Transduction pathways involved in rapid hormone receptor regulation in the mammary epithelium

Franklyn F. Bolander Jr.

Department of Biological Sciences, University of South Carolina, Columbia, South Carolina 29208

    ABSTRACT
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Abstract
Introduction
Procedures
Results
Discussion
References

Previous studies have shown that the envelope protein of the mouse mammary tumor virus (MMTV) rapidly upregulates prolactin (PRL) receptors by shifting them from internal pools to the cell surface and downregulates epidermal growth factor (EGF) receptors by inducing their internalization and degradation. This study shows that the effect on PRL receptors is mediated by the nitric oxide (NO)/cGMP pathway, since it can be mimicked by an NO donor or 8-bromo-cGMP and can be blocked by an NO synthase inhibitor. In contrast, the effect on EGF receptors is mediated by tyrosine phosphorylation and phosphatidylinositol 3-kinase (PI3K), since it can be blocked by either a tyrosine kinase inhibitor or by a PI3K inhibitor. Both of these pathways can be activated by a calcium ionophore and inhibited by calcium chelation. Therefore, it appears that the mouse mammary tumor virus envelope protein, like other retroviral envelope proteins, initially elevates cytoplasmic calcium, which can then stimulate both the NO/cGMP and the tyrosine phosphorylation/PI3K pathways, leading to PRL receptor upregulation and EGF receptor downregulation, respectively.

nitric oxide; cGMP; phosphatidylinositol 3-kinase; epidermal growth factor receptor; prolactin receptor

    INTRODUCTION
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Abstract
Introduction
Procedures
Results
Discussion
References

THIS LABORATORY has previously shown that, in mammary epithelium, the envelope protein (gp52) of the mouse mammary tumor virus (MMTV) can rapidly elevate prolactin (PRL) receptors by recruiting them from internal pools (7) and lower epidermal growth factor (EGF) receptors by inducing their internalization and destruction (5). These actions are presumed to facilitate viral propagation, since the pups are infected by MMTV in the milk, a product of the differentiated mammary gland. By upregulation of receptors for PRL, a differentiative hormone, and downregulation of those for EGF, a growth/anti-differentiative hormone, the MMTV can enhance mammary gland differentiation, milk production, and its own propagation. This work attempts to determine which transduction pathways are being used to effect these changes in receptor levels.

Several molecular mechanisms have been associated with receptor regulation. For example, the most intensive investigations in the field of receptor cycling have been done on the receptor tyrosine kinases, in which it has been shown that the EGF, insulin, insulin-like growth factor II (IGF-II), platelet-derived growth factor, and stem cell factor receptors all require tyrosine phosphorylation for receptor processing (15, 17, 24, 32, 33, 35). Although the PRL receptor is not a tyrosine kinase, it is closely associated with soluble tyrosine kinases (11) and therefore may be subject to a similar control. As a result, this pathway is a potential mediator of the actions of MMTV. The role of this phosphorylation can be examined by the use of genistein, a tyrosine kinase inhibitor, and pervanadate, a phosphotyrosine phosphatase inhibitor that would allow tyrosine phosphorylation to accumulate.

Another second messenger that has been implicated in receptor processing is phosphatidylinositol 3-kinase (PI3K), which is believed to be involved in vesicular trafficking. PI3K is required for the redistribution of the IGF-II, platelet-derived growth factor, and possibly the stem cell factor receptor (12, 15, 17, 35). However, it is not involved in the EGF-induced downregulation of its own receptor (32). The participation of PI3K in the MMTV-induced effects can be tested with wortmannin, a PI3K inhibitor.

Although the nitric oxide (NO)/cGMP pathway has not yet been associated with receptor relocation, previous studies have demonstrated that it can be activated by the envelope protein of MMTV (4) as well as by several other retroviral envelope proteins (8, 27). Therefore, this transduction system was also investigated. NO synthase (NOS), a calcium-dependent enzyme, can be inhibited by calcium depletion or by substrate antagonists; the pathway can be activated by calcium ionophores or NO donors, for example sodium nitroprusside (SNP). Finally, one of the major effectors for NO is cGMP, the synthesis of which is stimulated by NO; the role of this nucleotide can be examined employing any of several cGMP agonists.

With the use of these pharmacological tools, the contribution of these various transducing pathways to the effects of MMTV on PRL and EGF receptors can be evaluated.

