Srcasm Corrects Fyn-induced Epidermal Hyperplasia by Kinase Down-regulation*

Src family tyrosine kinases (SFKs) are important regulators of epithelial cell growth and differentiation. Characterization of cellular mechanisms that regulate SFK activity will provide insights into the pathogenesis of diseases associated with increased SFK activity. Keratin 14-Fyn (K14) transgenic mice were derived to characterize the effect of Fyn on epidermal growth and differentiation in vivo. The epidermis of K14-Fyn mice is thickened, manifests prominent scale, and exhibits features consistent with hyperproliferation. Increased epidermal Fyn levels correlate with activation of p44/42 MAP kinases, STAT-3, and PDK-1, key signaling molecules that promote epithelial cell growth. The Src-activating and signaling molecule (Srcasm) is a substrate of SFKs that becomes tyrosine-phosphorylated downstream of the EGF receptor. In vitro, increased Srcasm levels promote activation of endogenous Fyn and keratinocyte differentiation. To study the in vivo effect of Srcasm upon Fyn, double transgenic lines were derived. K14-Fyn/Srcasm transgenic mice did not manifest the hyperproliferative phenotype. In contrast, K14-Fyn/Srcasm-P transgenic mice, which express a nonphosphorylatable Srcasm mutant, maintained the hyperproliferative phenotype. Resolution of the hyperproliferative phenotype correlated with reduced Fyn levels in vivo in three experimental systems: transgenic mice, primary keratinocytes, and cell lines. Biochemical studies revealed that Srcasm-dependent Fyn down-regulation requires Fyn kinase activity, phosphorylation of Srcasm, and the Srcasm GAT domain. Therefore, Srcasm is a novel regulator of Fyn promoting kinase down-regulation in a phosphorylation-dependent manner. Srcasm may act as a molecular “rheostat” for activated SFKs, and cellular levels of Srcasm may be important for regulating epithelial hyperproliferation associated with increased SFK activity.

Activation of protein tyrosine kinases is an important mechanism for promoting epithelial cell growth (1,2). Increased Src family tyrosine kinase (SFK) 2 activity is present in many human cancers, including colonic and breast carcinomas (3)(4)(5). Increased SFK activity in tumors could result from activating mutations and/or impairment of down-regulatory mechanisms. However, activating mutations of SFKs are rare in these carcinomas, raising the hypothesis that impaired down-regulation of activated SFKs could account for increased tumoral SFK activity (6,7). Therefore, characterization of negative regulatory mechanisms that target activated SFKs may provide insights into carcinogenesis associated with increased SFK activity.
In skin, neoplasia is associated with increased cellular proliferation, epidermal hyperplasia, and increased EGF receptor activity (8). Many signaling pathways that are persistently activated in cutaneous neoplasia are also stimulated in psoriasis, a cutaneous disorder also associated with T-cell inflammation, epidermal hyperplasia, and increased EGF receptor activity (9 -11). Given these observations, delineation of SFK-regulatory mechanisms in keratinocytes should provide insights into the pathogenesis of cutaneous neoplasia and psoriasis (12)(13)(14).
In vitro studies of keratinocytes from Fyn-deficient mice demonstrate abnormalities in differentiation, suggesting an important role for Fyn in differentiation (15)(16)(17). Increased Fyn expression in primary murine keratinocyte cultures promotes differentiation and withdrawal from the cell cycle (18). To evaluate the in vivo effects of increased Fyn expression, K14-Fyn transgenic mice were derived and characterized. K14-Fyn mice demonstrate a thickened, hyperplastic, and scaly epidermis dependent on increased Fyn expression. The K14-Fyn epidermis manifests activation of p44/42 MAP kinases, STAT-3, and PDK-1, molecules associated with keratinocyte growth (19 -22). In addition, keratin 6 expression was up-regulated consistent with a hyperproliferative phenotype.
