC 645008) is an antitumor derivative in the 7H-benzo[e]pyrido[4,3-b]indole series which is now being tested in clinical trials. Intoplicine strongly binds DNA (KA = 2 x 10(5) M-1) and thereby increases the length of linear DNA. These properties are consistent with DNA unwinding by intoplicine. Intoplicine was found to be a dual topoisomerase I and II inhibitor, with DNA sites of enzyme inhibition being different for these two enzymes. In this study, 22 analogues of intoplicine were evaluated for their effects on topoisomerase I- and II-mediated DNA cleavage reactions by using enzymes purified from calf thymus. Site-specific DNA cleavage mediated by topoisomerase I was observed with 7H-benzo[e]pyrido[4,3-b]indole derivatives but not with 11H-benzo[g]-pyrido[4,3-b]indole derivatives. Site-specific DNA cleavage mediated by topoisomerase II occurred with derivatives having hydroxyl groups at the 3-position on the 7H-benzo[e]pyrido[4,3-b]indole ring or at the 4-position on the 11H-benzo[g]pyrido[4,3-b]indole ring. Study of the relationships between the in vivo antitumor activity on P388 leukemia and the topoisomerase I- and/or II-mediated DNA cleavage activity revealed that the most highly active antitumor compounds possessed both topoisomerase I-and II-inhibitory properties. Compounds selectively inhibiting either topoisomerase I or II were less active. These results suggest that dual topoisomerase I and II inhibition is critical for the antitumor activity of this new series of antitumor compounds.

Animals↗

PubMed1993

Analysis of topoisomerase II-mediated DNA cleavage of the c-myc gene during HL60 differentiation.

We have investigated the effect of mAMSA, a potent topoisomerase II inhibitor, on the c-myc proto-oncogene of the acute promyelocytic leukemia HL60 cell line during its differentiation. When HL60 cells were induced by dimethylsulfoxide (DMSO) to terminally differentiate, a rapid drop in the level of c-myc mRNA was observed, followed by an arrest of cell proliferation. In contrast, the level of topoisomerase II mRNA was transiently increased with a maximum at 6 h after DMSO addition and was then completely abolished after 48 h, indicating that topoisomerase II is activated during the onset of HL60 differentiation. In exponentially growing cells, treatment by mAMSA results in the formation of topoisomerase II-mediated double strand DNA breaks in the c-myc gene at positions where topoisomerase II would normally nick and reseal the two strands. In HL60 cells treated with both mAMSA and DMSO, the sites in the c-myc gene at which mAMSA had induced cleavage were not altered. However, a DNA cleavage site located at the end of the first c-myc exon (position +3100), was strongly stimulated by mAMSA and DMSO treatment. This site fell within a DNase I hypersensitive region encompassing the MYC intron factor 1 (MIF1) binding site, where transcription elongation is normally blocked during differentiation. These data indicate that a change of topoisomerase II binding to critical regulatory region of the c-myc gene is associated with the downregulation of this gene during differentiation.

Cell Differentiation↗

PubMed1993

Synthesis and evaluation of new 6-amino-substituted benzo[c]phenanthridine derivatives.

Different 7,8,9,10-tetrahydrobenzo[c]phenanthridin-6(5H)-ones (10a-e) were prepared by using a one-pot procedure which includes the preparation of various 6- and 7-alkoxy-1-naphthylisocyanates from 1-naphthylamines and triphosgene, followed by addition of 1-N-morpholino-1-cyclohexenes, and cyclization of the resulting amides upon heating in the presence of hydrogen chloride. Subsequent aromatization, chlorination, and substitution with (dimethylamino)alkylamines, followed by a demethylation or a selective desisopropylation, allowed us to synthesize the derivatives 6a-i and 7a-h bearing a [(dimethylamino)alkyl]amino side chain at their 6-position. These compounds, as the other analogs 5a-b, were devised to further study the structure-activity relationships in the benzo[c]phenanthridine family of antitumor alkaloids led by fagaronine (1a) and nitidine (1b). Topoisomerases I and II cleavable complex assay and evaluation of the cytotoxicity and antitumor properties were performed. In vitro cytotoxicity (L1210 and Calc 18) shows a relationship between the cytotoxicity of these compounds and their topoisomerase poisoning properties. However, all these compounds were devoid of significant antitumor effect on the P388 murine leukemia system.

Adenocarcinoma↗

PubMed1993

The antileukemic alkaloid fagaronine is an inhibitor of DNA topoisomerases I and II.

