V599EB-RAF is an oncogene in melanocytes

The oncogenic version of B-RAF, (V599E)B-RAF, is found in approximately 70% of human melanomas. However, the role that this oncogene plays in melanoma is unclear because (V559E)B-RAF is also found in approximately 80% of benign nevi. We have examined the role of oncogenic B-RAF in the early stages of melanoma by expressing (V599E)B-RAF in cultured melanocytes. In these cells, (V599E)B-RAF induced constitutive mitogen activated ERK-activating kinase (MEK) and extracellular signal-regulated kinase (ERK) signaling, 12-O-tetradecanoylphorbol-13-acetate-independent growth, and tumorigenicity in nude mice. Intriguingly, in RAS-transformed melanocytes, B-RAF depletion did not block MEK-ERK signaling or cell cycle progression. Similarly, B-RAF depletion blocked MEK-ERK signaling in human melanoma cells harboring oncogenic B-RAF, but not in melanoma cells harboring oncogenic RAS. Thus, although B-RAF can act as a potent oncogene in the early stages of melanoma by signaling through MEK and ERK, it is not required for this signaling in RAS-transformed melanocytes due to innate redundancy within the pathway. These findings have important implications for future therapeutic strategies.

The Brn-2 transcription factor links activated BRAF to melanoma proliferation

Malignant melanoma, an aggressive and increasingly common cancer, is characterized by a strikingly high rate (70%) of mutations in BRAF, a key component of the mitogen-activated protein (MAP) kinase signaling pathway. How signaling events downstream from BRAF affect the underlying program of gene expression is poorly understood. We show that the Brn-2 POU domain transcription factor is highly expressed in melanoma cell lines but not in melanocytes or melanoblasts and that overexpression of Brn-2 in melanocytes results in increased proliferation. Expression of Brn-2 is strongly upregulated by Ras and MAP kinase signaling. Importantly, the Brn-2 promoter is stimulated by kinase-activating BRAF mutants and endogenous Brn-2 expression is inhibited by RNA interference-mediated downregulation of BRAF. Moreover, silent interfering RNA-mediated depletion of Brn-2 in melanoma cells expressing activated BRAF leads to decreased proliferation. The results suggest that the high levels of Brn-2 expression observed in melanomas link BRAF signaling to increased proliferation.

MITF-M plays an essential role in transcriptional activation and signal transduction in Xiphophorus melanoma

The teleost Xiphophorus provides a genetically well-described model system to study the molecular processes underlying melanoma formation. As transcriptional deregulation is a widespread phenomenon in many tumors, we have studied the regulation of melanoma-specific gene expression in this fish. A central regulator of melanocyte specific gene expression, which is also a marker for melanomas, is the transcription factor microphthalmia-associated transcription factor (MITF). One of its targets, the tyrosinase gene, codes for a key enzyme in the melanin synthesis pathway. We could show that the promoter of the medaka tyrosinase gene is highly active in the Xiphophorus melanoma cell line PSM (platyfish-swordtail melanoma) but not in non-melanoma cells. Functional dissection of the promoter revealed that three E-boxes are essential for its pigment cell-specific activity. These binding sites for basic helix-loop-helix transcription factors are recognized by a nuclear protein from the melanoma cell line PSM, most likely MITF, as its exogenous delivery could activate the promoter in non-melanoma cells. The use of specific signalling inhibitors demonstrated that the activity of the tyrosinase promoter is negatively regulated by the melanoma-inducing receptor tyrosine kinase Xmrk in PSM cells. This repression is mediated by MAPkinase and dependent on E-box integrity, again implicating the involvement of MITF. The cumulative evidence indicates that in Xiphophorus, Xmrk suppresses differentiation signals relayed by MITF as part of the transformation process finally resulting in melanoma formation.

