LATENT TRANSFORMING-GROWTH-FACTOR BETA-1 ACTIVATION IN-SITU - QUANTITATIVE AND FUNCTIONAL EVIDENCE AFTER LOW-DOSE GAMMA-IRRADIATION

The biological activity of transforming growth factor beta 1 (TCF-beta ) is controlled by its secretion as a latent complex in which it is no ncovalently associated with latency-associated peptide (LAP). Activati on is the extracellular process in which TGF-beta is released from LAP , and is considered to be a primary regulatory control, We recently re ported rapid and persistent changes in TGF-beta immunoreactivity in co njunction with extracellular matrix remodeling in gamma-irradiated mou se mammary gland, Our hypothesis is that these specific changes in imm unoreactivity are indicative of latent TGF-beta activation, In the pre sent study, we determined the radiation dose response and tested wheth er a functional relationship exists between radiation-induced TGF-beta and collagen type III remodeling, After radiation exposures as low as 0.1 Gy, we detected increased TGF-beta immunoreactivity in the mammar y epithelium concomitant with decreased LAP immunostaining, which are events consistent with activation, Quantitative image analysis demonst rated a significant (P=0.0005) response at 0.1 Gy without an apparent threshold and a linear dose response to 5 Gy, However, in the adipose stroma, loss of LAP demonstrated a qualitative threshold at 0.5 Gy, Lo ss of LAP paralleled induction of collagen III immunoreactivity in thi s tissue compartment, We tested whether TGF-beta mediates collagen III expression by treating animals with TGF-beta panspecific monoclonal a ntibody, 1D11.16, administered i.p. shortly before irradiation, Radiat ion-induced collagen III staining in the adipose stroma was blocked in an antibody dose-dependent manner, which persisted through 7 days pos tirradiation, RNase protection assay revealed that radiation-induced e levation of total gland collagen III mRNA was also blocked by neutrali zing antibody treatment, These data provide functional confirmation of the hypothesis that radiation exposure leads to latent TGF-beta activ ation, support our interpretation of the reciprocal shift in immunorea ctivity as evidence of activation, and implicate TGF-beta as a mediato r of tissue response to ionizing radiation, The sensitivity of activat ion to low radiation doses points to a potential role for TGF-beta in orchestrating tissue response to oxidative stress, As such, radiation may be useful as a probe to delineate the consequences of latent TGF-b eta 1 activation in situ.