Breast Cancer Research and Treatment 22: 3-5, 1992. © 1992 Kluwer Academic Publishers. Printedin the Netherlands.
Can the insulin-like growth factors regulate breast cancer growth? Douglas Yee, M.D. University of Texas Health Science Center, San Antonio, Texas, USA
Key words:
Summary Many laboratories have demonstrated that polypeptide growth factors stimulate human cancer cell growth in experimental systems. Despite this observation, a central question remains: can inhibition of peptide growth factor action inhibit tumor growth in humans? To answer this question, several other concerns must first be addressed. Which growth factors are critical for tumor growth? What are the specific cellular effectors for each growth factor? Can feasible therapies be designed to interrupt growth factor pathways? This issue of Breast Cancer Research and Treatment explores the relevance of the insulin-like growth factors to breast cancer cell growth.
The idea that a single growth factor interacts with a single receptor to affect cell growth has proven to be false in virtually every growth factor system yet examined. The IGF system is no exception. Krywicki and Yee review the multiple members of the IGF family in the introductory chapter. To date, at least two ligands, three receptors, and six high affinity extracellular binding proteins have been described, making the potential interactions quite complex. Cullen et al. review the evidence that the IGFs are expressed in breast cancer cells and tissues. Although the IGFs are mitogens for many breast cancer cell lines, only a few cell lines express IGF-II and none have been found to express IGFI. However, this laboratory shows that IGF-II can act as an autocrine stimulator of breast cancer cell growth; when MCF-7 cells were infected with an
IGF-II retroviral expression vector, some degree of hormone independence was observed. Brtinner at al. make a similar observation in their athymic mouse model of breast cancer - - IGF-II expression was directly correlated with hormonal responsiveness in the T61 and MCF-7 tumor xenografts. Cullen et al. also found IGF expression in stromal fibroblasts obtained from malignant and benign breast biopsy specimens. Remarkably, IGF-II expression was more commonly seen in stromal fibroblasts obtained from malignant specimens, while IGF-I was expressed by fibroblasts grown from benign biopsy specimens. This observation is supported by the work of Paik, who used in situ hybridization to study the expression of IGF-I and IGFII. Again, IGF-I was found adjacent to normal breast lobules, while IGF-II was detected in some
Address for correspondence: Douglas Yee, M.D., Medicine/Oncology,Universityof Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio TX 78284-7884, USA
4
D Yee
malignant epithelial cells and their surrounding stroma. Since IGF-II is expressed during fetal development, Paik argues that this "switching" from IGF-I to IGF-II is a marker for a more fetal type of stromal cell associated with the cancer cells. The evidence could also support the idea that either autocrine or paracrine IGF-II expression stimulates breast cancer growth, perhaps by acting through its high affinity receptor, the type II IGF receptor. Vignon and Rochefort discuss the possibility that the type II receptor (which is the mannose6-phosphate receptor) can mediate biological functions. They demonstrate that the proteolytic enzyme cathepsin D can also bind to this receptor at a site distinct from the IGF-II binding site. This unexpected interaction between a growth factor pathway and a lysosomal enzyme routing pathway could have important biological consequences. Competition between IGF-II and other mannose-6-phosphate bearing proteins could perturb either pathway and lead to enhanced cell growth or invasion. The type I IGF receptor is homologous to the insulin receptor. Peyrat and Bonneterre review the evidence that the type I receptor can mediate some, if not all, of the mitogenic effects of IGF-I and IGF-II in breast cancer cell lines. Higher levels of type I receptor were present in estrogen receptor positive cells. These investigators also showed that type I receptor was present in breast cancer tissue specimens. Levels of the receptor were higher in malignant disease than in benign disease, suggesting a correlation with proliferative rate. Paradoxically, expression of type I IGF receptors in breast cancer specimens correlated with an improved prognosis, perhaps suggesting interactions between expression of this receptor and other factors. Goldfine et al. present data demonstrating a complex series of interactions between progesterone, insulin receptor, type I IGF receptor, and IGF-II. In T47D cells, progestins increased expression of insulin receptor and IGFII. IGF-II then interacted with the type I receptor
to down-regulate its expression. Thus, interactions between steroid hormones and IGF activity are evident, although some of the data is contradictory. Estrogen increases type I IGF receptor expression, so that high levels of this receptor may signify a functionally intact estrogen receptor. In contrast, estrogen and progesterone increase IGF-II expression which, at least in T47D cells, may then down-regulate type I IGF receptor expression. Further complexity is added when one considers the IGF binding proteins. The IGF binding proteins can alter the interactions between the IGFs and their receptors. Figueroa and Yee review the evidence that these binding proteins are expressed by breast cancer cells, and like the ligands and receptors, can be regulated by estrogen. Thus steroid hormones may regulate all the components of the IGF system by increasing expression of IGF-II, the type I IGF receptor, and at least some of the IGF binding proteins. Estrogen may also indirectly regulate IGF-II effects by upregulating cathepsin D and interfering with