Identification of a second egfr gene in Xiphophorus uncovers an expansion of the epidermal growth factor receptor family in fish

The epidermal growth factor receptor (EGFR) gives name to a family of receptors formed by four members in mammals (EGFR, ErbB2, ErbB3, and ErbB4). Members of this family can be activated to become potent oncogenes, and many human and animal tumors express qualitatively or quantitatively altered receptors from this group. We have isolated and characterized a second egfr gene in the melanoma model fish Xiphophorus. Both Xiphophorus egfra and egfrb duplicates are co-orthologs of the mammalian egfr gene. Database analysis showed that not only egfr but also erbB3 and erbB4 are present as duplicates in some fish species. They originated from ancient duplication events that might be consistent with the hypothesis of a fish-specific genome duplication. In Xiphophorus, the egfrb gene underwent a second duplication that generated the melanoma-inducing oncogene Xmrk. The study and comparison of some of the functional characteristics of both Xiphophorus EGF receptors, including expression profile, ligand-binding abilities, and intracellular signal transduction revealed that Xiphophorus Egfra not only shares common features with Egfrb and the human EGFR but also shows significant differences in its functional characteristics. The mechanism of maintenance of these duplicates remains to be clarified.

Melanoma development and pigment cell transformation in xiphophorus

As early as 1927, it was recognised that hybridisation of platyfish (Xiphophorus maculatus) and swordtails (Xiphophorus helleri) results in offspring that develop tumours according to Mendelian laws. Most obviously, the primary event, namely the cell lineage-specific overexpression of a structurally altered receptor tyrosine kinase, finds its parallel in many tumours of birds and mammals. Once expressed at high levels, this receptor, the Xiphophorus melanoma inducing receptor kinase Xmrk, shows constitutive activation. By using different pathways, Xmrk induces both proliferative as well as anti-apoptotic signalling in pigment cells finally leading to cell transformation, tumour induction, and progression. Analyses of the different signalling cascades induced by the Xmrk-receptor led to the identification of the src-kinase Fyn, the MAP kinases ERK1 and ERK2, the "Signal Transducer and Activator of Transcription" STAT5, and the PI3-kinase as its major downstream substrates. This review describes some of the genetic findings, as well as the results from the recent molecular analyses of the factors involved in the initiation and manifestation of pigment cell transformation and melanoma development in Xiphophorus.

Autocrine stimulation by osteopontin contributes to antiapoptotic signalling of melanocytes in dermal collagen

The growth of melanocytes, the pigment-producing cells of the skin, normally is restricted to the epidermis. Transformed melanocytes, which have invaded the dermis, however, have gained the ability to grow in this new environment and to counteract apoptosis induced by the dermal connective tissue. The expression of genes contributing to the survival of melanocytes in the dermal environment, therefore, might be involved in melanoma development. Using a differential display approach, we identified osteopontin as such a gene. In melanocytes, expression of the secreted adhesion protein OPN was up-regulated by the melanoma-inducing receptor tyrosine kinase Xmrk as well as by the fibroblast growth factor receptor, which plays a decisive role in human melanoma. Activation of both receptors triggered survival of melanocytes in three-dimensional dermal collagen gels. Competition experiments revealed that the presence of OPN in the medium as a result of receptor signaling was contributing to these effects. Addition of exogenous OPN allowed melanocytes to adhere, spread, and survive in three-dimensional collagen gels, whereas in the absence of OPN, the cells underwent apoptosis. The integrin alpha(v)beta(3) known to be involved in melanoma cell survival and growth was identified as an OPN receptor, which points to an OPN-mediated cross-talk between growth factor receptors and this integrin receptor in melanocytes. In summary, we could show that in melanocytes growth factor receptor-induced secretion of OPN can promote antiapoptotic signaling and mediate appropriate interactions with the extracellular matrix in an autocrine way. Our findings suggest a new role of growth factor receptors of the family of receptor tyrosine kinases in processes associated with melanoma development and progression.

Activation of STAT5 triggers proliferation and contributes to anti-apoptotic signalling mediated by the oncogenic Xmrk kinase

Extensive studies of primary tumors and tumor derived cell lines revealed that inappropriate activation of specific STATs (particularly of STAT3 and STAT5) occurs with high frequency in a wide variety of human cancers. We reported recently that the melanoma inducing EGFR-related receptor Xmrk specifically induces constitutive activation of STAT5 in fish melanoma cells. However, little is known about the role of STAT5 in solid tumours in general and its function in melanoma in particular. Recent examinations suggest that activated STAT signalling participates in oncogenesis by stimulating cell proliferation and preventing apoptosis. As an initial approach to understanding the consequences of Xmrk induced STAT5 signalling we used the well characterized pro B-cell line Ba/F3 as a sensitive system to analyse mitogenic as well as anti-apoptotic signalling. We identified STAT5 activation as being involved in both growth and survival signalling triggered by the Xmrk kinase possibly due to STAT5 induced expression of pim-1 and bcl-x. We also found a new mechanism of activation of STAT5 by receptor tyrosine kinases, whereby direct interaction of the receptor kinase domain with the STAT protein in a phosphotyrosine independent way led to activation of STAT5 in terms of DNA binding and target gene expression.

