Activation of the lutropin/choriogonadotropin receptor in MA-10 cells leads to the tyrosine phosphorylation of the focal adhesion kinase by a pathway that involves Src family kinases

Hesperetin Inhibits TGF-β1-Induced Migration and Invasion of Triple Negative Breast Cancer MDA-MB-231 Cells via Suppressing Fyn/Paxillin/RhoA Pathway

Triple-negative breast cancer is an aggressive subtype of breast cancer with poor clinical outcomes and poor prognosis. Hesperetin is an active component extracted from Citrus fruits and Traditional Chinese Medicine has a wide range of pharmacological effects. Here, we assessed the anti-migration and anti-invasive effects and explored inhibitory mechanisms of hesperetin on metastasis of human triple negative breast cancer MDA-MB-231 cells. Cell viability experiments revealed that 200 μM hesperetin has a clear inhibitory effect on MDA-MB-231 cells. TGF-β1 treatment induces apparent tumor progression in MDA-MB-231 cells including aberrant wound-healing and invasion ability, which is effectively suppressed by hesperetin co-treatment. Additionally, hesperetin inhibited the TGF-β1-mediated actin stress fiber formation. Western blot results showed that hesperetin suppressed the TGF-β1-mediated (i) activation of Fyn, (ii) phosphorylation of paxillin at Y31, Y88, and Y118 sites, (iii) the increased expression of RhoA, and (iv) activation of Rho-kinase. We demonstrated the increased interaction of Fyn with paxillin and RhoA protein in the TGF-β1-induced metastasis of MDA-MB-231 cells. Small interfering RNA Fyn inhibited phosphorylation of paxillin (Y31) and activation of Rho-kinase induced by TGF-β1. In conclusion, hesperetin has a significant inhibitory effect on migration and invasion of MDA-MB-231 cells induced by TGF-β1, which might be attributed to inhibiting the Fyn/paxillin/RhoA pathway.

Notch signaling represses GATA4-induced expression of genes involved in steroid biosynthesis

Notch2 and Notch3 and genes of the Notch signaling network are dynamically expressed in developing follicles, where they are essential for granulosa cell proliferation and meiotic maturation. Notch receptors, ligands, and downstream effector genes are also expressed in testicular Leydig cells, predicting a potential role in regulating steroidogenesis. In this study, we sought to determine if Notch signaling in small follicles regulates the proliferation response of granulosa cells to FSH and represses the up-regulation steroidogenic gene expression that occurs in response to FSH as the follicle grows. Inhibition of Notch signaling in small preantral follicles led to the up-regulation of the expression of genes in the steroid biosynthetic pathway. Similarly, progesterone secretion by MA-10 Leydig cells was significantly inhibited by constitutively active Notch. Together, these data indicated that Notch signaling inhibits steroidogenesis. GATA4 has been shown to be a positive regulator of steroidogenic genes, including STAR protein, P450 aromatase, and 3B-hydroxysteroid dehydrogenase. We observed that Notch downstream effectors HEY1, HEY2, and HEYL are able to differentially regulate these GATA4-dependent promoters. These data are supported by the presence of HEY/HES binding sites in these promoters. These studies indicate that Notch signaling has a role in the complex regulation of the steroidogenic pathway.

Expression of focal adhesion kinase in mouse cumulus-oocyte complexes, and effect of phosphorylation at Tyr397 on cumulus expansion

We investigated the expression of focal adhesion kinase (FAK) in mouse cumulus-oocyte complexes (COCs), as well as the role of FAK phosphorylation at Tyr397 during oocyte maturation. The effect of inhibiting FAK phosphorylation at Tyr397 during in vitro maturation (IVM) on subsequent fertilization and preimplantation embryo development was also examined. Western blotting analyses revealed that total and Tyr397-phosphorylated FAK were expressed in vivo in both cumulus cells and oocytes. Immunocytochemical studies localized this kinase throughout the cytoplasm of cumulus cells and oocytes; in particular, Tyr397-phosphorylated FAK tended to accumulate in regions where cumulus cells contact each other. Interestingly, the in vivo level of Tyr397 phosphorylation in cumulus cells was significantly lower after compared to before cumulus expansion. Addition of FAK inhibitor 14, which specifically blocks phosphorylation at Tyr397, stimulated oocyte meiotic maturation and cumulus expansion during IVM in the absence of follicle-stimulating hormone (FSH). Reverse-transcriptase PCR showed that the mRNA expression of hyaluronan synthase 2 (Has2), a marker of cumulus expansion, was significantly induced in cumulus cells. Subsequent in vitro fertilization and culture showed that more oocytes developed to the blastocyst stage when they were treated with FAK inhibitor 14 during IVM, although the blastocyst total cell number was lower than in oocytes stimulated with FSH. These results indicate that FAK is involved in the maturation of COCs; specifically, phosphorylation at Tyr397 may regulate cumulus expansion via the expression of Has2 mRNA in cumulus cells, which could affect the developmental competence of oocytes.