    EXPERIMENTAL PROCEDURES
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Abstract
Introduction
Procedures
Results
Discussion
References

Materials. Mouse EGF (lot 908374) was purchased from Collaborative Biomedical Products (Bedford, MA), and ovine PRL (oPRL-19) was kindly provided by the Hormone Distribution Program (National Institute of Diabetes and Digestive and Kidney Diseases, Bethesda, MD). HEPES, genistein, wortmannin, 8-bromo-cGMP, A-23187, EGTA, sodium orthovanadate, hydrogen peroxide, catalase, lactoperoxidase, and BSA were obtained from Sigma Chemical (St. Louis, MO). NG-monomethyl-L-arginine (L-NMMA) and SNP were purchased from Alexis (San Diego, CA). Medium 199 with Earle's salts was from GIBCO (Grand Island, NY), and collagenase type I (179 U/mg) was obtained from Worthington Biochemicals (Freehold, NJ). Na125I (carrier free) was purchased from NEN (Boston, MA).

The MMTV envelope protein was prepared in the author's laboratory by the method of Marcus et al. (20); milk from C3H/HeN MMTV+ mice was used as the source for the virus. Pervanadate was prepared from orthovanadate and hydrogen peroxide according to the procedure of Pumiglia et al. (23) and used immediately.

Organ culture. Virgin mice (C3H/HeN MMTV+ and MMTV-) were obtained from the Frederick Cancer Research Facility (Frederick, MD). The mice (MMTV-) were killed by cervical dislocation, and explants were prepared from the fourth pair of mammary glands under sterile conditions, as previously described (16). Explants were cultured on siliconized lens paper in medium 199 containing 20 mM HEPES (pH 7.6) and combinations of the following reagents, as required by the individual experiment: SNP (10 µM), 8-bromo-cGMP (10 µM), L-NMMA (100 µM), genistein (50 µM), pervanadate (100 µM), wortmannin (100 nM), A-23187 (32.5 nM), EGTA (3 mM), and/or the MMTV envelope protein (1 µg/ml). The tissue was incubated under air at 37°C.

The concentrations of the above reagents were determined empirically using mammary epithelium cultured for 30 min, as described above. Under these conditions, 32.5 nM A-23187 increased 45Ca2+ fluxes 8.5-fold (3). During the short incubation period without hormones, neither pervanadate nor genistein affected 32P incorporation into proteins immunoprecipitated with anti-phosphotyrosine antibody, but both significantly affected the actions of PRL. PRL increased tyrosine phosphorylation 440%, as determined by the method of Said and Medina (25); 100 µM pervanadate amplified this effect 2.5-fold, whereas 50 µM genistein reduced PRL stimulation by 77%. PRL also stimulated PI3K 370%, as measured by the method of Whitman et al. (34). Again, 100 nM wortmannin had no effect under basal conditions, but it did completely suppress the stimulation by PRL. Finally, the effect of L-NMMA on NOS was ascertained indirectly by determination of cGMP levels with the use of an RIA, as previously described (4). The MMTV envelope protein stimulated cGMP levels 220%, whereas L-NMMA totally blocked this elevation.

Cell isolation and receptor assay. EGF and insulin receptors were measured on an epithelial cell-enriched fraction isolated from mammary explants, as previously described (31). Briefly, the tissue was finely minced and digested with collagenase [1.5 mg/ml of medium 199 containing 20 mM HEPES (pH 7.6) and 4% (wt/vol) BSA] at 37°C. During this incubation, the tissue fragments were pipetted through successively smaller bore pipettes. After 30 min, the cells were centrifuged and washed three times in medium 199 containing 20 mM HEPES (pH 7.6) and 2% BSA.

EGF and insulin were iodinated by a modification (6) of the lactoperoxidase method of Miyachi et al. (22). The resulting 125I-labeled hormone was used in binding studies, as previously described (2). Receptor assays were performed after a 30-min incubation; this time period was chosen because it is the shortest interval during which one can easily see receptor redistribution within the mammary epithelial cell.

Protein was determined by the method of Lowry et al. (19), and binding data were analyzed by the method of Scatchard (26). Because of the low epithelial content of mouse mammary glands, five to six animals were required to generate enough cells to construct a single Scatchard plot; all experiments were replicated six times, and resulting data were subjected to ANOVA.