Activation of p59(Fyn) leads to melanocyte dedifferentiation by influencing MKP-1-regulated mitogen-activated protein kinase signaling

Malignant melanoma is a cancer whose incidence is rising rapidly, but the mechanism by which normal melanocytes become malignant in vivo is still little understood. In the course of melanoma progression, a fraction of cells often becomes depigmented, which reflects the loss of the balance between mitogenic activities and differentiation in those pigment cells. A key factor involved in differentiation in pigment cells is mitogen-activated protein kinase (MAPK). However, because both activation and inhibition of MAPK signaling is known to correlate with differentiation, its function in pigment cells is still unclear. We investigated the role of MAPK signaling in pigment cells using the melanoma-inducing receptor tyrosine kinase Xmrk. Xmrk signaling in mouse melanocytes suppressed differentiation and induced a transformed phenotype. We found that this was based on sustained MAPK activation caused by low and transient expression of MAPK-phosphatase MKP-1. The Src kinase p59(Fyn) was thereby identified as being crucial for the receptor-mediated suppression of differentiation by down-regulating MKP-1 expression. Our findings reveal a novel mechanism of regulating the balance between differentiation and proliferation based on a Src kinase-modified MAPK activity. Moreover, they point to a new role for Src kinases in dedifferentiation and transformation of pigment cells.

Apoptosis suppression by Raf-1 and MEK1 requires MEK- and phosphatidylinositol 3-kinase-dependent signals

Two Ras effector pathways leading to the activation of Raf-1 and phosphatidylinositol 3-kinase (PI3K) have been implicated in the survival signaling by the interleukin 3 (IL-3) receptor. Analysis of apoptosis suppression by Raf-1 demonstrated the requirement for mitochondrial translocation of the kinase in this process. This could be achieved either by overexpression of the antiapoptotic protein Bcl-2 or by targeting Raf-1 to the mitochondria via fusion to the mitochondrial protein Mas p70. Mitochondrially active Raf-1 is unable to activate extracellular signal-related kinase 1 (ERK1) and ERK2 but suppresses cell death by inactivating the proapoptotic Bcl-2 family member BAD. However, genetic and biochemical data also have suggested a role for the Raf-1 effector module MEK-ERK in apoptosis suppression. We thus tested for MEK requirement in cell survival signaling using the interleukin 3 (IL-3)-dependent cell line 32D. MEK is essential for survival and growth in the presence of IL-3. Upon growth factor withdrawal the expression of constitutively active MEK1 mutants significantly delays the onset of apoptosis, whereas the presence of a dominant negative mutant accelerates cell death. Survival signaling by MEK most likely results from the activation of ERKs since expression of a constitutively active form of ERK2 was as effective in protecting NIH 3T3 fibroblasts against doxorubicin-induced cell death as oncogenic MEK. The survival effect of activated MEK in 32D cells is achieved by both MEK- and PI3K-dependent mechanisms and results in the activation of PI3K and in the phosphorylation of AKT. MEK and PI3K dependence is also observed in 32D cells protected from apoptosis by oncogenic Raf-1. Additionally, we also could extend these findings to the IL-3-dependent pro-B-cell line BaF3, suggesting that recruitment of MEK is a common mechanism for survival signaling by activated Raf. Requirement for the PI3K effector AKT in this process is further demonstrated by the inhibitory effect of a dominant negative AKT mutant on Raf-1-induced cell survival. Moreover, a constitutively active form of AKT synergizes with Raf-1 in apoptosis suppression. In summary these data strongly suggest a Raf effector pathway for cell survival that is mediated by MEK and AKT.