IGFII/type II IGF receptor interactions. Although the effect of estrogen on the components and interactions of the IGF system are not precisely defined, the net effect is clear: cell growth is enhanced. What is the evidence that the IGFs are related to these growth effects? Pollak et al. find that serum IGF-I levels are decreased by tamoxifen treatment. The patients assigned to tamoxifen adjuvant therapy for breast cancer (NSABP trial B-14) had lower serum IGFI levels than non-treated patients. Even in individual patients, IGF-I levels are similarly reduced after starting tamoxifen. Although these observations do not prove that lowering of IGF-I levels by tamoxifen results directly in decreased breast cancer growth, they do provide suggestive evidence that endocrine sources of IGF-I can be lowered by anti-estrogens. Arteaga provides the most direct evidence that interruption of the IGF pathway can lead to decreased tumor growth. Blockade of the type I IGF receptor with a
IFGs and breast cancer
monoclonal antibody was used to inhibit the growth of MDA-MB-231 cells in athymic mice. Curiously, the most IGF-responsive cell line in vitro, MCF-7, was not inhibited by this strategy. Since estrogen increases levels of IGF-II in MCF7 cells, it is possible that the failure of type I receptor antibody blockade resulted from an IGFII/type II IGF receptor interaction, or from an internal autocrine loop that was not blocked by pharmacologic administration of antibody. Despite this anomaly, this experiment provided the most direct evidence that an anti-IGF treatment strategy could be effective in preventing breast tumor growth. Where do these studies leave us with regard to the question of the importance of the IGF system and its potential as a therapeutic target? Clearly both IGF-I and IGF-II are mitogens for breast cancer cells in vitro, and estrogen alters expression of some components of the system. Moreover, blockade of the type I IGF receptor can inhibit the growth of some tumor cells in ex-
5
perimental systems. However, several other questions must be answered before an anti-type I IGF receptor strategy can be embraced with much enthusiasm. Is IGF-II central to breast cancer growth, and if so, what function does the type II IGF receptor have? Are the IGF binding proteins important in determining IGF action? Can they be used, as we have suggested, as IGF neutralizing agents? Is estrogen-induced growth regulated by alterations in the expression of the components of the IGF system and is this how anti-estrogens function? Will combination steroid hormone and anti-IGF treatments synergize? Will factors, such as expression of type I IGF receptor expression, determine IGF responsiveness in vivo? These questions, and many others, need to be asked of any potential anti-growth factor strategy. In the next several years, research from several laboratories will help to answer these questions, and hopefully, lead to better treatments for the patient with breast cancer.
Can the insulin-like growth factors regulate breast cancer growth? Douglas Yee, M.D. University of Texas Health Science Center, San Antonio, Texas, USA
Key words:
Summary Many laboratories have demonstrated that polypeptide growth factors stimulate human cancer cell growth in experimental systems. Despite this observation, a central question remains: can inhibition of peptide growth factor action inhibit tumor growth in humans? To answer this question, several other concerns must first be addressed. Which growth factors are critical for tumor growth? What are the specific cellular effectors for each growth factor? Can feasible therapies be designed to interrupt growth factor pathways? This issue of Breast Cancer Research and Treatment explores the relevance of the insulin-like growth factors to breast cancer cell growth.
The idea that a single growth factor interacts with a single receptor to affect cell growth has proven to be false in virtually every growth factor system yet examined. The IGF system is no exception. Krywicki and Yee review the multiple members of the IGF family in the introductory chapter. To date, at least two ligands, three receptors, and six high affinity extracellular binding proteins have been described, making the potential interactions quite complex. Cullen et al. review the evidence that the IGFs are expressed in breast cancer cells and tissues. Although the IGFs are mitogens for many breast cancer cell lines, only a few cell lines express IGF-II and none have been found to express IGFI. However, this laboratory shows that IGF-II can act as an autocrine stimulator of breast cancer cell growth; when MCF-7 cells were infected with an
IGF-II retroviral expression vector, some degree of hormone independence was observed. Brtinner at al. make a similar observation in their athymic mouse model of breast cancer - - IGF-II expression was directly correlated with hormonal responsiveness in the T61 and MCF-7 tumor xenografts. Cullen et al. also found IGF expression in stromal fibroblasts obtained from malignant and benign breast biopsy specimens. Remarkably, IGF-II expression was more commonly seen in stromal fibroblasts obtained from malignant specimens, while IGF-I was expressed by fibroblasts grown from benign biopsy specimens. This observation is supported by the work of Paik, who used in situ hybridization to study the expression of IGF-I and IGFII. Again, IGF-I was found adjacent to normal breast lobules, while IGF-II was detected in some
Address for correspondence: Douglas Yee, M.D., Medicine/Oncology,Universityof Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio TX 78284-7884, USA
4
D Yee