    RESULTS
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Abstract
Introduction
Procedures
Results
Discussion
References

The envelope protein of MMTV has previously been demonstrated to induce RNA synthesis in mammary epithelium via the NO/cGMP pathway (4), and Fig. 1 shows that these same transducers can recruit PRL receptors to the cell surface during the 30-min incubation period. SNP, an NO donor, and 8-bromo-cGMP both mimic the envelope protein, whereas L-NMMA, an NOS inhibitor, blocks the effects of MMTV. Tyrosine phosphorylation does not appear to be involved, because pervanadate, a phosphotyrosine phosphatase inhibitor, does not shift PRL receptors, and genistein, a tyrosine kinase inhibitor, does not block the effects of MMTV. Finally, the inhibition of PI3K by wortmannin also has no effect on MMTV induction of PRL receptors.


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Fig. 1.   Effect of modulators of nitric oxide (NO)/cGMP, tyrosine kinase, and phosphatidylinositol 3-kinase (PI3K) pathways on plasma membrane prolactin (PRL) receptors. Receptors were measured 30 min after addition of envelope protein (mouse mammary tumor virus, MMTV); other culture conditions and compound concentrations are given in EXPERIMENTAL PROCEDURES. Data are means ± SE of 6 separate experiments. SNP, sodium nitroprusside; L-NMMA, NG-monomethyl-L-arginine.

In contrast to the stimulation of PRL receptors, EGF receptors are downregulated by the MMTV envelope protein (5). Surprisingly, the NO/cGMP pathway does not appear to play any role in this process, because neither SNP nor 8-bromo-cGMP can induce internalization, and L-NMMA cannot block the effect of MMTV (Fig. 2). There is evidence that tyrosine phosphorylation is involved, since genistein can block the MMTV-triggered downregulation. However, pervanadate cannot mimic the effects of the MMTV envelope protein. Although pervanadate can augment PRL-induced tyrosine phosphorylation (see EXPERIMENTAL PROCEDURES), it was unable to affect tyrosine phosphorylation during a 30-min incubation. Similar results have been reported in rabbit mammary epithelium, in which no effects of pervanadate were seen on gene expression before a 24-h exposure (1), and in mouse mammary epithelium, in which no effects of this compound were observed on mitosis before 5 days (21). Apparently, the basal activity of tyrosine kinases in the mammary gland is so low that the inhibition of phosphotyrosine phosphatases is not sufficient to elevate net tyrosine phosphorylation during short incubation periods. Like tyrosine phosphorylation, PI3K also appears to be important, as wortmannin blocks the downregulation induced by the MMTV envelope protein.


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Fig. 2.   Effect of modulators of NO/cGMP, tyrosine kinase, and PI3K pathways on plasma membrane epidermal growth factor (EGF) receptors. Receptors were measured 30 min after addition of envelope protein (MMTV); other culture conditions and compound concentrations are given in EXPERIMENTAL PROCEDURES. Data are means ± SE of 6 separate experiments.

Calcium can integrate all of these transduction pathways. First, calcium can activate NOS, the product of which, NO, can stimulate the soluble guanylate cyclase. In addition, calcium can activate several soluble tyrosine kinases, the substrates of which include docking proteins for PI3K. Therefore, it was of interest to examine the role of calcium in receptor regulation. Cytosolic calcium levels were elevated by the calcium ionophore A-23187, which allows extracellular calcium to leak into the cell. Calcium stores were depleted by using A-23187 in combination with the calcium chelator EGTA. With EGTA in the medium, the calcium concentration is reversed, and A-23187 now transports calcium out of the cell. Cells were preincubated with A-23187 and EGTA for 30 min before the addition of the MMTV envelope protein. The elevation of cytoplasmic calcium was indeed able both to increase the plasma membrane receptors for PRL and decrease those for EGF (Fig. 3). Furthermore, calcium-depleted cells were unable to respond to MMTV with respect to receptor redistribution.


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Fig. 3.   Effect of modulators of cellular calcium on plasma membrane PRL (left) and EGF receptors (right). Receptors were measured 30 min after addition of envelope protein (MMTV); other culture conditions and compound concentrations are given in EXPERIMENTAL PROCEDURES. Data are means ± SE of 6 separate experiments.

Finally, all agonists at maximally effective concentrations were equipotent to the MMTV envelope protein (Figs. 1-3) and were not additive with the envelope protein (Fig. 3 and unpublished observations), suggesting that MMTV and these effectors are utilizing the same pathway.