The antileukemic alkaloid, fagaronine, is a potent differentiation inducer of various hematopoietic cell lines. We show here that fagaronine is a DNA base-pair intercalator with a K(app) of 2.1 x 10(5) M-1 for calf thymus DNA. Fagaronine inhibits the catalytic activity of purified calf thymus topoisomerase I as shown by relaxation of supercoiled plasmid DNA followed by electrophoresis in neutral as well as in chloroquine-containing gels. The catalytic activity of topoisomerase I is inhibited at concentrations above 30 microM. Fagaronine also inhibits the catalytic activity of purified calf thymus topoisomerase II at concentrations above 25 microM as shown by decatenation of kinetoplast DNA. Fagaronine stabilizes the covalent DNA-enzyme reaction intermediate (the cleavable complex) between topoisomerase I and linear pBR322 DNA at concentrations up to 1 microM. Further increase of the fagaronine concentration leads to a progressive decrease in the cleavable complex formation, which is totally inhibited at 100 microM. In contrast, up to 1 microM fagaronine has no effect on cleavable complex formation between purified calf thymus topoisomerase II and linear pBR322 DNA, whereas cleavable complex formation is inhibited at higher concentrations. Exposure to fagaronine results in an increase in DNA-protein complex formation in intact P388 murine leukemia cells. P388CPT5 cells, which have an altered topoisomerase I activity, are 4-fold resistant to the growth inhibitory effects of fagaronine compared to the parental cell line. Similarly, DC-3F/9-OH-E Chinese hamster fibrosarcoma cells, which have an altered topoisomerase II activity, are about 5-fold resistant to the growth inhibitory effects of fagaronine. We conclude that fagaronine is an inhibitor of both DNA topoisomerase I and II and propose that this might play a role in the cytotoxic activity.

Alkaloids↗

PubMed1993

Molecular and cellular interactions between intoplicine, DNA, and topoisomerase II studied by surface-enhanced Raman scattering spectroscopy.

The surface-enhanced Raman scattering spectra of the new antitumoral agent, intoplicine (RP 60475, NSC 645008), and those of its complexes with DNA and topoisomerase II in vitro and in K562 cancer cells were obtained. Intoplicine was found to unwind DNA and to inhibit purified calf thymus topoisomerase II via a stabilization of the ternary cleavable complex. The intensity of the surface-enhanced Raman scattering spectrum of intoplicine was not modified by the addition of plasmid pBR322 or calf thymus DNA. In the complex of this antitumor agent with topoisomerase II, the signal of intoplicine was completely abolished, indicating that at least some portion of intoplicine binds to an internal part of the enzyme. During the formation of the ternary complex, intoplicine was released from the interior of the protein and formed hydrogen bonds via its hydroxyl and/or amino groups. Similar modifications of the intoplicine spectra were found by microsurface-enhanced Raman scattering spectroscopy of the compound in the nucleus of treated K562 cells. In contrast, intoplicine was found to be in a free form in the cytoplasm.

Animals↗

PubMed1993

Altered topoisomerase I activity and recombination activating gene expression in a human leukemia cell line resistant to doxorubicin.

We examined the expression of the genes encoding topoisomerases I and II and those associated with V(D)J [variable(diversity)joining] recombination in two human T-cell acute lymphoblastic leukemia (T.ALL) cell lines, CEM and CEM/DOX. In CEM/DOX cells, which are resistant to doxorubicin, the topoisomerase I gene was found to be 4-fold overexpressed and nuclear topoisomerase I relaxation activity was 2-fold greater in CEM/DOX than in CEM cells. Furthermore, the cleavable complex reaction induced by camptothecin, a specific topoisomerase I inhibitor, was found to be 2.5-increased in the presence of topoisomerase I extracted from CEM/DOX, in comparison to that in CEM cells. Conversely, the topoisomerase II mRNA levels, nuclear decatenation activities and (mAMSA) 4'(9-acridinylamino)methanesulfon-m-anisidide-induced cleavable complex formation in CEM/DOX were similar to those of the doxorubicin-sensitive cells. The results indicate that topoisomerase I activity is elevated in CEM/DOX cells. Nevertheless, CEM/DOX cells were 11-fold more resistant to camptothecin than were CEM cells, and cross-resistance to camptothecin was not reversed by verapamil. Furthermore, using an intact cell assay for DNA-protein complexes, we found that camptothecin-stimulated cleavable complexes formed in CEM/DOX cells were increased in correlation with the elevated topoisomerase I activity. These results suggest that camptothecin resistance in CEM/DOX cells is due to different mechanism(s) than topoisomerase- or P-glycoprotein-associated multidrug resistance. The recombination activating gene, RAG1, which is one of the components of the site-specific V(D)J recombination complex, was 20-fold overexpressed in CEM/DOX cells. In contrast, RAG2 and T160 gene transcripts, other components of the V(D)J complex, were at best poorly detected in both sensitive and resistant cells. No specific V(D)J recombinase activity was found in CEM or CEM/DOX cells when the pJH201 transfection assay was used. The results indicate that CEM/DOX cells failed to generate V(D)J recombination although RAG1 gene is overexpressed. The mechanism of the RAG1 gene activation was not gene amplification, and no rearrangement was detected in the RAG1 gene locus. RAG1 presents homology with the yeast gene HPR1, itself homologous to yeast topoisomerase I and responsible for the control of recombination in somatic cells. Since DNA topoisomerases are themselves involved in the control of DNA topology, recombination and DNA repair, the possible coactivation of RAG1 and topoisomerase I genes in CEM/DOX cells is discussed.