Srcasm, the Src-activating and signaling molecule, is an SFK substrate that is tyrosine-phosphorylated secondary to EGF and transforming growth factor-␣ stimulation of primary human keratinocytes. Srcasm modulates p44/42 MAP kinase signaling in an EGF-dependent manner (14,23). Increased Srcasm levels activate endogenous Fyn, promote differentiation, and decrease the S-phase fraction of primary human keratinocytes, even after EGF stimulation. Srcasm levels are decreased in cutaneous squamous cell carcinoma and associated precursor lesions (14). These data suggest that Srcasm is an important regulator of SFKs in keratinocytes that promotes keratinocyte differentiation.
The in vivo relationship between Fyn and Srcasm was evaluated by generating double transgenic lines co-expressing Fyn with native Srcasm or Srcasm-P, a mutant lacking Fyn phosphorylation sites. K14-Fyn/Srcasm mice did not exhibit a hyperproliferative phenotype, whereas the K14-Fyn/Srcasm-P mice did. Increased Srcasm, but not Srcasm-P, expression in K14-Fyn/Srcasm mice correlated with decreased Fyn levels. Biochemical studies delineate a mechanism of Srcasm-dependent Fyn down-regulation that requires Fyn kinase activity, the Fyn phosphorylation sites of Srcasm, and the Srcasm GAT (GGA and Tom-1) domain. Srcasm efficiently down-regulates constitutively activated Fyn mutants but not kinase-inactive mutants. High levels of Srcasm also interfere with the downregulatory process, suggesting a biphasic relationship between activated SFKs and Srcasm. Therefore, Srcasm is a novel molecular "rheostat" for activated SFKs that limits cellular proliferation and promotes differentiation.

MATERIALS AND METHODS
Expression Vector Construction and Generation of Transgenic Mice-Murine HA-Srcasm and Fyn cDNAs were cloned into a human keratin 14 expression vector (23)(24)(25). The Srcasm-P mutant contained Tyr to Phe mutations at tyrosines 392, 440, 441, and 457. All K14 transgene cassettes were excised from the targeting vector and purified via Tris acetate-EDTAagarose electrophoresis. C57BL/6 ϫ CBA-fertilized oocytes were microinjected with the K14 transgene cassettes using standard protocols of the University of Pennsylvania Transgenic Core Facility.
Characterization of Transgenic Mice-Tail genomic DNA was isolated, and a genomic PCR was performed using the following primers: coding primer (K14 promoter), 5Ј-ATCTTG-AGAACTTCAGGG-3Ј; noncoding primer (Srcasm), 5Ј-GGT-GACCCACAGAGGTAG-3Ј. This strategy produced a 900-bp Srcasm transgene product. To detect Fyn transgenes, the primer pair of K14 promoter coding (5Ј-AAC GTG CTG GTT ATT GTG CTG-3Ј) and Fyn noncoding (5Ј-TTC CGT CCG TGC TTC ATA GT-3Ј was used to amplify a 400-bp product. Transgenic founders and littermate controls were crossed with C57BL/6 mice to generate lines. F1 transgenic progeny were crossed with C57BL/6 mice to maintain lines. The skin of mice was examined at least biweekly after birth; mice exhibiting severe runting were monitored daily. The phenotypes described were maintained through at least 7 generations. In the K14-Fyn A line, 368 mice were observed, with 74 of 203 genotype-positive mice expressing a positive phenotype; epidermal hyperkeratosis of less than 1 cm 2 was not considered a positive phenotype. The phenotype, as defined, exhibited incomplete penetrance and was expressed in 36% of K14-Fyn genotype-positive mice in both lines and all crosses. No epidermal hyperkeratosis was seen in 165 genotype-negative mice. In the K14-Fyn/Srcasm crosses, 148 mice were observed. One of 46 K14-Fyn/Srcasm mice exhibited a phenotype, whereas 13 of 32 K14-Fyn mice in these crosses had a phenotype. 43 K14-Srcasm and 27 littermate controls did not exhibit any epidermal hyperkeratosis. In the K14-Fyn/Srcasm-P crosses, 108 mice were observed. 