IGFBP-6 regulates SH2D4A expression to promote breast cancer cell cycle progression in response to progesterone.

Insulin-like growth factor binding protein-6 (IGFBP-6) is induced by progesterone in breast cancer cells and regulates progesterone receptor (PR) via negative feedback. To further understand the mechanisms by which IGFBP-6 regulates PR and downstream signaling, proteomic analysis was performed in progesterone-treated T47D breast cancer cells following siRNA-mediated knockdown of IGFBP-6. Of the 8638 unique proteins identified, 29 proteins were downregulated, and 14 proteins were upregulated by progesterone treatment after IGFBP-6 knockdown. The decreased proteins were mostly identified as regulators of G2/M phase of the cell cycle. SH2D4A is induced by progesterone only when IGFBP-6 was high. To understand the role of SH2D4A in progesterone-induced signaling, SH2D4A was knocked down using siRNA prior to progesterone treatment. Knockdown of IGFBP-6 or SH2D4A leads to cell cycle arrest in G1, and both SH2D4A and IGFBP-6 regulate cyclin B. Treatment of cells with abemaciclib and nocodazole caused cell cycle arrest in G1 and G2/M, respectively, and decreased IGFBP-6, suggesting that IGFBP-6 is regulated in a cell cycle-dependent manner. These results identify a pathway linking progesterone to IGFBP-6 and SH2D4A, and to cell cycle progression in breast cancer cells.