Hematopoiesis and breast epithelial proliferation and differentiation represent processes of terminal differentiation leading to cell death in the case of hematopoiesis and of reversible differentiation and proliferation in the case of the breast epithelium. When viewed in the context of aging, each mimics aspects of cellular aging where other factors such as number of cell divisions and oxidant stress and damage are thought to limit cellular life span but nonetheless are likely to have effects through many of the same cell signaling processes. When viewed in the context of the treatment of malignant diseases, pharmacologic manipulation of signaling pathways responsible for controlling the balance between differentiation, proliferation and cell death in conjunction with chemotherapeutic agents may well provide methods to increase the specificity of conventional agents thus increasing both efficacy and reducing toxicity. Both are critical components needed to improve treatment in patients with co-morbidities most frequently associated with aging where the balance between benefit vs risk of intervention becomes increasingly problematic with age. CDNA micro-arrays are currently being utilized to examine global expression changes in estrogen responsive breast epithelial proliferation using the MCF-7 and T47D human breast tumor cell models. Multiple changes have been identified and are currently being analyzed. In addition, a critical change previously identified is being analyzed in greater depth, that of estrogen induced regulation of the transcription factor proto-oncogene c-myb. Interest in c-myb in breast epithelium and tumors derives from the finding that c-myb is expressed in more than 60% of clinical breast tumor specimens and that while expression is positively correlated with estrogen receptor (ER) and progesterone receptor (PR) status, significant numbers (approx. 30%) of ER-/PR- tumors also express myb. This laboratory has subsequently begun to examine mechanisms regulating myb expression as well as consequences of expression in both ER+ and - tumor cell lines. Recent studies indicate that: i) c-myb is expressed in all ER+ breast tumor cell lines examined to date and is also expressed in some ER-/PR- cell lines, providing models in which c-myb regulation and function can be studied; ii) myb is regulated in response to estrogen in the ER+ cell line MCF-7 following withdrawal and restimulation though a direct effect of the estrogen receptor and iii) the mechanism of regulation of myb expression is radically different from hematopoietic models where we have shown that most if not all regulation is at the level of the transcription attenuator within intron 1. In the case of both myb expressing and non-expressing breast tumors, the promoter remains active with no evidence of attenuator function based on run-on assays. Thus, c-myb would be expected to be expressed uniformly unless regulation was occurring by a post-transcriptional mechanism without precedence in hematopoietic models of myb regulation. This is also true of decreases and increases in steady state c-myb expression in response to estrogen withdrawal and restimulation in MCF-7 cells. If the regulatory mechanism involving attenuator control of c-myb expression in hematopoietic model systems examined to date represents the """"""""normal"""""""" mechanism, absence of attenuator function in breast tumor cell lines that do or do not express myb at detectable levels could indicate that an early defect in breast epithelial cell transformation may be development of a defect in attenuator function involving either cis or trans mechanisms. Beyond an interest in myb regulation, an equally important question from a biologic perspective is what effect myb expression has on breast tumor cell behavior. In the case of myb expression in ER+ cell lines, transfectants are currently being developed. The hypothesis to be tested is fairly clear in this case. It has been known for many years that one mechanism by which estrogen stimulates growth of estrogen dependent tumors is by induction of a """"""""second"""""""" wave of growth factors and their receptors including IGF-1. It is now known that a specific function of c-myb in some cell types is stimulation of IGF-1 and IGF-1R expression. Thus, constitutive expression of c-myb would be expected to make cells such as MCF-7 estrogen independent and resistant to antiestrogens such as Tamoxifen. If true, then Tamoxifen resistance could occur through any mechanism that would uncouple estrogen regulation from myb expression. Development of this breast tumor model will also provide an epithelial cell system for elucidation of transcription targets of c-myb action and whether these are lineage and/or tissue specific when compared with those of the hematopoietic system and provide an important added dimension to work examining chemotherapeutic agent effects coupled with modification of signal transduction pathways in proliferation, differentiation and cell death.