15 of 34 K14-Fyn/Srcasm-P and 6 of 17 K14-Fyn mice maintained the phenotype. 35 K14-Srcasm-P and 20 littermate controls demonstrated no epidermal hyperplasia. All mice were handled in accord with University of Pennsylvania Institutional Animal Care and Use Committee Protocol 705452. A two-sided Fisher's exact test was used to determine the statistical significance of phenotype expression. Histologic and Immunohistochemical Analysis-Skin samples from the back, abdomen, ear, tail, and footpad were fixed overnight in 10% buffered formalin. Tissues were subjected to standard processing followed by paraffin embedding. Five-micron-thick sections were stained with hematoxylin and eosin. For immunohistochemistry, ear or back skin sections of 1-week-old mice were deparaffinized, rehydrated, blocked with 10% goat serum, and then incubated with the specified primary antibody, biotinylated secondary antibodies, streptavidinhorseradish peroxidase or alkaline phosphatase conjugate, and colorimetric substrates. Hematoxylin or Nuclear Fast Red was used for counter-staining. Sections were incubated with 3% H 2 O 2 solution to block endogenous peroxidase activity when streptavidin-horseradish peroxidase was used. Prior to BrdUrd or Ki-67 immunostaining, antigen retrieval was performed by boiling sections in 10 mM sodium citrate solution (pH 6.0) for 10 min. Photomicrographs were obtained using a Leica DC300 digital camera coupled to a Zeiss Axiophot microscope; all paired photos were obtained under identical conditions. The frequency of BrdUrd and Ki-67 staining in control and K14-Fyn mice was quantified by determining the percentage of positive nuclei. For BrdUrd staining: control, 12% positive nuclei, n ϭ 382; K14-Fyn, 21% positive nuclei, n ϭ 620. For Ki-67 staining: control, 52 positive nuclei/344 cells; K14-fyn, 431 positive nuclei/1544 cells. A Pearson Chi-squared test was applied to determine the statistical significance of positive staining percentages.
Primary Keratinocyte Culture and Adenoviral Infection-Primary cultures of murine keratinocytes were obtained from pups less than 3 days old. Isolated skin was subjected to trypsin digestion (0.25%, overnight at 4°C) and agitation to generate a cell suspension. Cells were plated on rat type I collagen-coated dishes, cultured in MCDB-153 medium with supplements (14), and maintained at 34°C in an atmosphere of 8% CO 2 . Keratinocytes were used at passage 3 or 4; cells at ϳ60 -70% confluence were infected with Ad-Fyn (murine Fyn) or control virus at the indicated m.o.i., and were analyzed 16 h after infection.
Immunoblotting-Cell and tissue lysates were prepared using a radioimmune precipitation assay buffer with protease and phosphatase inhibitors (14). Lysates were incubated on ice for 15 min and then cleared by centrifugation at 14,000 ϫ g for 10 min at 4°C. Supernatants were assayed for protein content using the MicroBCA protein assay kit (Pierce Chemical Co.). Aliquots of lysate were separated by SDS-PAGE and transferred to PolyScreen (PerkinElmer Life Sciences). Western blotting was conducted in a standard manner with the indicated antibodies and developed using an enhanced chemiluminescence kit as described by the manufacturer (Lumilight Plus, Roche Applied Science). Band densitometry was performed using a Canon LiDE 50 flatbed scanner and Scion image analysis software. Values are standardized to actin levels.
Quantitative RT-PCR for Fyn Transcript-Transfected cells were lysed in TRIzol reagent, and total RNA was isolated. Total RNA was subjected to reverse transcription using poly(dT). Equivalent amounts of cDNA were subjected to quantitative RT-PCR to detect Fyn transcript using Sybr Green technology on an MJ Research Opticon 2 system.