Ligand-independent dimerization and activation of the oncogenic Xmrk receptor by two mutations in the extracellular domain

Overexpression of the oncogenic receptor tyrosine kinase ONC-Xmrk is the first step in the development of hereditary malignant melanoma in the fish Xiphophorus. However, overexpression of its proto-oncogene counterpart (INV-Xmrk) is not sufficient for the oncogenic function of the receptor. Compared with INV-Xmrk, the ONC-Xmrk receptor displays 14 amino acid changes, suggesting the presence of activating mutations. To identify such activating mutations, a series of chimeric and mutant receptors were studied. None of the mutations present in the intracellular domain was found to be involved in receptor activation. In the extracellular domain, we found two mutations responsible for activation of the receptor. One is the substitution of a conserved cysteine (C578S) involved in intramolecular disulfide bonding. The other is a glycine to arginine exchange (G359R) in subdomain III. Either mutation leads to constitutive dimer formation and thereby to activation of the ONC-Xmrk receptor. Besides, the presence of these mutations slows down the processing of the Xmrk receptor in the endoplasmic reticulum, which is apparent as an incomplete glycosylation.

Activation of phosphatidylinositol 3-kinase by a complex of p59fyn and the receptor tyrosine kinase Xmrk is involved in malignant transformation of pigment cells

Malignant melanoma in the fish Xiphophorus is induced by overexpression of the Xmrk-oncogene, encoding a subclass I receptor tyrosine kinase. The mutationally activated Xmrk protein triggers constitutive mitogenic signalling in fish melanoma cells. In recent studies we showed that in melanoma cells phosphatidylinositol (PtdIns) 3-kinase, as well as p59fyn, has elevated levels of kinase activity. Both bind directly to different phosphotyrosine residues in the Xmrk receptor C-terminus through their SH2 domains. To analyse the mechanism of regulation of these Xmrk-associated kinases in melanoma we characterized the protein-protein interactions between PtdIns 3-kinase, p59fyn and the Xmrk receptor in detail. A ternary complex in which the p85 subunit of PtdIns 3-kinase is associated with p59fyn as well as with Xmrk was identified. Contrary to complexes described for other receptors, the adaptor protein p120Cbl was not involved in these interactions. Thus, we describe here a new mechanism of activation of PtdIns 3-kinase by a receptor of the epidermal growth factor receptor family in which p59fyn acts as an adaptor as well as an activator of PtdIns 3-kinase. Activation of PtdIns 3-kinase activity by fyn was also found in vivo. The fact that this was only detectable in highly transformed Xmrk overexpressing melanomas but not in benign lesions points to the essential role of the Xmrk receptor in this mechanism of regulation.

PI3-kinase is involved in mitogenic signaling by the oncogenic receptor tyrosine kinase Xiphophorus melanoma receptor kinase in fish melanoma

Overexpression of the mutationally activated receptor tyrosine kinase Xiphophorus melanoma receptor kinase (Xmrk) initiates formation of hereditary malignant melanoma in the fish Xiphophorus. In melanoma as well as in a melanoma-derived cell line (PSM) this receptor is highly activated resulting in constitutive Xmrk-mediated mitogenic signaling. In order to analyze mitogenic signaling triggered by Xmrk a possible involvement of phosphatidylinositol 3 (PI3)-kinase in Xmrk signal transduction was examined. Constitutive binding of the p85 adapter subunit of PI3-kinase to the Xmrk receptor was detected in PSM melanoma cells. Further analyses in BHK cells expressing a Xmrk chimera (HER-mrk) showed that p85 association with the intracellular part of Xmrk was dependent on autophosphorylation of the receptor. In vitro binding studies revealed that the interaction is mediated mainly through the N-terminal SH2 domain of p85 which directly binds to a sequence motif around phosphorylated Tyr-983 in the Xmrk carboxy-terminus. In accordance with recruitment of p85 by Xmrk in PSM cells, the PI3-kinase downstream target Akt was found to be highly phosphorylated on Ser-473, indicating efficient PI3-kinase signaling in melanoma cells. PI3-kinase activation was also detected in Xiphophorus melanoma. Moreover, malignant melanomas exhibited an increased level of PI3-kinase activity which was about three times higher than that in benign pigmented lesions. Inhibition of PI3-kinase activity in PSM melanoma cells by both Wortmannin and LY294002 blocked entry into S-phase. Together these data demonstrate that PI3-kinase is a substrate of the oncogenic Xmrk receptor and plays a significant role in mitogenic signaling of melanoma cells and the formation of malignant melanoma in Xiphophorus.