    DISCUSSION
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Abstract
Introduction
Procedures
Results
Discussion
References

Most hormones downregulate their own receptors, especially those binding receptor tyrosine kinases or G protein-coupled receptors. In the former case, it has been shown that tyrosine receptor phosphorylation is essential for internalization (12, 15, 24, 32, 33, 35); in addition, PI3K activation is often required (12, 15). These two processes are frequently linked, since PI3K contains an src-homology 2 domain that binds phosphotyrosines; this interaction not only allosterically stimulates the enzyme but also brings it to the plasma membrane near its substrates. The exact role of PI3K in receptor regulation is not clear, but PI3K appears to be involved with membrane trafficking, probably through protein kinase B, protein kinase Czeta , or other transducers that its product is known to affect (29). Therefore, it is not surprising that the MMTV envelope protein also utilizes this pathway to induce EGF receptor internalization. It is interesting that EGF itself requires receptor tyrosine phosphorylation but not PI3K stimulation to downregulate its own receptor (32), suggesting that different stimuli can have similar effects on receptor regulation via different mechanisms.

Fewer investigations have been done on receptor upregulation via recruitment from internal pools, probably because most receptors are already at the cell surface, so that few microsomal receptors are available to recruit. However, PRL does have a large reservoir of internal receptors (7). In contrast to the situation with the EGF receptor, the relocalization of the PRL receptor appears to be mediated by the NO/cGMP pathway (Fig. 4). It is not clear why two different pathways are employed by the envelope protein to move hormone receptors within a cell. The use of two different pathways may be required to ensure directional specificity. However, IGF-II recruits its own receptors to the cell surface in a PI3K-dependent manner (17), suggesting that the PI3K pathway can be used to shuttle receptors in either direction. Alternatively, the mechanisms may be receptor specific: EGF and IGF-II bind receptor tyrosine kinases, whereas PRL interacts with a cytokine receptor.


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Fig. 4.   A hypothetical pathway integrating data in this study. NOS, NO synthase; PKB, protein kinase B; PKCzeta , protein kinase Czeta ; Y-PO4, a phosphotyrosine docking site.

Although studies of receptor exocytosis are limited, there are several examples of nonreceptor membrane proteins being induced to migrate to the cell surface in response to cyclic nucleotides: the renal water channel, aquaporin-2 (10), the epithelial chloride channel (28), and the cystic fibrosis transmembrane conductance regulator (18) are all translocated to the plasmalemma in response to the elevation of cAMP. The effect on aquaporin-2 is mediated by direct phosphorylation by the cAMP-dependent protein kinase (protein kinase A, PKA), but the effect on the chloride channel is PKA independent. The mechanism for the effect on cystic fibrosis transmembrane conductance regulator is unknown. It is interesting to note that cGMP, which induces PRL receptor redistribution, also has the potential to activate PKA at high physiological concentrations (9, 14) in addition to stimulating its own kinase, the cGMP-dependent protein kinase.

Finally, there are also a few examples of calcium-regulated membrane protein redistribution. Acetylcholine activation of the muscarinic receptor in the parotid gland recruits the aquaporin-5 channel to the cell surface by a calcium-dependent, but protein kinase C-independent, mechanism (13). In addition, Ret, a member of the receptor complex for the glial cell line-derived neurotropic factor, requires calcium for posttranslational processing and migration to the cell surface (30). Neither study examined the role of the NO/cGMP pathway in their respective system.

Receptor number is an important factor in determining the sensitivity of a cell toward hormones. Although the concentration can be regulated by altering receptor synthesis, this usually requires several hours. Receptor redistribution is a mechanism that can dramatically change receptor number at the cell surface within minutes. For many receptor tyrosine kinases, this process appears to be mediated by components of the tyrosine phosphorylation/PI3K pathway. However, this study has shown that, for at least one cytokine receptor, another transduction system predominates: the NO/cGMP pathway.

    ACKNOWLEDGEMENTS

The technical assistance of William McAmis is gratefully appreciated.

    FOOTNOTES

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Address for reprint requests: F. F. Bolander, Jr., Dept. of Biological Sciences, Univ. of South Carolina, Columbia, SC 29208.

Received 15 April 1998; accepted in final form 23 June 1998.

    REFERENCES
Top
Abstract
Introduction
Procedures
Results
Discussion
References

1.   Bayat-Sarmadi, M., C. Puissant, and L. M. Houdbine. The effects of various kinase and phosphatase inhibitors on the transmission of the prolactin and extracellular matrix signals to rabbit alpha S1-casein and transferrin genes. Int. J. Biochem. Cell Biol. 27: 707-718, 1995[Medline].

