Down-regulation of insulin-like growth factor binding proteins and growth modulation in hepatoma cells by retinoic acid

We observed that all-trans-retinoic acid (RA) down-regulated insulin-like g rowth factor binding proteins (IGFBPs) in cultured human hepatoma cells (He p 3B, PLC/PRF/5, and Hep G2); therefore, we characterized the role of this down-regulation in cell growth, Treatment with 10 mu mol/L RA revealed a ra pid decrease in IGFBP-3 within 2 days, and continued treatment with RA for 6 days resulted in a time-dependent stimulation of Hep 3B cell growth. Howe ver, RA treatment decreased IGFBP-1 in PLC/PRF/5 cells and in Hep G2 cells, and the growth-stimulatory activity of RA. was transient and less prominen t, and was finally obliterated in both cell lines. The addition of 5 ng/mL or 50 ng/mL insulin-like growth factors (IGFs) did not change the growth ef fects elicited by RA, The addition of IGFBP-3 (1,000 ng/mL) inhibited the g rowth of Hep 3B cells and counteracted the growth-stimulatory activity of R A, but not completely, suggesting that RA has direct growth-stimulatory act ivity and that this is enhanced by autocrine down-regulation of IGFBP-3. IG FBP-3 also inhibited the growth of PLC/PRF/5 cells and of Hep G2 cells. Tre atment with phosphorylated IGFBP-1 (1,000 ng/mL) alone or with RA did not a ffect the growth of PLC/PRF/5 cells or Hep G2 cells, However, addition of d ephosphorylated IGFBP-1, derived from in vivo dephosphorylation of the phos phorylated form, stimulated the growth of both cell lines, independent of i nteraction with IGF-I, From these observations, we propose that RA down-reg ulates IGFBPs, which in turn causes autocrine modulation of cell growth ind ependent of IGF in hepatoma cells in vitro or in vivo. In addition, RA regu lates IGFBPs at the posttranscriptional (Hep 3B cells and Hep G2 cells) or transcriptional level (PLC/PRF/5 cells) in a cell-specific manner.