In rodents, PRL exposure enhances the development of chemically induced mammary cancers (1
). Development of chemically induced mammary cancers is dependent upon the number of terminal end buds and the degree of cell proliferation at the time of chemical exposure (293
). Terminal end buds disappear with differentiation of the mammary gland. Differentiation of the mammary gland correlates with the onset of sexual maturity. A differentiated mammary gland that contains fewer terminal end buds is less susceptible to the action of chemical carcinogens (294
). Exposure to estrogen with secondary increases in circulating PRL levels is able to restore susceptibility to chemical carcinogens in parous mice (295
). In summary, numerous studies point to a role for PRL in increasing receptiveness to chemical carcinogens in rodent mammary glands.
In the mammary gland, PRL stimulates phosphorylation and activation of Jak2 and the Stat5a and Stat5b proteins. Stat5a plays a more prominent role than Stat5b in the mammary gland (296
). Jak2 is the major Janus kinase activated by PRL in mammary epithelial cells (297
). The SOCS family of proteins act in a classical negative feed-back loop to down-regulate PRL-induced Jak/Stat activation (267
). Activation of Stat5a and Stat5b in the mammary gland also can be controlled by other mechanisms. For example, local factors, and not changes in circulating levels of PRL secretion, are responsible for the inactivation of Stat5a and Stat5b during mammary gland involution (303
). Finally, PRL also can signal through the MAPK pathway with stimulation of mammary epithelial cell proliferation (147
The development of transgenic technology offered the opportunity to study the role of PRL in mammary gland cancer development through gain-of-function and loss-of-function mouse models. In gain-of-function mouse models, a trans-gene encoding a selected protein either can be overexpressed in a tissue that normally demonstrates expression of that particular protein or introduced into a tissue that does not normally express that particular protein. In loss-of-function models, the function of a selected protein is lost by preventing expression of the protein through a germ-line disruption of the gene encoding the protein or through expression of a dominant negative form of the selected protein that interrupts gene function. These models permit study of specific elements within the PRL-signaling pathway in the context of an intact animal (). To date, specific investigations have focused on gain and loss of PRL function, loss of PRLR function, loss of Jak2 function, loss of Stat5a and Stat5b function, and gain and loss of SOCS1 activity. Germ-line loss of Jak2 function results in late embryonic lethality, necessitating the use of embryonic mammary gland transplants for study of its specific role in mammary gland development and carcinogenesis (205
). Redundancy in the MAPK pathways complicates study of the role of specific proteins in PRL-related cancer development in the intact animal.
Effects on specific signaling molecules in the PRL pathway on mammary gland development and mammary gland cancer in mouse models
Studies of specific signaling molecules in the PRL pathway have illustrated the dose responsiveness of the signaling cascade in the intact animal. For example, loss of one functional copy of the PRLR gene is sufficient to interrupt lactation after a first pregnancy (306
). Loss of Stat5a function through germ-line disruption of the Stat5a gene results in impaired lactation after the first pregnancy, but lactation can be recovered with increased activation of the Stat5b gene during subsequent lactation periods (307
). Similarly, the first pregnancy-associated lactation failure found in mice carrying only one functional copy of the PRLR gene is rescued by a germ-line deletion of one SOCS1 allele (308
Overexpression of PRL in transgenic mice with increased activation of the PRLR is sufficient to induce the formation of mammary cancers at 11–15 months of age (309
). In contrast, no tumors were noted in parallel transgenic controls expressing bovine GH (309
), suggesting that unlike PRL, the contribution of GH to mammary neoplasia may be indirect. Supporting this hypothesis is the phenotype observed in lit−/−
mice have a functional mutation in the GnRH receptor, demonstrate markedly decreased levels of both GH and IGF, and evidence significant reductions in the growth of mammary tumor xenografts (311
Loss of PRL function through germ-line disruption of the PRL gene aborts mammary gland development by impairing ductal arborization and lobular budding and reduces the growth of Polyoma middle T antigen-induced mammary cancers (313
In mice, germ-line disruption of only one PRLR allele is sufficient to impair lactation after the first pregnancy (111
). Loss of PRLR function in mammary epithelial cells by disruption of both PRLR gene impairs mammary lobular development during pregnancy (316
). Significantly, loss of PRLR function also can have an indirect effect on mammary gland development. Mammary gland transplant experiments have demonstrated that wild-type mammary epithelial cells transplanted into the mammary fat pad of mice carrying germ-line deletions of the PRLR genes do not undergo normal development during puberty (316
). These mice now can be used to study the specific role of the PRLR in mammary cancer development. The use of mammary gland transplant experiments will allow investigators to isolate the role of the PRLR in mammary epithelial cells from systemic effects resulting from loss of PRLR function in other tissues (317
The role of Jak2 in mammary epithelium was studied using mammary transplants of Jak2-null epithelium into the mammary gland fat pads of wild-type mice (318
). Loss of Jak2 function through disruption of both Jak2 genes in the mammary epithelium results in impaired mammary gland development during pregnancy. Although ductal tissues formed normally, there was no development of secretory epithelium during pregnancy. This indicates that Jak2 is required for pregnancy-induced mammary gland development through the placental lactogen- and PRL-signaling pathways.
Germ-line disruption of the Stat5a gene and complete loss of Stat5a expression not only impair lactation but also result in decreased survival of mammary epithelial cells (210
). Complete loss of Stat5a promotes apoptosis of TGFα over-expressing mammary epithelial cells during mammary gland involution and delays development of TGFα -induced mammary hyperplasia and cancer in a mouse model (210
). Germ-line disruption of just one Stat5a allele results in reduced Stat5a expression levels in mammary epithelial cells (319
). Reduced Stat5a expression levels result in significantly increased levels of apoptosis of mammary adenocarcinoma cells and delay tumorigenesis in the whey acidic protein-TAg mouse model of mammary gland cancer progression (319
). Germ-line disruption of both Stat5a and Stat5b genes results in the loss of both Stat5a and Stat5b in mammary epithelial cells (320
); as a consequence, these cells fail to differentiate into alveolar cells during pregnancy.
Gain of SOCS1 function through expression of a SOCS1-encoding transgene in mammary epithelial cells results in decreased levels of Stat5 activation and impaired lactation (321
). Loss of SOCS1/CIS1/SSI1 function by germ-line disruption of the SOCS1/CIS1 genes increases levels of Stat5 activation and accelerates mammary gland development during pregnancy (308
). Haploid loss of SOCS1 function is sufficient to rescue the lactation defect in haploid-deficient PRLR mice (308
). Changes in either proliferation or survival of mammary epithelial cells were not determined directly in the gain of SOCS1/CIS1/SSI1 function mouse model and no molecules in the MAPK pathway were studied. In contrast, the MAPK pathway has been examined in the loss of SOCS1 function mice. In these mice a decrease in phosphorylated ERK1/2 was reported. Thus, further analysis of these signaling pathways and associated functions in these mouse models should further delineate the role of SOCS1 in mammary cancer development.