Phenotype of Fyn and Srcasm Transgenic
Mice-Two independent K14-Fyn founder mice exhibited epidermal scaling with hair loss on the snouts and hindquarters (Fig. 1a). K14-Fyn progeny exhibited diffuse epidermal scale at 2-3 days post-natal, which was prominent by 7 days (Fig. 1b). Approximately, 10% of the affected pups died by 1 week of age; such pups were runted and exhibited decreased feeding activity. Over a period of few weeks, diffuse scaling coalesced into prominent hyperkeratotic plaques lacking hair; these plaques usually resolved by 8 weeks (Fig. 1c). The scaling phenotype was observed through 7 generations and was not seen in transgene-negative mice (n ϭ 165). No spontaneous epidermal tumors were seen in 25 phenotype-positive, unstimulated mice carried to 6 months. Affected pups also exhibit runting (Con, littermate controls). c, localized hyperkeratotic plaques lacking hair were seen at 2 weeks. d, K14-Fyn and K14-Fyn/Srcasm-P mice maintain the thickened skin phenotype. K14-Srcasm, K14-Srcasm-P, and K14-Fyn/Srcasm mice lack the skin phenotype.
Histologic Analysis of K14-Fyn Mice-Epidermal sections from K14-Fyn mice demonstrated marked hyperplasia and hyperkeratosis (Fig. 2a). Keratinocytes in the K14-Fyn epidermis were larger and exhibited nuclear atypia (Fig. 2a, high  power). A mixed inflammatory infiltrate containing lymphocytes and neutrophils was present in the superficial dermis of K14-Fyn mice but not in control mice (Fig. 2a). Increased Ki-67 staining and BrdUrd labeling of keratinocytes in the K14-Fyn epidermis was consistent with hyperproliferation ( Fig. 2a). The differences in positive staining between K14-Fyn and control mice for Ki-67 (p Ͻ 0.001) and BrdUrd (p ϭ 0.001) staining were statistically significant (Fig. 2d).

Srcasm Corrects Fyn-induced Epidermal Hyperplasia
Subtle activation of SFKs and mildly increased Fyn levels could be detected via immunohistochemistry in the K14-Srcasm mice but not in the control or K14-Srcasm-P mice. These findings suggest that endogenous SFK levels coupled with supraphysiologic Srcasm levels may result in impaired down-regulation of activated SFKs. Such findings are supported by previous experiments in primary human keratinocytes (14). These data suggest that with endogenous Fyn levels, increased native Srcasm expression may increase levels of activated SFKs; however, with supraphysiologic levels of SFK activation, as in the K14-Fyn mice, increased Srcasm levels promote Fyn down-regulation. Immunohistochemical studies of the K14 transgenic lines were supported by Western blot analysis of epidermal lysates. Lysates from K14-Fyn mice demonstrated elevated levels of Fyn, activated SFKs, and keratin 6 compared with lysates from control, K14-Srcasm, and K14-Srcasm-P mice (Fig. 3b). Lysates from K14-Fyn/Srcasm mice contained lower levels of Fyn and activated SFKs than K14-Fyn mice; the levels of keratin 6 were reduced but remained higher than in control mice (Fig. 3b). In vivo, increased Srcasm expression is associated with decreased Fyn lev-els; however, increased expression of Srcasm-P does not promote Fyn down-regulation.
Srcasm-dependent Fyn down-regulation was studied in primary murine keratinocytes derived from neonatal control, K14-Srcasm, and K14-Srcasm-P transgenic mice. Infection of K14-Srcasm keratinocytes with Fyn adenovirus resulted in lower Fyn levels than those detected in parallel infections of control and K14-Srcasm-P keratinocytes (Fig. 4). Fyn levels in K14-Srcasm keratinocytes were restored to control levels by pretreatment with 10 mM NH 4 Cl for 60 min before lysis. Parallel experiments were attempted using primary keratinocyte cultures from K14-Fyn mice transduced with Srcasm or Srcasm-P adenoviruses. However, primary cultures from K14-Fyn mice were difficult to establish and maintain, given that increased Fyn activity in murine keratinocytes promotes cell cycle withdrawal (18). These experimental results suggest that the functional integrity of the endosomal/lysosomal pathway may influence Srcasm-dependent Fyn down-regulation.