2.   Bolander, F. F. Enhanced hormonal responsiveness in mammary glands from parous mice: molecular mechanisms. Mol. Cell. Endocrinol. 35: 221-227, 1984[Medline].

3.   Bolander, F. F. Possible roles of calcium and calmodulin in mammary gland differentiation in vitro. J. Endocrinol. 104: 29-34, 1985[Abstract].

4.   Bolander, F. F. Second messengers induced by the envelope protein of a retrovirus. Mol. Cell. Endocrinol. 129: 27-32, 1997[Medline].

5.  Bolander, F. F. The regulation of mammary hormone receptor metabolism by a retroviral envelope protein. J. Mol. Endocrinol. In press.

6.   Bolander, F. F., and R. E. Fellows. Growth hormone covalently bound to Sepharose or glass: analysis of ligand release rates and characterization of soluble radiolabeled products. Biochemistry 14: 2938-2943, 1975[Medline].

7.   Bolander, F. F., E. Ginsburg, and B. K. Vonderhaar. The regulation of mammary prolactin receptor metabolism by a retroviral envelope protein. J. Mol. Endocrinol. 19: 131-136, 1997[Abstract/Free Full Text].

8.   Fails, A. D., T. W. Mitchell, J. L. Rojko, and L. R. Whalen. An oligopeptide of the feline leukemia virus envelope glycoprotein is associated with morphological changes and calcium dysregulation in neural growth cones. J. Neurovirol. 3: 179-191, 1997[Medline].

9.   Forte, L. R., P. K. Thorne, S. L. Eber, W. J. Krause, R. H. Freeman, S. H. Francis, and J. D. Corbin. Stimulation of intestinal Cl- transport by heat-stable enterotoxin: activation of cAMP-dependent protein kinase by cGMP. Am. J. Physiol. 263 (Cell Physiol. 32): C607-C615, 1992[Abstract/Free Full Text].

10.   Fushimi, K., S. Sasaki, and F. Marumo. Phosphorylation of serine 256 is required for cAMP-dependent regulatory exocytosis of the aquaporin-2 water channel. J. Biol. Chem. 272: 14800-14804, 1997[Abstract/Free Full Text].

11.   Goffin, V., and P. A. Kelly. The prolactin/growth hormone receptor family: structure/function relationships. J. Mam. Gland Biol. Neoplasia 2: 7-17, 1997.

12.   Gommerman, J. L., R. Rottapel, and S. A. Berger. Phosphatidylinositol 3-kinase and Ca2+ influx dependence for ligand-stimulated internalization of the c-Kit receptor. J. Biol. Chem. 272: 30519-30525, 1997[Abstract/Free Full Text].

13.   Ishikawa, Y., T. Eguchi, T. Skowronski, and H. Ishida. Acetylcholine acts on M3 muscarinic receptors and induces the translocation of aquaporin5 water channel via cytosolic Ca2+ elevation in rat parotid glands. Biochem. Biophys. Res. Commun. 245: 835-840, 1998[Medline].

14.   Jiang, H., J. B. Shabb, and J. D. Corbin. Cross-activation: overriding cAMP/cGMP selectivities of protein kinases in tissues. Biochem. Cell Biol. 70: 1283-1289, 1993.

15.   Joly, M., A. Kazlauskas, F. S. Fay, and S. Corvera. Disruption of PDGF receptor trafficking by mutation of its PI-3 kinase binding sites. Science 263: 684-687, 1994[Medline].

16.   Juergens, W. C., F. E. Stockdale, Y. J. Topper, and J. J. Elias. Hormonal-dependent differentiation of mouse mammary gland in vitro. Proc. Natl. Acad. Sci. USA 54: 629-634, 1965[Medline].

17.   Körner, C., and T. Braulke. Inhibition of IGF II-induced redistribution of mannose 6-phosphate receptors by the phosphatidylinositol 3-kinase inhibitor, wortmannin. Mol. Cell. Endocrinol. 118: 201-205, 1996[Medline].

18.   Lehrich, R., S. G. Aller, P. Webster, C. R. Marino, and J. N. Forrest. Vasoactive intestinal peptide, forskolin, and genistein increase apical CFTR trafficking in the rectal gland of the spiny dogfish, Squalus acanthias: acute regulation of CFTR trafficking in an intact epithelium. J. Clin. Invest. 101: 737-745, 1998[Abstract/Free Full Text].