Involvement of Src family of kinases and cAMP phosphodiesterase in the luteinizing hormone/chorionic gonadotropin receptor-mediated signaling in the corpus luteum of monkey

BACKGROUND

In higher primates, during non-pregnant cycles, it is indisputable that circulating LH is essential for maintenance of corpus luteum (CL) function. On the other hand, during pregnancy, CL function gets rescued by the LH analogue, chorionic gonadotropin (CG). The molecular mechanisms involved in the control of luteal function during spontaneous luteolysis and rescue processes are not completely understood. Emerging evidence suggests that LH/CGR activation triggers proliferation and transformation of target cells by various signaling molecules as evident from studies demonstrating participation of Src family of tyrosine kinases (SFKs) and MAP kinases in hCG-mediated actions in Leydig cells. Since circulating LH concentration does not vary during luteal regression, it was hypothesized that decreased responsiveness of luteal cells to LH might occur due to changes in LH/CGR expression dynamics, modulation of SFKs or interference with steroid biosynthesis.

METHODS

Since, maintenance of structure and function of CL is dependent on the presence of functional LH/CGR its expression dynamics as well as mRNA and protein expressions of SFKs were determined throughout the luteal phase. Employing well characterized luteolysis and CL rescue animal models, activities of SFKs, cAMP phosphodiesterase (cAMP-PDE) and expression of SR-B1 (a membrane receptor associated with trafficking of cholesterol ester) were examined. Also, studies were carried out to investigate the mechanisms responsible for decline in progesterone biosynthesis in CL during the latter part of the non-pregnant cycle.

RESULTS AND DISCUSSION

The decreased responsiveness of CL to LH during late luteal phase could not be accounted for by changes in LH/CGR mRNA levels, its transcript variants or protein. Results obtained employing model systems depicting different functional states of CL revealed increased activity of SFKs [pSrc (Y-416)] and PDE as well as decreased expression of SR-B1 correlating with initiation of spontaneous luteolysis. However, CG, by virtue of its heroic efforts, perhaps by inhibition of SFKs and PDE activation, prevents CL from undergoing regression during pregnancy.

CONCLUSIONS

The results indicated participation of activated Src and increased activity of cAMP-PDE in the control of luteal function in vivo. That the exogenous hCG treatment caused decreased activation of Src and cAMP-PDE activity with increased circulating progesterone might explain the transient CL rescue that occurs during early pregnancy.

Reactive oxygen species (ROS) play a critical role in the cAMP-induced activation of Ras and the phosphorylation of ERK1/2 in Leydig cells

Activation of the LH receptor (LHR) in Leydig cells results in the phosphorylation of ERK1/2 by cAMP-dependent and cAMP-independent pathways. Here we examine the mechanisms by which cAMP stimulates ERK1/2 phosphorylation. We show that the stimulation of steroidogenesis is not necessary or sufficient to stimulate the phosphorylation of ERK1/2 but that other cAMP-dependent mitochondrial functions are involved. Using MA-10 cells as a model, we showed that cAMP analogs increase reactive oxygen species (ROS) formation and that an uncoupler of oxidative phosphorylation and a ROS scavenger prevent this increase. These two compounds also inhibit the increase in ERK1/2 phosphorylation provoked by cAMP analogs, thus suggesting that the cAMP-induced phosphorylation of ERK1/2 is mediated by mitochondrial ROS. In agreement with this hypothesis we also show that a reduction in glutathione levels, which alters the redox state of MA-10 cells, potentiates the effect of cAMP on ERK1/2 phosphorylation. Measurements of the dephosphorylation of ERK and the activation of Ras showed that the ROS scavenger prevents the cAMP-provoked activation of Ras and that cAMP, with or without a ROS scavenger, has little or no effect on the dephosphorylation of ERK. Lastly, we show that the uncoupler of oxidative phosphorylation and the ROS scavenger also prevent the ability of cAMP analogs to increase ERK1/2 phosphorylation in primary cultures of mouse Leydig cells. We conclude that, in Leydig cells, cAMP enhances the phosphorylation of ERK1/2 via a mitochondria-derived, ROS-dependent activation of Ras.