STAT-3 and PDK-1 Activation Correlate with Fyn Levels-STAT-3 activation is associated with keratinocyte hyperplasia, murine epidermal tumors, human squamous cell carcinomas, and psoriasis (20,21,29). Therefore, the epidermis of the transgenic lines was evaluated for STAT-3 activation. Increased nuclear and plasma membrane staining for activated STAT-3 was identified in the K14-Fyn hyperplastic epidermis but not in littermate controls, linking increased Fyn levels with STAT-3 activation in keratinocytes (Fig. 5). Levels of activated STAT-3 decreased in K14-Fyn/ Srcasm mice, suggesting that Srcasm-induced Fyn down-regulation is associated with normalization of activated STAT-3 levels. Levels of nuclear and plasma membrane phospho-STAT-3 remained high in the skin of K14-Fyn/Srcasm-P mice.
Characterization of Srcasm-dependent Fyn Down-regulation-Transfection studies in COS cells were performed to characterize Srcasm-dependent Fyn down-regulation. Increasing Srcasm levels leads to a decrease in Fyn levels (Fig.  6a). However, as Srcasm levels continue to rise, Fyn levels rebound from a nadir, rise mildly, but do not reach control levels (Fig. 6a). The data from these experiments suggest a biphasic relationship between Fyn levels and Srcasm levels.
Co-transfection of Fyn with plasmids constituting a doxycycline-inducible Srcasm expression system demonstrated that small increases in Srcasm levels induced significant Fyn down-regulation; this expression system is "leaky," yielding subtle increases in Srcasm expression in the absence of exogenous doxycycline (Fig. 6b). At relatively high Srcasm levels, Fyn levels rebounded in a manner similar to that seen in Fig. 6a. These experiments demonstrate a temporal relationship between increased Srcasm expression and Fyn down-regulation.
Inhibition of Srcasm-dependent Fyn Down-regulation-COS cells pretreated with 10 mM ammonium chloride for 60 min before lysis did not exhibit Srcasm-dependent Fyn down-regulation, confirming findings seen in primary murine keratinocytes (Fig. 7a). Treatment of transfected COS cells with the lysosomal protease inhibitors E64 and leupeptin for up to 4 h prior to lysis did not appreciably alter Srcasmdependent Fyn down-regulation (Fig. 7a). Incubation of transfected COS cells with proteasomal inhibitors lactacystin, MG132, and ALLN for up to 4 h before lysis mildly inhibited Srcasm-dependent Fyn down-regulation. These data suggest that Srcasm-dependent Fyn downregulation requires an intact endosomal/lysosomal pathway and may require proteasomal function.
The ability of EGF to modulate Srcasm-dependent Fyn down-regulation was evaluated. EGF treatment alone did not  . Lysates subjected to SDS-PAGE and Western blot analysis to detect Fyn, Srcasm, and ␤-actin. n ϭ 3. b, inducible Srcasm expression promotes Fyn down-regulation. COS cells were transfected with Fyn plasmid and control vectors or CMV-rtTA and TRE (tetracyclineresponse element)-Srcasm plasmids. Some cells were incubated with doxycycline-containing (ng/ml) medium (Dox) for 6 h. Lysates were subjected to SDS-PAGE and Western blot analysis to detect Fyn, Srcasm, GFP, and ␤-actin. Data are representative of four experiments. c, biphasic relationship between Fyn and Srcasm levels. A densitometric analysis of the data in a, after standardization to ␤-actin signals, is represented graphically. appreciably effect Fyn down-regulation by Srcasm. However, EGF treatment of cells appeared to lessen the inhibitory effect of ammonium chloride on Fyn down-regulation (Fig. 7b). Therefore, EGF stimulation may promote Srcasm-dependent Fyn down-regulation under specific conditions.
