IGF-I mediates mammary ductal development through stimulation of terminal end bud (TEB) development; however, no published data exist on the mechanism through which this occurs. The mechanism of IGF-I action on the TEB was studied by determining the requirement for the IGF-I receptor (IGF-IR) in IGF-I-dependent ductal development. We hypothesized that loss of the IGF-IR would disrupt mammary ductal development through a combination of decreased proliferation or increased apoptosis. Because IGF-IR null mice die at birth, embryonic mammary gland transplantation was used to study the effects of a disrupted IGF-IR gene. Analyses of grafts after 4 or 8 wk of development demonstrated a limited growth potential of the null mammary epithelium in virgin hosts. Bromodeoxyuridine labeling and terminal deoxynucleotidyltransferase-mediated deoxy-UTP nick-end labeling showed that cell proliferation was significantly decreased in null TEBs, but apoptosis was not. In addition, both the size and number of TEBs were reduced in null outgrowths. In pregnant hosts, null ductal growth was stimulated beyond the level seen in virgin hosts. These findings directly establish a proliferation-dependent role for the IGF-IR in the cells of the TEB. Additionally, this study indicates that pregnancy-dependent compensatory mechanisms can stimulate mammary development in the absence of an IGF-IR.

Mammary tumorigenesis was analysed in transgenic mice which overexpress des(1-3)hIGF-I (WAP-DES) and/or a mutant form of p53 (p53 172R-H ). Nonlactating, multiparous WAP-DES mice exhibited hyperplastic lesions termed mammary interepithelial neoplasia (MIN) which constitutively expressed WAP-DES. By 23 months of age, 53% of the WAP-DES mice developed mammary adenocarcinomas. A 75% reduction in both apoptosis and proliferation was observed in the normal mammary glands of WAP-DES mice. Mammary tumor incidence in WAP-DES/p53 bitransgenic mice was similar to that of WAP-DES and 2 ± 3-fold greater than that of nontransgenic and p53 172R-H females. Tumor latency, however, was reduced by 8 months in bitransgenic mice as compared to mice of the other three genotypes. Aneuploidy was frequently observed in tumors from bitransgenic and p53 172R-H mice, but not from mice expressing only the WAP-DES transgene. Expression of IGFBP3 was elevated in tumors from WAP-DES, but not bitransgenic mice, indicating an alteration in the p53/IGF-I axis. These studies indicate that overexpression of des(1-3)hIGF-I increases the frequency of MIN and stochastic mammary tumors and that the appearance of tumors displaying genomic instability is accelerated by mutant p53 172R-H . Oncogene (2000) 19, 889 ± 898.

IGF-I mediates mammary ductal development through stimulation of terminal end bud (TEB) development; however, no published data exist on the mechanism through which this occurs. The mechanism of IGF-I action on the TEB was studied by determining the requirement for the IGF-I receptor (IGF-IR) in IGF-I-dependent ductal development. We hypothesized that loss of the IGF-IR would disrupt mammary ductal development through a combination of decreased proliferation or increased apoptosis. Because IGF-IR null mice die at birth, embryonic mammary gland transplantation was used to study the effects of a disrupted IGF-IR gene. Analyses of grafts after 4 or 8 wk of development demonstrated a limited growth potential of the null mammary epithelium in virgin hosts. Bromodeoxyuridine labeling and terminal deoxynucleotidyltransferase-mediated deoxy-UTP nick-end labeling showed that cell proliferation was significantly decreased in null TEBs, but apoptosis was not. In addition, both the size and number of TEBs were reduced in null outgrowths. In pregnant hosts, null ductal growth was stimulated beyond the level seen in virgin hosts. These findings directly establish a proliferation-dependent role for the IGF-IR in the cells of the TEB. Additionally, this study indicates that pregnancy-dependent compensatory mechanisms can stimulate mammary development in the absence of an IGF-IR.

Insulin receptor substrates (IRS) are central integrators of hormone, cytokine, and growth factor signaling. IRS proteins can be phosphorylated by a number of signaling pathways critical to normal mammary gland development. Studies in transgenic mice that overexpress IGF-I in the mammary gland suggested that IRS expression is important in the regulation of normal postlactational mammary involution. The goal of these studies was to examine IRS expression in the mouse mammary gland and determine the importance of IRS-1 to mammary development in the virgin mouse. IRS-1 and -2 show distinct patterns of protein expression in the virgin mouse mammary gland, and protein abundance is dramatically increased during pregnancy and lactation, but rapidly lost during involution. Consistent with hormone regulation, IRS-1 protein levels are reduced by ovariectomy, induced by combined treatment with estrogen and progesterone, and vary considerably throughout the estrous cycle. These changes occur without similar changes in mRNA levels, suggesting posttranscriptional control. Mammary glands from IRS-1 null mice have smaller fat pads than wild-type controls, but this reduction is proportional to the overall reduction in body size. Development of the mammary duct (terminal endbuds and branch points) is not altered by the loss of IRS-1, and pregnancy-induced proliferation is not changed. These data indicate that IRS undergo complex developmental and hormonal regulation in the mammary gland, and that IRS-1 is more likely to regulate mammary function in lactating mice than in virgin or pregnant mice.