Amsacrine↗

PubMed1993

Sequential modifications of topoisomerase I activity in a camptothecin-resistant cell line established by progressive adaptation.

The DNA-topoisomerase I (Topo I) inhibitor, camptothecin (CPT), is a plant alkaloid with an important antitumor activity. In order to investigate the cellular mechanism leading to the development of the resistance to this agent, we have established by progressive adaptation a P388 subline resistant to CPT. After 5 months of continuous drug exposure, the resistance index reached a value of 20 and the resistant cell line, P388CPT0.3, was maintained in the presence of CPT. CPT-induced single strand breaks measured by alkaline elution were found drastically reduced in the resistant cell line. Topo I activity and CPT-induced DNA cleavage were measured on cells at different steps of resistance. We first observed that the Topo I activity was strongly decreased. In addition, the resistant cells recovered the ATP-independent relaxation activity after 3 months of exposure to CPT, but still kept a reduced CPT-induced DNA cleavage. Further evaluations at the final stage of the resistance induction have indicated that cells presented a CPT-resistant form of Topo I. Rearranged Topo I gene on one allele and a reduced Topo I transcription were also observed in resistant cells. The putative role of the rearrangement was discussed. These data show that the resistance mechanism has evolved from a decreased Topo I activity to an altered form of the enzyme, and suggest that multiple mechanisms of Topo I modifications could contribute to CPT resistance.

Animals↗

PubMed1993

Amphibian Pleurodeles waltl fibronectin: cDNA cloning and developmental expression of spliced variants.

Partial cDNA clones encoding approximately the carboxy terminal half of Pleurodeles fibronectin (FN) were isolated. They account for 4.7 Kbp of the 3' region of the FN mRNA. The cDNA nucleotide sequence comprises all three alternatively spliced segments designated EIIIA, EIIIB and V-segment, respectively. All three segments are included in FN mRNA synthesized during early embryogenesis whereas, from the tailbud stage onward the V-region was partially excluded. The isolation of Pleurodeles cDNA clones including the three different spliced EIIIA, EIIIB and V segment raises new possibilities for the study of the precise role of specific regions of FN in early amphibian development.

Alternative Splicing↗

PubMed1993

[Regionalization of the expression of tenascin as a response to the inducers of mesoderm].

Tenascin (TN) is an extracellular matrix (ECM) glycoprotein which possesses antiadhesive properties and thus is able to modulate cell interactions with molecules of the ECM. In Xenopus embryos, TN is expressed dorsally in a very restricted pattern. We have studied the distribution of TN mRNA in tailbud-stage embryos by in situ hybridization. We show that TN transcripts are principally expressed in myotome cells. No TN mRNA could be detected in lateral plate mesoderm. This had led us to study TN expression in response to mesodermal inducers. Analysis of the distribution of TN after mesodermal induction of blastula animal caps with activin A and bFGF or after ectopic expression of XBra shows that TN expression is elicited when dorsal or posterior structures are formed. Our results show that TN regionalization in Xenopus embryos depends on mesoderm patterning.

Activins↗

PubMed1993

Dual topoisomerase I and II inhibition by intoplicine (RP-60475), a new antitumor agent in early clinical trials.

