Taniya Sarkar*, Wei Zhao*, and Nurul H. Sarkar*
*Medical College of Georgia, Department of Immunology and Microbiology,
Augusta, GA 30912-2400.
Correspondence should be addressed to: Nurul H. Sarkar, PhD.
Submitted for publication: August 1995
Keywords: Breast cancer, oncogene, jun , human, rodent
The discovery that normal cells contain genes which have the ability to induce tumors marks a
turning point in cancer research (1). These cancer causing genes are called protooncogenes or
oncogenes. To date, more than 30 oncogenes have been discovered, and it is believed that there
are more oncogenes yet to be identified. Although most of the presently known oncogenes have
been found to be associated with a variety of animal tumors, formal demonstration of the
involvement of many of these genes in tumorigenesis is lacking. However, numerous
results now suggest that alterations in either the structure, copy number, or expression of these
genes lead to the transformation of a variety of normal cells including mammary cells to malignant
tumor cells (2).
The development of spontaneous mammary tumors in mice is frequently associated with
chronic infection of host mammary tissue by a retrovirus called mouse mammary tumor virus
(MMTV) which induces mammary tumors by acting as an insertional mutagen within the mammary
epthelial cells. The expression of at least three cellular genes, int -1 (also designated by Wnt- 1;
3), int - 2 (Fgf- 3; 4), and int -3 (5), which are not normally expressed in the mammary gland, are
activated in mammary tumors as a consequence of the integration of an MMTV proviral genome
into flanking cellular DNA sequences (5-8). One of the oncogenes, int -2 has been implicated in
human breast cancer because it is amplified in breast tumor tissues but not in normal tissues (9). In
addition, a number of other oncogenes, such as erb B2 (also known as c-neu or HER -2), and
p53, have also been implicated in human breast carcinogenesis (10-16). It is of interest to note that
many oncogenes are believed to act singly or in combination with other oncogenes in inducing
different types of tumors in different hosts (17). For an example, erb B2 was initially isolated
from rat neuroglioblastomas (18), but now it has been implicated in both human breast cancer and
ovarian cancer (19,20). Similarly the ras oncogene that was initially cloned from a human bladder
carcinoma (21) has now been shown to be associated with a variety of tumors including mammary
tumors in rats (22,23). It is possible, therefore, that human breast carcinogenesis may result from
the activaton of a number of the presently known oncogenes, including the BCR I gene (for review
see 24), as well as other oncogenes yet to be identified.
Having established that both mammary tumors and normal mammary tissue expresses jun C,
an attempt was made to determine if they also express jun B, a closely related member of the jun
family. As shown in figures 2 and 3, indeed both mammary tumors and normal mammary glands of
mice express high levels of jun B (also see Table I). Interestingly, however, lactating mammary
glands of mice seem to express a higher level of jun B compared to non-lactating mammary glands.
This observation is similar to what was observed for jun C. Additionally, the level of jun
B RNA in mammary tussues of MMTV-negative mouse strains, BALB/c and C57BL, is low, but
following MMTV infection the mammary glands of these mice (BALB/cfC3H and C57BLfGR)
appeared to express more of the jun B transcripts.
In contrast to the above observations, the normal mammary glands of WLCO mice, derived
by breeding from a pair of wild mice, which neither carry endogenous/exogenous MMTV nor
develop mammary tumors were found to express the lowest amount of jun B RNA (at least 20-30
fold less than that expressed by several conventional laboratory strains of mice). On the other
hand, mammary tumors of another wild-mouse-derived strain of mice (JYG), infected with MMTV
(D. Morris, personal communication) expressed jun B in abundance
(Fig. 4). Studies of the
expression of jun B in rat tissue showed that both mammary tumors and mammary glands
contained jun B RNA. The levels of RNA, however, were found to be slightly higher in
mammary tumors (1.2-1.7x) than in normal tissue.
In view of the notion that a number of oncogenes, are involved in human mammary
tumorigenesis, we determined whether or not human breast tumors express jun .. Total cellular
RNA from human breast tumors were isolated, size fractionated by agarose gel electrophoresis,
transferred to filters and hybridized with jun B probe. Our results show that all of the five tumors
tested expressed the 2.1 kb jun B (Fig. 5, Panel A) transcript. In addition, the jun B probe, used
in the present study, detected two other species of RNA, 1.8 kb and 1.5 kb in sizes, in 4 of the
5 human tumors. This observation is unique in that none of the several normal or cancerous
tissues from mouse and rats that we analyzed showed the presence of the 1.8 kb and 1.5 kb jun B
specific transcripts. It should be noted that no previous studies have reported these transcripts in
any other tissues.
The results presented in this report show that mammary tissues from mouse, rat, and human
contain constitutive levels of jun B transcripts. This is not surprising since in a previous survey
that did not include mammary tissue, jun B was found to be present in a variety of mouse tissues
(28, 44, 45). What is interesting is that lactating mammary glands and mammary tumors of
certain strains of mice, and the mammary tumors of rats and humans were found to contain higher
levels of jun B. In addition, our results also show correspondence between the levels of the
expression of jun B and MMTV. Analyses of the human breast tumors also revealed an interesting
finding: 4 of the 5 tumors tested contained two additional 1.5 and 1.8 kb jun B transcripts in
addition to the known 2.1 transcript.
Thanks are due to Dr. A. Hossain of the Medical College of Georgia for his help and advice.
This work was partly supported by grants from the National Institutes of Health (CA- 45127),
American Institute for Cancer Research (92B31), and the American Cancer Society (BC- 494).