19.   Lowry, O. H., N. J. Rosebrough, A. J. Farr, and R. J. Randall. Protein measurements with the Folin phenol reagent. J. Biol. Chem. 193: 265-275, 1951[Free Full Text].

20.   Marcus, S. L., R. Kopelman, and N. H. Sarkar. Simultaneous purification of murine mammary tumor virus structural proteins: analysis of antigenic reactivities of native gp34 by radioimmunocompetition assays. J. Virol. 31: 341-349, 1979[Medline].

21.   McIntyre, B. S., K. P. Briski, H. L. Hosick, and P. W. Sylvester. Effects of protein tyrosine phosphatase inhibitors on EGF- and insulin-dependent mammary epithelial cell growth. Proc. Soc. Exp. Biol. Med. 217: 180-187, 1998[Abstract].

22.   Miyachi, Y., J. L. Vaitukaitis, E. Nieschlag, and M. B. Lipsett. Enzymatic radioiodination of gonadotropins. J. Clin. Endocrinol. Metab. 34: 23-28, 1972[Medline].

23.   Pumiglia, K. M., L. F. Lau, C. K. Huang, S. Burroughs, and M. B. Feinstein. Activation of signal transduction in platelets by the tyrosine phosphatase inhibitor pervanadate (vanadyl hydroperoxide). Biochem. J. 286: 441-449, 1992[Medline].

24.   Reynet, C., M. Caron, J. Magré, J. Picard, G. Cherqui, and J. Capeau. Insulin receptor autophosphorylation sites tyrosines 1162 and 1163 control both insulin-dependent and insulin-independent receptor internalization pathways. Cell. Signal. 6: 35-46, 1994[Medline].

25.   Said, T. K., and D. Medina. Tyrosine phosphorylation in mouse mammary hyperplasias. Carcinogenesis 16: 923-930, 1995[Abstract].

26.   Scatchard, G. The attractions of proteins for small molecules and ions. Ann. NY Acad. Sci. 51: 660-672, 1949.

27.   Scorziello, A., T. Florio, A. Bajetto, and G. Schettini. Intracellular signalling mediating HIV-1 gp120 neurotoxicity. Cell. Signal. 10: 75-84, 1998[Medline].

28.   Shintani, Y., and Y. Marunaka. Regulation of chloride channel trafficking by cAMP via protein kinase A-independent pathway in A6 renal epithelial cells. Biochem. Biophys. Res. Commun. 223: 234-239, 1996[Medline].

29.   Toker, A., and L. Cantley. Signalling through the lipid products of phosphoinositide-3-OH kinase. Nature 387: 673-676, 1997[Medline].

30.   Van Weering, D. H. J., T. C. Moen, I. Braakman, P. D. Baas, and J. L. Bos. Expression of the receptor tyrosine kinase Ret on the plasma membrane is dependent on calcium. J. Biol. Chem. 273: 12077-12081, 1998[Abstract/Free Full Text].

31.   Vonderhaar, B. K., I. S. Owens, and Y. J. Topper. An early effect of prolactin on the formation of alpha -lactalbumin by mouse mammary epithelial cells. J. Biol. Chem. 248: 467-471, 1973[Abstract/Free Full Text].

32.   Wang, Z., and M. F. Moran. Requirement for the adapter protein GRB2 in EGF receptor endocytosis. Science 272: 1935-1939, 1996[Abstract].

33.   Ware, M. F., D. A. Tice, S. J. Parsons, and D. A. Lauffenburger. Overexpression of cellular Src in fibroblasts enhances endocytic internalization of epidermal growth factor receptor. J. Biol. Chem. 272: 30185-30190, 1997[Abstract/Free Full Text].

34.   Whitman, M., D. R. Kaplan, B. Schaffhausen, L. Cantley, and T. M. Roberts. Association of phosphatidylinositol kinase activity with polyoma middle-T competent for transformation. Nature 315: 239-242, 1985[Medline].

35.   Yee, N. S., C. W. M. Hsiau, H. Serve, K. Vosseller, and P. Besmer. Mechanism of down-regulation of c-kit receptor: roles of receptor tyrosine kinase, phosphatidylinositol 3'-kinase, and protein kinase C. J. Biol. Chem. 269: 31991-31998, 1994[Abstract/Free Full Text].


Am J Physiol Endocrinol Metab 275(4):E553-E557
0002-9513/98 $5.00 Copyright © 1998 the American Physiological Society




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