The mechanisms of action of intoplicine (RP-60475), a 7H-benzo[e]pyrido[4,3-b]indole derivative that is presently in early clinical trials, have been investigated. Intoplicine induced both topoisomerase I- and II-mediated DNA strand breaks, using purified topoisomerases. The topoisomerase cleavage site patterns induced by intoplicine were unique, relative to those of camptothecin, 4'-(9-acridinylamino)methanesulfon-m-anisidide (m-AMSA), and other known topoisomerase inhibitors. Both topoisomerase I- and II-induced DNA breaks decreased at drug concentrations higher than 1 microM, which is consistent with the DNA-intercalating activity of intoplicine. DNA damage was investigated in KB cells in culture by using alkaline elution. Intoplicine induced single-strand breaks (SSB) in a bell-shaped manner with respect to drug concentration (maximum frequency at 1 microM approximately 220 rad-equivalents). SSB formation was fast, whereas reversal after drug removal was slow. Similar bell-shaped curves were obtained for DNA double-strand breaks (DSB) and DNA-protein cross-links. SSB and DNA-protein cross-link frequencies were approximately equal, and no protein-free breaks were detectable, indicating the protein concealment of the breaks, as expected for topoisomerase inhibition. Comparison of SSB and DSB frequencies indicated that intoplicine produced a significant amount of SSB not related to DSB, which is consistent with concomitant inhibition of both DNA topoisomerases I and II in cells. Data derived from resistant cell lines indicated that multidrug-resistant cells were cross-resistant to intoplicine but that m-AMSA- and camptothecin-resistant cells were sensitive to intoplicine. Hence, intoplicine might circumvent topoisomerase I-mediated and topoisomerase II-mediated resistance by poisoning both enzymes simultaneously.

Animals↗

PubMed1992

Effects of Taxotere on murine and human tumor cell lines.

Taxotere (RP 56976, NSC 628503), an analog of taxol, is an inhibitor of depolymerisation of microtubules and is currently in Phase I clinical trials. Comparisons of the cytotoxicities of Taxotere and taxol have been studied on several murine (P388, SVras) and human cell lines (Calc18, HCT116, T24, N417, KB). Taxotere was found more potent than taxol (1.3-12 fold), a result which could be explained by its higher affinity than taxol for microtubules. In agreement with its postulated mechanism of action, Taxotere is more cytotoxic on proliferating than on non proliferating N417 cells and does not inhibit cellular DNA, RNA and protein synthesis. Taxotere gives partial cross resistance on P-glycoprotein resistant P388/DOX cell line, in contrast to taxol which gives a complete cross resistance. On the other hand, no cross resistances were observed on Calc18/AM and P388/CPT5 cell lines, bearing modified activities of topoisomerase II and topoisomerase I, respectively. These results underline the higher cytotoxic activity of Taxotere compared to taxol, and the lack of cross resistance of that class of agent with the topoisomerase I and II-related multidrug resistance phenotypes.

Alkaloids↗

PubMed1992

Differential effects of amsacrine and epipodophyllotoxins on topoisomerase II cleavage in the human c-myc protooncogene.

Amsacrine and demethylepipodophyllotoxins (etoposide and teniposide) are potent topoisomerase II inhibitors which have optimum activity in different cancers. To investigate whether these differences are due to different activity on cellular oncogenes, drug-induced topoisomerase II cleavage sites were mapped and sequenced in the human c-myc protooncogene. In the presence of purified murine L1210 topoisomerase II, amsacrine induces prominent cleavage in the P2 promoter (site 2499/2502). Footprinting experiments indicate that topoisomerase II binds to the entire promoter region (approximately 20 base pairs on the sides of the P2 site). In the case of teniposide or etoposide, cleavage is more diffuse and markedly less at the P2 site. Mapping of cleavage sites in human small cell lung carcinoma cells (NCI N417) also shows that cleavage in the P2 promoter region is induced preferentially by amsacrine but not by demethylepipodophyllotoxins. Thus, selective gene damage among topoisomerase II inhibitors may contribute to differential anticancer activity.

Amsacrine↗

PubMed1992

Tenascin: a potential modulator of cell-extracellular matrix interactions during vertebrate embryogenesis.