Activation of the lutropin/choriogonadotropin receptor inhibits apoptosis of immature Leydig cells in primary culture

We used proliferating primary cultures of immature rat Leydig cells expressing the recombinant human LH/choriogonadotropin (CG) receptor (LHR) to test the hypothesis that activation of this receptor inhibits apoptosis. We also compared the effects of LH/CG with epidermal growth factor (EGF) and IGF-I because these have been previously shown to stimulate proliferation and/or inhibit apoptosis in Leydig cells. Human CG (hCG), EGF, and IGF-I stimulated the phosphorylation of ERK1/2 and Akt in primary cultures of immature rat Leydig cells. These three hormones also robustly stimulated thymidine incorporation and inhibited drug-induced apoptosis. Using selective inhibitors of ERK1/2 (UO126) or Akt phosphorylation (LY294002), we show that the ERK1/2 and Akt cascades are both involved in the hCG- and EGF-dependent proliferation of Leydig cells, but only the ERK1/2 cascade is involved in their antiapoptotic actions. The same strategy showed that the proliferative and antiapoptotic actions of IGF-I are mediated entirely by the Akt pathway. These results show that activation of the LHR inhibits apoptosis in Leydig cells and that it does so through stimulation of the ERK1/2 pathway.

Dax-1 and steroid receptor RNA activator (SRA) function as transcriptional coactivators for steroidogenic factor 1 in steroidogenesis

The nuclear receptor steroidogenic factor 1 (SF-1) is essential for adrenal development and steroidogenesis. The atypical orphan nuclear receptor Dax-1 binds to SF-1 and represses SF-1 target genes. Paradoxically, however, loss-of-function mutations of Dax-1 also cause adrenal hypoplasia, suggesting that Dax-1 may function as an SF-1 coactivator under some circumstances. Indeed, we found that Dax-1 can function as a dosage-dependent SF-1 coactivator. Both SF-1 and Dax-1 bind to steroid receptor RNA activator (SRA), a coactivator that functions as an RNA. The coactivator TIF2 also associates with Dax-1 and synergistically coactivates SF-1 target gene transcription. A naturally occurring Dax-1 mutation inhibits this transactivation, and the mutant Dax-1-TIF2 complex mislocalizes in living cells. Coactivation by Dax-1 is abolished by SRA knockdown. The expression of the steroidogenic gene products steroidogenic acute regulatory protein (StAR) and melanocortin 2 receptor is reduced in adrenal Y1 cells following the knockdown of endogenous SRA. Similarly, the knockdown of endogenous Dax-1 downregulates the expression of the steroidogenic gene products CYP11A1 and StAR in both H295R adrenal and MA-10 Leydig cells. These findings reveal novel functions of SRA and Dax-1 in steroidogenesis and adrenal biology.

The luteinizing hormone receptor-activated extracellularly regulated kinase-1/2 cascade stimulates epiregulin release from granulosa cells

We examine the pathways involved in the luteinizing hormone receptor (LHR)-dependent activation of the epidermal growth factor (EGF) network using cocultures of LHR-positive granulosa cells and LHR-negative test cells expressing an EGF receptor (EGFR)-green fluorescent protein fusion protein. Activation of the LHR in granulosa cells results in the release of EGF-like growth factors that are detected by measuring the phosphorylation of the EGFR-green fluorescent protein expressed only in the LHR-negative test cells. Using neutralizing antibodies and real-time PCR, we identified epiregulin as the main EGF-like growth factor produced upon activation of the LHR expressed in immature rat granulosa cells, and we show that exclusive inhibition or activation of the ERK1/2 cascade in granulosa cells prevents or enhances epiregulin release, respectively, with little or no effect on epiregulin expression. These results show that the LHR-stimulated ERK1/2 pathway stimulates epiregulin release.

FYN is overexpressed in human prostate cancer

OBJECTIVE

To test the hypothesis that FYN, a member of the SRC family of kinases (SFKs), is up-regulated in prostate cancer, as FYN is functionally distinct from other SFKs, and interacts with FAK and paxillin (PXN), regulators of cell morphology and motility.

