Endogenous IGFBP-3 is required for both growth factor-stimulated cell proliferation and cytokine-induced apoptosis in mammary epithelial cells.

TNF-alpha and IGF-I exert opposing effects on mammary epithelial cell (MEC) growth and survival. However, both increase IGF binding protein-3 (IGFBP-3) expression, a multifunctional protein that plays both IGF-dependent as well as independent roles in these processes. We have reported that IGF-I utilizes the PI3-K and MAPK pathways to induce IGFBP-3 expression in bovine MEC. Here we show that TNF-alpha requires the SAPK pathway p38, but not JNK, to induce IGFBP-3 expression. Contrary to reports in cancer cell lines, TNF-alpha retained its ability to decrease DNA synthesis in cells transfected with IGFBP-3 siRNA. It also retained its ability to inhibit IGF-I-stimulated DNA synthesis in these cells. In contrast, the ability of IGF-I to increase DNA synthesis was attenuated with IGFBP-3 knockdown. IGFBP-3 knockdown also decreased basal DNA synthesis, indicating that a certain level of IGFBP-3 may be required for cell proliferation. While TNF-alpha alone failed to induce apoptosis, it increased cell death when added with the JNK agonist anisomycin (ANS). TNF-alpha and ANS were unable to induce apoptosis when either IGFBP-3 or JNK-2 was knocked-down, suggesting that both JNK and IGFBP-3 may interact with a downstream molecule central to apoptosis. There are reports that IGFBP-3 promotes either cell proliferation or apoptosis in different cell systems. However, this is the first report that endogenous IGFBP-3 is required for the action of both stimulatory and inhibitory factors within the same cell line. Therefore, the actions of IGFBP-3 are not pre-determined, but instead governed by cellular context such as JNK activation.

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