A two-step selection approach for the identification of ligand-binding determinants in cytokine receptors

We have developed a novel cell-based method for the isolation and selection of mutant cytokine receptors with defects in ligand binding and applied it to the human interleukin-4 receptor. The experimental procedure is based upon the functional heterologous expression of receptor mutants in eukaryotic cells followed by a two-step selection procedure. Positive selection for cells that express receptor variants is achieved by means of an agonistic antibody that mediates cell survival through receptor dimerization. An IL-4-coupled toxin is subsequently used to select against cells expressing wild-type receptors. Cells expressing mutant receptors that are unable to bind the cytotoxic ligand survive and can be amplified. The procedure allows the isolation of rare receptor variants from cell pools containing predominantly wild-type cells. This method, which should be equally applicable to similar receptor systems, was used to demonstrate the importance of a critical charged amino acid residue in the human IL-4 receptor alpha-subunit for IL-4-induced receptor activation.

Multiple binding sites in the growth factor receptor Xmrk mediate binding to p59fyn, GRB2 and Shc

Melanoma formation in Xiphoporus is initiated by overexpression of the EGFR-related receptor tyrosine kinase Xmrk (Xiphoporus melanoma receptor kinase). This receptor is activated in fish melanoma as well as in a melanoma-derived cell line (PSM) resulting in constitutive Xmrk-mediated mitogenic signaling. In order to define the underlying signaling pathway(s), triggered by the activated Xmrk receptor, we attempted to identify its physiological substrates. Examination of the Xmrk carboxyterminus for putative tyrosine autophosphorylation sites revealed the presence of potential binding motifs for GRB2 as well as for Shc. Binding of these adaptor proteins to the Xmrk receptor was detected in vitro and in cells expressing the mrk kinase. The GRB2 and Shc interactions with the receptor could be disrupted individually by phosphotyrosine peptides containing putative Xmrk autophosphorylation sites, indicating direct binding of both proteins. Recruitment of GRB2 by the constitutively activated Xmrk receptor led to strong MAP kinase activation in Xiphoporus melanoma cells. We also identified a high-affinity binding site for src-kinases (pYEDL) in the Xmrk carboxyterminus. Competition experiments with phosphopeptides comprising this site confirmed that it is used for high-affinity binding of Xiphoporus fyn (Xfyn) to Xmrk in melanoma cells. Thus, Xmrk can initiate different signaling pathways by using multiple substrate-binding sites to trigger proliferation of pigment cells.

Signalling by the oncogenic receptor tyrosine kinase Xmrk leads to activation of STAT5 in Xiphophorus melanoma

Overexpression of the mutationally activated Xmrk receptor initiates the formation of hereditary malignant melanoma in the fish Xiphophorus. In addition to transcriptional overexpression a cell-type specific signal transduction is essential for Xmrk mediated tumor formation. To elucidate the consequence of Xmrk signalling and to identify target proteins that characterize the tumor phenotype, we analysed proteins that are strongly tyrosine phosphorylated in the fish melanoma cell line PSM. One of the most prominent phosphotyrosine proteins was found to be the signal transducer and activator of transcription STAT5. In a heterologous cell system (murine pro B-cells), activation of the Xmrk kinase in a chimeric receptor induced tyrosine phosphorylation, nuclear translocation and DNA binding of STAT5. Following receptor stimulation, expression of the STAT5 specific target genes cis, osm and pim-1 was induced. In Xiphophorus PSM cells STAT5 was found to be preferentially localized in the nucleus, but treatment with tyrphostin AG555, a specific Xmrk kinase-inhibitor, blocked nuclear localization. In these cells as well as in Xiphophorus melanoma expression of pim-1 and constitutive DNA-binding activity of STAT5 was detectable. This constitutive activity was higher in malignant than in benign melanomas, indicating that STAT5 activation is correlated with the malignancy of these tumors.

Receptor tyrosine kinase Xmrk mediates proliferation in Xiphophorus melanoma cells

Over-expression of the oncogenic receptor tyrosine kinase (RTK) Xmrk is sufficient to induce formation of hereditary malignant melanoma in the fish Xiphophorus. In the melanoma tissue as well as in a melanoma-derived cell line (PSM), the Xmrk protein shows strong tyrosine phosphorylation, indicating either ligand-independent or autocrine activation of its kinase domain. However, it is unknown whether the constitutively activated Xmrk receptor itself directly triggers the proliferative signals, thus leading to uncontrolled growth of the pigment cells. In order to evaluate the role of Xmrk in proliferation of melanoma cells, we inhibited its kinase activity by using a Xmrk specific tyrphostin. At a concentration of 10 microM, tyrphostin AG555 led to a decrease of the Xmrk-induced DNA synthesis to 10% in NIH 3T3 Hm cells, whereas serum dependent 3H-thymidine incorporation was unaffected. In fish melanoma cells, the drug efficiently blocked DNA synthesis and cellular growth. This anti-proliferative activity correlated with the potency of AG555 to inhibit Xmrk autophosphorylation, indicating that the Xmrk receptor is the major determinant of mitogenic signaling in Xiphophorus melanoma cells.

