Introduction: The hyperplastic enlarged lobular unit (HELU) is a common alteration in adult female human breast and is the earliest histologically identifiable lesion with premalignant potential. Growth and differentiation in normal epithelium are regulated by estrogen and progesterone, whose effects are mediated through estrogen receptor (ER)-alpha and progesterone receptor (PR). We assessed correlations between growth (proliferation and apoptosis), endogenous hormone levels (using age as a surrogate for menopausal/estrogen status), and ER-alpha/PR expression in HELUs versus adjacent normal terminal duct lobular units (TDLUs) to gain insight into potentially premalignant hyperplasia.
Methods: Proliferation (Ki67 antigen), ER-alpha, and PR were assessed by immunohistochemistry, apoptosis using the TUNEL (terminal transferase-mediated dUTP nick end-labeling) assay, and nuclear colocalization of ER-alpha and Ki67 by dual-labeled immunofluorescence in HELUs and adjacent TDLUs (n = 100-584, depending on the factor) from 324 breasts. All factors were quantified under direct microscopic visualization. ER-alpha/PR expression was semiquantified by estimating the proportion of positive cells (0 = none, 1 = or <1/100, 2 = 1/100 to 1/10, 3 = 1/10 to 1/3, 4 = 1/3 to 2/3, and 5 = or >2/3). Ki67, TUNEL, and colocalization of ER-alpha and Ki67 were scored by absolute counting (%positive).
Results: ER-alpha and PR expression were significantly elevated in HELUs versus adjacent TLDUs (average score: 4.5 versus 3.1 and 3.5 versus 2.1; P < 0.0001). Proliferation was also significantly higher in HELUs versus TDLUs (average 6.3% versus 2.0%; P < 0.0001). In contrast, apoptosis was significantly lower in HELUs versus TDLUs (average 0.61% versus 0.22%; P < 0.0001). Changes in proliferation and receptor expression were similar between premenopausal and postmenopausal TDLUs and HELUs, suggesting that hyperplastic cells remain responsive to regulation by estrogen. The proportion of ER-positive/proliferating cells was much higher in HELUs than TDLUs (27.6% vs. 4.9%; P < .0001).
Conclusion: Development of HELUs is associated with increased proliferation and decreased cell death relative to normal cells. ER-alpha and PR are highly elevated in HELUs, which may contribute to the hyperplasia because they mediate hormonal regulation of growth. An understanding of the fundamental causes of increased levels of receptors and growth may lead to new strategies to prevent breast cancer.