malignant epithelial cells and their surrounding stroma. Since IGF-II is expressed during fetal development, Paik argues that this "switching" from IGF-I to IGF-II is a marker for a more fetal type of stromal cell associated with the cancer cells. The evidence could also support the idea that either autocrine or paracrine IGF-II expression stimulates breast cancer growth, perhaps by acting through its high affinity receptor, the type II IGF receptor. Vignon and Rochefort discuss the possibility that the type II receptor (which is the mannose6-phosphate receptor) can mediate biological functions. They demonstrate that the proteolytic enzyme cathepsin D can also bind to this receptor at a site distinct from the IGF-II binding site. This unexpected interaction between a growth factor pathway and a lysosomal enzyme routing pathway could have important biological consequences. Competition between IGF-II and other mannose-6-phosphate bearing proteins could perturb either pathway and lead to enhanced cell growth or invasion. The type I IGF receptor is homologous to the insulin receptor. Peyrat and Bonneterre review the evidence that the type I receptor can mediate some, if not all, of the mitogenic effects of IGF-I and IGF-II in breast cancer cell lines. Higher levels of type I receptor were present in estrogen receptor positive cells. These investigators also showed that type I receptor was present in breast cancer tissue specimens. Levels of the receptor were higher in malignant disease than in benign disease, suggesting a correlation with proliferative rate. Paradoxically, expression of type I IGF receptors in breast cancer specimens correlated with an improved prognosis, perhaps suggesting interactions between expression of this receptor and other factors. Goldfine et al. present data demonstrating a complex series of interactions between progesterone, insulin receptor, type I IGF receptor, and IGF-II. In T47D cells, progestins increased expression of insulin receptor and IGFII. IGF-II then interacted with the type I receptor
to down-regulate its expression. Thus, interactions between steroid hormones and IGF activity are evident, although some of the data is contradictory. Estrogen increases type I IGF receptor expression, so that high levels of this receptor may signify a functionally intact estrogen receptor. In contrast, estrogen and progesterone increase IGF-II expression which, at least in T47D cells, may then down-regulate type I IGF receptor expression. Further complexity is added when one considers the IGF binding proteins. The IGF binding proteins can alter the interactions between the IGFs and their receptors. Figueroa and Yee review the evidence that these binding proteins are expressed by breast cancer cells, and like the ligands and receptors, can be regulated by estrogen. Thus steroid hormones may regulate all the components of the IGF system by increasing expression of IGF-II, the type I IGF receptor, and at least some of the IGF binding proteins. Estrogen may also indirectly regulate IGF-II effects by upregulating cathepsin D and interfering with IGFII/type II IGF receptor interactions. Although the effect of estrogen on the components and interactions of the IGF system are not precisely defined, the net effect is clear: cell growth is enhanced. What is the evidence that the IGFs are related to these growth effects? Pollak et al. find that serum IGF-I levels are decreased by tamoxifen treatment. The patients assigned to tamoxifen adjuvant therapy for breast cancer (NSABP trial B-14) had lower serum IGFI levels than non-treated patients. Even in individual patients, IGF-I levels are similarly reduced after starting tamoxifen. Although these observations do not prove that lowering of IGF-I levels by tamoxifen results directly in decreased breast cancer growth, they do provide suggestive evidence that endocrine sources of IGF-I can be lowered by anti-estrogens. Arteaga provides the most direct evidence that interruption of the IGF pathway can lead to decreased tumor growth. Blockade of the type I IGF receptor with a
IFGs and breast cancer
monoclonal antibody was used to inhibit the growth of MDA-MB-231 cells in athymic mice. Curiously, the most IGF-responsive cell line in vitro, MCF-7, was not inhibited by this strategy. Since estrogen increases levels of IGF-II in MCF7 cells, it is possible that the failure of type I receptor antibody blockade resulted from an IGFII/type II IGF receptor interaction, or from an internal autocrine loop that was not blocked by pharmacologic administration of antibody. Despite this anomaly, this experiment provided the most direct evidence that an anti-IGF treatment strategy could be effective in preventing breast tumor growth. Where do these studies leave us with regard to the question of the importance of the IGF system and its potential as a therapeutic target? Clearly both IGF-I and IGF-II are mitogens for breast cancer cells in vitro, and estrogen alters expression of some components of the system. Moreover, blockade of the type I IGF receptor can inhibit the growth of some tumor cells in ex-
5
perimental systems. However, several other questions must be answered before an anti-type I IGF receptor strategy can be embraced with much enthusiasm. Is IGF-II central to breast cancer growth, and if so, what function does the type II IGF receptor have? Are the IGF binding proteins important in determining IGF action? Can they be used, as we have suggested, as IGF neutralizing agents? Is estrogen-induced growth regulated by alterations in the expression of the components of the IGF system and is this how anti-estrogens function? Will combination steroid hormone and anti-IGF treatments synergize? Will factors, such as expression of type I IGF receptor expression, determine IGF responsiveness in vivo? These questions, and many others, need to be asked of any potential anti-growth factor strategy. In the next several years, research from several laboratories will help to answer these questions, and hopefully, lead to better treatments for the patient with breast cancer.