Tenascin is a large oligomeric extracellular matrix (ECM) glycoprotein whose expression is highly restricted during vertebrate development. It has a characteristic hexameric quaternary structure with six arms linked to a central globular domain. Each arm contains a single polypeptide with the central globular domain formed by the covalent association of the N-terminal ends of the six polypeptides. Tenascin first appears during development, associated with the neural crest cell migration pathways of mammalian, avian and amphibian embryos. During later development, it is observed at sites of cartilage, bone and tendon formation. Tenascin expression also occurs in defined areas in the developing nervous system and in condensing mesenchyme, in response to epithelio-mesenchymal interactions. The function of tenascin in these different morphogenetic processes is not yet clearly understood. Tenascin can promote neurite outgrowth in vitro and can inhibit cell interactions with fibronectin. Results based on antibody mapping and molecular cloning indicate that these properties involve two distinct cell binding sites. Together with its highly regulated expression in the embryo, these properties suggest that tenascin plays a key role in the control of cell migration and differentiation during development.

Animals↗

PubMed1992

Expression of tenascin mRNA in mesoderm during Xenopus laevis embryogenesis: the potential role of mesoderm patterning in tenascin regionalization.

In Xenopus embryos, the extracellular matrix (ECM) protein tenascin (TN) is expressed dorsally in a very restricted pattern. We have studied the spatial and temporal expression of TN mRNA in tailbud-stage embryos by RNAase protection and in situ hybridization using a cDNA probe for Xenopus TN obtained by PCR amplification. We report that TN transcripts are principally expressed in cells dispersed around the neural tube and notochord as well as in myotome and sclerotome cells. No TN mRNA could be detected in lateral plate mesoderm, but expression was detectable beneath tail fin epidermis. In a second series of experiments, we studied the expression of TN mRNA and protein in combinations between animal and vegetal stage-6 blastomeres and in stage-8 blastula animal caps treated with activin A or basic fibroblastic growth factor (b-FGF). Isolated animal cap tissue cultured alone differentiates into epidermis, which expresses neither TN protein nor TN mRNA. TN expression is, however, elicited in response to isolated dorsal vegetal blastomeres and in response to high concentrations of activin, both of which treatments lead to formation of muscle and/or notochord. Low concentrations of activin, and ventral vegetal blastomeres, treatments that induce mesoderm of ventral character, are poor inducers of TN. However, b-FGF, which also induces ventral mesoderm, elicits strong expression. These results indicate that TN regionalization is a complex process, dependent both on the pattern of differentiation of mesodermal tissues and on the agent with which they are induced. The data further show that "ventral mesoderm" induced by low concentrations of activin is distinct from that induced by b-FGF, and imply that activin induces ventral mesoderm of the trunk while b-FGF induces posterior mesoderm of the tailbud.

Animals↗

PubMed1992

Differential expression and regulation of two distinct fibroblast growth factor receptors during early development of the urodele amphibian Pleurodeles waltl.

Fibroblast growth factor (FGF) has been shown to be involved in mesoderm induction during amphibian development. Its presence in the embryo suggests that FGF is an endogenous inducer. By polymerase chain reaction (PCR) methods and by screening a Pleurodeles waltl tail-bud cDNA library with a cDNA probe for human FGF receptor, we have isolated two full-length cDNA clones, which we designate PFR1 and PFR4 based on their homology to the human FGF receptors FGFR-1 and FGFR-4. Both cDNA clones encode Pleurodeles FGF receptors that share characteristics common to members of the FGF receptor superfamily. The deduced amino acid sequence of PFR1 is 85% identical overall with the human fms-like-gene (FLG). PFR4 is most closely related to the human FGFR-4 (66% overall identity). The tyrosine kinase catalytic domains of both receptors are remarkably conserved. The two receptors show distinct patterns of regulation during early development. PFR1 first appears as a maternally derived mRNA and mRNA levels remain constant during early developmental stages. However, PFR4 mRNA is first expressed at the late blastula stage, which suggests that its expression is a result of zygotic transcription. Furthermore, northern blot analysis indicates that PFR1 mRNA is distributed evenly in the early gastrula while PFR4 mRNA is predominantly localized to the presumptive ectoderm. At tail-bud stage, PFR1 transcripts are localized primarily to the neural and mesodermal tissues, PFR4 transcripts are most abundantly expressed in neural tissue, and more transcripts are detected in lateral plate mesoderm than in the somites. When animal cap explants of blastulae are cultured in the presence of mesoderm-inducing factors, PFR1 mRNA levels are maintained by bFGF and activin A. In contrast, PFR4 mRNA levels are significantly down-regulated. These observations suggest a differential expression and regulation of FGF receptors in early amphibian development.

Amino Acid Sequence↗