Activation of the Xmrk proto-oncogene of Xiphophorus by overexpression and mutational alterations

Xmrk is a receptor tyrosine kinase closely related to the human EGF receptor. In the teleost fish Xiphophorus two versions of the Xmrk gene exist, an oncogene (ONC) and a proto-oncogene (INV). While ONC-Xmrk is the melanoma-inducing gene, INV-Xmrk appears not to be involved in transformation of pigment cells. To elucidate the mechanism that converts the proto-oncogene into a transforming oncogene a comparative analysis of the structure, expression and function of both versions of the gene was performed. In contrast to ONC-Xmrk which is expressed at high levels in melanoma cells, the proto-oncogene INV-Xmrk is ubiquitously expressed at very low levels indicating overexpression as one possible reason for tumorigenicity by ONC-Xmrk. As sequence comparison of the proto-oncogene and the oncogene revealed a number of amino acid changes, a possible effect of these mutations on the activation of the ONC-Xmrk receptor was determined. A constitutive activation of the oncogenic receptor was found and ectopic expression of INV-Xmrk after microinjection into medakafish embryos did not lead to the high tumour rate in transgenic fish as observed for the oncogene. Our data therefore suggest that overexpression of the receptor alone is not sufficient for melanoma induction, but that in addition activating mutations in ONC-Xmrk are responsible for its full tumorigenic potential.

Signal transduction by the oncogenic receptor tyrosine kinase Xmrk in melanoma formation of Xiphophorus

Melanoma formation in Xiphophorus is initiated by overexpression of an oncogenic version of the EGFR-related receptor tyrosine kinase Xmrk (Xiphophorus melanoma receptor kinase). High steady-state levels of Xmrk oncogene mRNA are found in malignant melanoma; however, this overabundance of transcripts appears to be not sufficient for manifestation of the full oncogenic potential of Xmrk. In addition, several amino acid exchanges cause the oncogenic receptor to be highly active, resulting in a strong tyrosine phosphorylation even without growth factor stimulation. Besides the receptor itself a Xmrk-specific signal transduction seems to be a critical part of the transformation machinery. Expression experiments in transgenic fish indicate that the Xmrk-mediated intracellular signalling is contributing to the cell-type specificity in development of hereditary melanoma in Xiphophorus.

Association between the melanoma-inducing receptor tyrosine kinase Xmrk and src family tyrosine kinases in Xiphophorus

Melanoma formation in the fish Xiphophorus is an in vivo model for the function of receptor tyrosine kinases (RTKs) in tumor development. The overexpression and high activity of the RTK Xmrk (Xiphophorus melanoma receptor kinase) is responsible for the formation of hereditary malignant melanoma in this fish, but the mechanism by which Xmrk signals cell proliferation has not been elucidated. Remarkably, in earlier experiments an elevated level of a pp60c-src related kinase activity was found in the melanomas. In order to evaluate the significance of src family SH2 domain interactions in the intracellular signalling of Xmrk, we determined its relative binding affinity to the ubiquitous general RTK substrate, PLC gamma, and to the Xiphophorus cytoplasmic kinases Xsrc, Xfyn and Xyes. Recombinant Xmrk purified from baculovirus infected Sf9 cells bound with high affinity to the SH2 domains of PLC gamma and Xfyn in vitro. The affinity of Xmrk to Xsrc and Xyes SH2 domains was 5- to 10-fold lower. Coprecipitation experiments revealed that the Xmrk/Xfyn interaction occurred also in melanoma cells. Moreover, stimulation of the Xmrk kinase activity was paralleled by an increase in Xfyn activity. These results suggest that in malignant melanoma of Xiphophorus the highly activated Xmrk may enhance the activity of Xfyn through direct interaction and that both kinases are linked in a signal transduction pathway.