Mitogenic growth factors in breast fluid obtained from healthy women: evaluation of biological and extraneous sources of variability.
Peptide growth factors (GFs), including epidermal GF (EGF) and transforming GF-alpha (TGF-alpha), are presumed to play an important role in the local regulation of breast cell proliferation. Breast fluid collected by nipple aspiration provides a potential means to assess the concentration of these factors in contact with the ductal epithelium. Although identification of immunoreactive EGF-like GFs in breast fluid has been reported previously, we performed this study to evaluate the sensitivity and reliability of newer RIA methods and to characterize the sources and amounts of both intra- and intersubject variability. We also evaluated the relationship of breast fluid EGF and TGF-alpha levels to each other and to plasma levels of estradiol and progesterone. Breast fluid and plasma samples were obtained two to four times at weekly intervals from 18 healthy, premenopausal women. EGF and TGF-alpha were measured by competitive binding RIA. Both GFs were detected with good precision in all breast fluid samples analyzed, using dilutions as low as 1:100 for EGF (1 microliter) and 1:25 for TGF-alpha (4 microliters). The correlations between the right and left breasts, sampled concurrently, were r = 0.78 (P = 0.003) for EGF and r = 0.89 (P = 0.0001) for TGF-alpha. For both GFs, the variation between women was substantially greater than the variation between breasts or over time in an individual woman, particularly for EGF, for which there were 100-fold differences between women in mean levels. When samples from multiple women were analyzed together, we found no apparent relationships between EGF and TGF-alpha levels or between either GF level and menstrual cycle phase or plasma hormone concentrations. However, in random effects analyses, EGF levels within an individual were significantly associated overall with both TGF-alpha (P = 0.02) and plasma estradiol levels (P = 0.01). These data, which are the first comprehensive results on the feasibility of measuring mitogenic GFs in breast fluid, support the conclusion that women secrete consistent and individually distinct levels of EGF and TGF-alpha and that, in at least some women, EGF secretion in vivo covaries with both TGF-alpha in breast fluid and circulating estradiol.