MATERIALS AND METHODS

Through data-mining in Oncomine (http://www.oncomine.org), cell-line profiling with immunoblotting, quantitative reverse transcription and polymerase chain reaction (RT-PCR) and immunohistochemical analysis, we described FYN expression in prostate cancer. The analysis included 32 cases of prostate cancer, nine of prostatic intraepithelial neoplasia (PIN) and 19 normal prostates. Samples were scored for the percentage of stained glands and intensity of staining (from 0 to 3). Each sample was assigned a composite score generated by multiplying percentage and intensity.

RESULTS

Data-mining showed an eight times greater FYN expression in prostate cancer than in normal tissue; this was specific to FYN and not present for other SFKs. Expression of FYN in prostate cancer cell lines (LNCaP, 22Rv1, PC3, DuPro) was detected using quantitative RT-PCR and immunoblotting. Expression of FYN and its signalling partners FAK and PXN was detected in human tissue. Comparing normal with cancer samples, there was a 2.1-fold increase in median composite score for FYN (P < 0.001) 1.7-fold increase in FAK (P < 0.001), and a doubling in PXN (P < 0.05). There was a 1.7-fold increase in FYN (P < 0.05) and a 1.6-fold increase in FAK (P < 0.01) in cancer compared with PIN.

CONCLUSIONS

These studies support the hypothesis that FYN and its related signalling partners are up-regulated in prostate cancer, and support further investigation into the role of the FYN as a therapeutic target.

Arrestin-3 is essential for the activation of Fyn by the luteinizing hormone receptor (LHR) in MA-10 cells

Recent studies showed that Fyn is a mediator of the LHR-induced activation of the ERK1/2 cascade in MA-10 cells. Since the LHR is a G protein-coupled receptor and the Src family of kinases can be activated by some Galpha subunits and by the non-visual arrestins we investigated the role of these signaling molecules in the LHR-provoked activation of Fyn. Small interfering RNAs (siRNAs) that target two Galpha subunits that participate in LHR signaling (Galpha(s) and Galpha(11)) and one that targets arrestin-3 were co-transfected with the hLHR in MA-10 cells. We then determined the effects of these siRNAs on the LHR-provoked activation of Fyn, the phosphorylation of FAK (a prominent Fyn substrate) and the release of EGF-like growth factors (a Fyn-mediated process). Expression of the siRNA against Galpha(s) decreased the level of Galpha(s) and LHR-stimulated cAMP production by approximately 50% but did not affect LHR-stimulated Fyn activation or FAK phosphorylation. Likewise, expression of the siRNA against Galpha(11) decreased the level of Galpha(11) and LHR-stimulated inositol phosphate production by approximately 50% but did not affect LHR-stimulated Fyn activation or FAK phosphorylation. Expression of the siRNA against arrestin-3 decreased the level of arrestin-3 and the rate of internalization of hCG by approximately 50% and it also inhibited the LHR-provoked stimulation of Fyn, the phosphorylation of FAK and the release of EGF-like growth factors. These results show that, in MA-10 cells, the hLHR activates Fyn through an arrestin-3-dependent pathway and that this pathway is a mediator of the hLHR-provoked release of EGF-like growth factors.

Mutations of the lutropin/choriogonadotropin receptor that do not activate the phosphoinositide cascade allow hCG to induce aromatase expression in immature rat granulosa cells

Using primary cultures of immature rat granulosa cells and adenoviral infections we expressed two mutants of the human lutropin receptor (hLHR) that do not activate the phosphoinositide cascade. One mutant (hLFF) has the extracellular domain of the hLHR and the transmembrane and intracellular domains of the hFSHR. The other (hLHR-L457D) has a leucine to aspartate mutation in residue 457 of transmembrane helix 3. When expressed in immature rat granulosa cells the hLHR stimulates cAMP and inositol phosphate accumulation, transactivates the epidermal growth factor receptor (EGFR), elicits a transient increase in Akt phosphorylation, and a sustained increase in ERK1/2 phosphorylation but aromatase expression is not enhanced. When expressed at comparable densities, hLFF and hLHR-L457D support cAMP accumulation and transient Akt phosphorylation but do not support inositol phosphate accumulation, EGFR transactivation or a sustained phosphorylation of ERK1/2. Cells expressing either of these two mutants respond to hCG with increased aromatase expression. We also show that addition of hCG to cells expressing the hLHR antagonizes the effects of hFSH on aromatase expression whereas addition of hCG to cells expressing the hLHR-L457D mutant does not. These results show that activation of the phosphoinositide cascade is upstream of EGFR transactivation and ERK1/2 phosphorylation and that this pathway is a negative regulator of aromatase expression in granulosa cells.