Structure-Function Analysis of Srcasm-dependent Fyn Down-regulation-A structure-function analysis of Srcasm was performed to identify domains and residues required for Srcasmdependent Fyn down-regulation. The Srcasm GAT domain binds monoubiquitinated proteins, and this post-translational modification appears to be important for down-regulating EGF receptor and SFK signaling (32)(33)(34)(35). A mutant Srcasm lacking the GAT domain failed to induce Fyn down-regulation; in contrast, this mutant Srcasm appeared to interfere with Fyn down-regulation (Fig. 8a). Similarly, co-transfection of Srcasm-P with Fyn did not decrease Fyn levels but increased Fyn levels relative to controls. These data demonstrate that the Srcasm GAT domain and Fyn phosphorylation sites are required for Srcasm-dependent Fyn down-regulation. To exclude a potential transcriptional effect of Srcasm upon Fyn mRNA levels, quantitative RT-PCR for Fyn transcript was performed on mRNA isolated from all transfection conditions. Fyn transcript levels did not vary consistently or significantly across transfection conditions, and mRNA levels for glyceraldehyde-3-phosphate dehydrogenase also remained constant in all samples (Fig. 8b). Fyn transcript levels did not correlate with Fyn protein levels. Therefore, Srcasm-dependent Fyn down-regulation does not involve modulation of Fyn transcript levels.
The Role of Fyn Kinase Activity in Srcasm-dependent Fyn Downregulation-Because Fyn phosphorylation of Srcasm appears necessary for Srcasm-dependent Fyn down-regulation, the role of Fyn kinase activity in Srcasm-dependent Fyn down-regulation was evaluated. Co-transfection of activated Fyn Y528F with native Srcasm results in down-regulation of the activated kinase similar to that seen with native Fyn (Fig. 9a). Under similar conditions, Srcasm-P did not induce down-regulation of activated Fyn but promoted accumulation of the activated Fyn kinase. Native Srcasm does not efficiently down-regulate kinase-deficient Fyn K296R relative to native Fyn (Fig. 9b). These data demonstrate that efficient Srcasm-dependent Fyn down-regulation requires Fyn kinase activity and Srcasm phosphorylation by Fyn.

DISCUSSION
The K14-Fyn transgenic mouse is a novel model of epidermal hyperplasia that may provide insights into cutaneous disorders associated with keratinocyte hyperproliferation. Characterization of the dermal and epidermal inflammatory infiltrate will reveal if the K14-Fyn mouse mimics inflammatory disorders such as psoriasis. The hyperproliferative epidermis of the K14-Fyn mice demonstrates increased STAT-3 activation, a finding consistent with other models of psoriasis and cutaneous squamous cell carcinoma (20,21,29). Fyn appears to be an upstream activator of STAT-3 in keratinocytes, and phospho-STAT-3 may transmit some Fyn-initiated signals to the nucleus.
The epidermal hyperplasia present in K14-Fyn transgenic  mice requires elevated Fyn levels. As Fyn levels decrease with increasing Srcasm levels, the epidermal hyperplasia normalized. Spontaneous squamous cell carcinoma formation was not seen in 25 mice followed for 6 months; this cohort was not subjected to any procarcinogenic stimuli. Additional studies will determine whether K14-Fyn transgenic mice represent a model for characterizing cutaneous neoplasia. Increasing Srcasm levels decreases Fyn levels in three different experimental systems: double transgenic mice, adenoviral infection of primary keratinocytes, and transfection of cell lines. Phenotype maintenance in K14-Fyn/Srcasm-P mice supports the hypothesis that Srcasm-dependent Fyn down-regulation requires Fyn phosphorylation of Srcasm. Biochemical characterization of Srcasm-dependent Fyn down-regulation in COS cells demonstrates that Srcasm levels influence the degree of down-regulation. The biphasic relationship between Srcasm and Fyn levels suggests that Srcasm may associate with other molecules to promote degradation of Fyn; in such a model, high Srcasm levels may dilute the pool of these down-regulatory molecules thereby decreasing the efficiency of down-regulation. It will be interesting to determine whether molecules such as Tollip (Toll-interacting protein), TSG101, Cbl, or other monoubiquitinated proteins play a role in the down-regulation of activated SFKs (35)(36)(37).