Hormone-induced expression of tumor necrosis factor alpha-converting enzyme/A disintegrin and metalloprotease-17 impacts porcine cumulus cell oocyte complex expansion and meiotic maturation via ligand activation of the epidermal growth factor receptor

The epidermal growth factor (EGF)-like growth factors, amphiregulin (AREG) and epiregulin (EREG), are expressed in murine cumulus oocyte complexes (COCs) where they impact the function of cumulus cells and oocyte maturation during LH-mediated ovulation. Because TNFalpha-converting enzyme (TACE)/a disintegrin and metalloprotease-17 (ADAM17) is essential for ectodomain shedding of AREG and EREG from the surface of other cell types, the expression and function of TACE/ADAM17 was analyzed in a porcine COC culture system in which FSH- and LH-mediated expansion and oocyte meiotic maturation have been well characterized and shown to occur between 20 and 40 h. In this model, Areg, Ereg, and Tace/Adam17 mRNAs increased significantly with maximal levels observed between 5 and 20 h of culture with FSH plus LH. TACE/ADAM17 protein and protease activity were up-regulated markedly at 10 h and maintained to 40 h. Treatment of COCs with the TACE/ADAM17-selective inhibitor TNFalpha-processing inhibitor-2 (TAPI-2) significantly suppressed in a time-dependent manner downstream targets of EGF receptor activation such as ERK1/2 phosphorylation, Ptgs2, Has2, and Tnfaip6 mRNA expression, hormone-induced COC expansion, and meiotic maturation of the oocytes. Addition of EGF to COCs cultured in the presence of FSH/LH reversed the inhibitory effects of TAPI-2 on these ovulation-related processes. Gonadotropin-induced phosphorylation of ERK1/2 was also inhibited in rat granulosa cells treated with TAPI-2 or after transfection with Tace/Adam17 small interfering RNA. Induced expression of Tnfaip6 mRNA was also reduced by Tace/Adam17 small interfering RNA. Thus, TACE/ADAM17 is induced and the activity is involved in porcine COC expansion as well as oocyte meiotic maturation through the activation of EGF receptor in cumulus cells.

Oncostatin-M inhibits luteinizing hormone stimulated Leydig cell progenitor formation in vitro

BACKGROUND

The initial steps of stem Leydig cell differentiation into steroid producing progenitor cells are thought to take place independent of luteinizing hormone (LH), under the influence of locally produced factors such as leukaemia inhibitory factor (LIF), platelet derived growth factor A and stem cell factor. For the formation of a normal sized Leydig cell population in the adult testis, the presence of LH appears to be essential. Oncostatin M (OSM) is a multifunctional cytokine and member of the interleukin (IL)-6 family that also includes other cytokines such as LIF. In the rat OSM is highly expressed in the late fetal and neonatal testis, and may thus be a candidate factor involved in Leydig cell progenitor formation.

METHODS

Interstitial cells were isolated from 13-day-old rat testes and cultured for 1, 3 or 8 days in the presence of different doses of OSM (range: 0.01 to 10 ng/ml) alone or in combination with LH (1 ng/ml). The effects of OSM and LH on cell proliferation were determined by incubating the cultures with [3H]thymidine or bromodeoxyuridine (BrdU). Developing progenitor cells were identified histochemically by the presence of the marker enzyme 3beta-hydroxysteroid dehydrogenase (3beta-HSD).

RESULTS

OSM, when added at a dose of 10 ng/ml, caused a nearly 2-fold increase in the percentage of Leydig cell progenitors after 8 days of culture. Immunohistochemical double labelling experiments with 3beta-HSD and BrdU antibodies showed that this increase was the result of differentiation of stem Leydig cells/precursor cells and not caused by proliferation of progenitor cells themselves. The addition of LH to the cultures consistently resulted in an increase in progenitor formation throughout the culture period. Surprisingly, when OSM and LH were added together, the LH induced rise in progenitor cells was significantly inhibited after 3 and 8 days of culture.

CONCLUSION

Taken together, the results of the present study suggest that locally produced OSM may not only play a role in the regulation of Sertoli cell proliferation and the initiation of spermatogenesis but may also play a role in the regulation of Leydig cell progenitor formation by keeping the augmenting effects of LH on this process in abeyance.

A co-culture system reveals the involvement of intercellular pathways as mediators of the lutropin receptor (LHR)-stimulated ERK1/2 phosphorylation in Leydig cells

Co-cultures of lutropin receptor (LHR) positive and negative Leydig cells were used to test the hypothesis that the LHR provokes phosphorylation of the extracellular regulated kinases (ERK1/2) using intracellular and intercellular pathways. Addition of hCG to MA-10 cells (LHR positive) stimulates phosphorylation of the EGF receptor (EGFR) and ERK1/2 whereas addition of hCG to I-10 cells (LHR negative) does not. Addition of hCG to co-cultures of MA-10 and I-10 cells rapidly stimulates the phosphorylation of the EGFR and ERK1/2 in I-10 cells, however. Transfection of interfering constructs shows that the LHR-mediated activation of Fyn in MA-10 cells is necessary for the phosphorylation of the EGFR and ERK1/2 in I-10 cells. This pathway can also be demonstrated in MA-10 cells but the phosphorylation of ERK1/2 in MA-10 cells also involves a second pathway mediated by protein kinase A (PKA). We propose that the LHR-mediated stimulation of the ERK1/2 cascade in Leydig cells depends on two independent pathways. One is intracellular and is mediated by PKA. The second is mediated by Fyn and it involves the release of soluble factors that act to phosphorylate the EGFR in an autocrine/paracrine fashion.

Lutropin/choriogonadotropin stimulate the proliferation of primary cultures of rat Leydig cells through a pathway that involves activation of the extracellularly regulated kinase 1/2 cascade

Primary cultures of progenitor and immature rat Leydig cells were established from the testes of 21- and 35-d-old rats, respectively. The cell population remained homogeneous after 4-6 d in culture as judged by staining for 3beta-hydroxysteroid dehydrogenase, but the cells were unable to bind 125I-human chorionic gonadotropin (hCG) or to respond to hCG with classical LH receptor (LHR)-mediated responses, including cAMP and inositol phosphate accumulation, steroid biosynthesis, or the phosphorylation of ERK1/2. Infection of primary cultures with recombinant adenovirus coding for beta-galactosidase showed that approximately 65% of the cells are infected. Infection with adenovirus coding for the human LHR (hLHR) allowed for expression of the hLHR at a density of approximately 25,000 receptors per cell and allowed the cells to respond to hCG with increases in cAMP and inositol phosphate accumulation, steroid biosynthesis, and the phosphorylation of ERK1/2. Although progenitor and immature cells were able to respond to hCG with an increase in progesterone, only the immature cells responded with an increase in testosterone. In addition to these classical LHR-mediated responses, the primary cultures of progenitor or immature rat Leydig cells expressing the recombinant hLHR proliferated robustly when incubated with hCG, and this proliferative response was sensitive to an inhibitor of ERK1/2 phosphorylation. These studies establish a novel experimental paradigm that can be used to study the proliferative response of Leydig cells to LH/CG. We conclude that activation of the LHR-provoked Leydig cell proliferation requires activation of the ERK1/2 cascade.

Activation of the lutropin/choriogonadotropin receptor in MA-10 cells stimulates tyrosine kinase cascades that activate ras and the extracellular signal regulated kinases (ERK1/2)

We show that activation of the recombinant lutropin/choriogonadotropin receptor (LHR) in mouse Leydig tumor cells (MA-10 cells) leads to the tyrosine phosphorylation of Shc (Src homology and collagen homology) and the formation of complexes containing Shc and Sos (Son of sevenless), a guanine nucleotide exchange factor for Ras. Because a dominant-negative mutant of Shc inhibits the LHR-mediated activation of Ras and the phosphorylation of ERK1/2, we conclude that the LHR-mediated phosphorylation of ERK1/2 is mediated, at least partially, by the classical pathway used by growth factor receptors. We also show that the endogenous epidermal growth factor receptor (EGFR) present in MA-10 cells is phosphorylated upon activation of the LHR. The LHR-mediated phosphorylation of the EGFR and Shc, the activation of Ras, and the phosphorylation of ERK1/2 are inhibited by expression of a dominant-negative mutant of Fyn, a member of the Src family kinases (SFKs) expressed in MA-10 cells and by PP2, a pharmacological inhibitor of the SFKs. These are also inhibited, but to a lesser extent, by AG1478, an inhibitor of the EGFR kinase. We conclude that the SFKs are responsible for the LHR-mediated phosphorylation of the EGFR and Shc, the formation of complexes containing Shc and Sos, the activation of Ras, and the phosphorylation of ERK1/2.