Srcasm-dependent Fyn down-regulation in COS cells requires not only phosphorylation of Srcasm by Fyn but also a functional Srcasm GAT domain. Srcasm contains VHS and GAT domains, which are found in a number of proteins associated with endosomal trafficking (23,35,37,38). The Srcasm GAT domain binds to monoubiquitinated proteins and to Tollip in a mutually exclusive manner (35). Immunofluorescence studies in cell lines demonstrate that Fyn and Srcasm co-localize to the multivesicular body (37). Therefore, Srcasm lies at a signaling nexus involving monoubiquitinated proteins, growth factor/cytokine signaling, and the endosomal/lysosomal pathway (14,39,40). Given these characteristics, Srcasm appears to play an important role in terminating SFK-dependent signals downstream of growth factors and cytokine receptors (Fig. 10). In fact, previous data show that increased Srcasm levels promote keratinocyte differentiation and that cutaneous squamous cell carcinoma and related precursor lesions exhibit decreased Srcasm levels (14). Characterization of the cellular mechanisms associated with Srcasm down-regulation in carcinomas may provide new insights into how SFK activity is increased in carcinomas in the absence of activating mutations.
Ammonium chloride treatment reliably inhibited Srcasmdependent Fyn down-regulation; this treatment will raise lysosomal pH, thereby globally affecting lysosomal protease activity and promoting secretion of lysosomal proteases (41,42). In addition, cells exposed to ammonium chloride exhibit altered intracellular membrane trafficking and receptor complex disassembly within the endosomal-lysosomal pathway (43,44). The inability of E64 and leupeptin to alter Srcasm-dependent Fyn down-regulation may reflect incomplete inhibition of all lysosomal proteases. Proteasomal inhibitors mildly inhibited Srcasm-dependent Fyn down-regulation, suggesting that Srcasm may, in part, promote Fyn down-regulation through the proteasome. Current studies characterizing how Srcasm interacts with the intracellular membrane surfaces of the endosomal-lysosomal pathway to promote Fyn down-regulation should provide novel insights into SFK signal regulation.
Srcasm, but not Srcasm-P, down-regulates the activated Fyn Y528F kinase to levels similar to that seen with native Fyn. Relatively high Srcasm levels promote activation of endogenous SFKs (14) or down-regulate supraphysiologic levels of activated SFKs, thereby promoting keratinocyte differentiation. Persistent SFK signaling occurs with relatively low Srcasm levels leading to enhanced cell proliferation. The ratio of Srcasm to activated SFKs appears important for efficient kinase down-regulation.
These results parallel findings seen in Cbl-dependent Fyn down-regulation, where Fyn tyrosine kinase activity is critical in promoting kinase down-regulation (32,36). The relationship between Srcasm and Cbl in regulating the levels of Src family kinases will be important to explore (45,46).
The data presented support a novel mechanism for regulating activated SFKs through modulating Srcasm levels. The ratio of SFK-to-Srcasm appears important for determining whether there is increased or decreased kinase activity (Fig. 10). The ability of Srcasm to attenuate increased Fyn activity has profound effects upon keratinocyte growth and differentiation as manifested by the phenotypic variation of the various transgenic strains. Further work to delineate the molecular partners in Srcasm-dependent Fyn down-regulation should provide important insights into epithelial diseases and their biology.