Insulin-like growth factor binding protein-3 (IGFBP-3), an antiproliferative and proapoptotic
protein, has been shown to be upregulated by growth inhibitory concentrations of
androgens in LNCaP human
prostate cancer (PCa) cells, but the mechanism of regulation and the role of
IGFBP-3 in the modulation of PCa cell proliferation are unknown. In this study, we have examined the effects of a range of concentrations of the
synthetic androgen R1881 on
IGFBP-3 expression and cell growth in LNCaP cells. We have also investigated the role of
androgen-stimulated
IGFBP-3 in
androgen-induced growth inhibition. We show that low doses of
R1881 stimulate LNCaP cell proliferation, but do not induce
IGFBP-3 expression, whereas high doses of
R1881 that inhibit cell growth, significantly increase expression of
IGFBP-3. Importantly, we demonstrate that the combination of
calcitriol and
androgens not only synergistically upregulates
IGFBP-3 expression but also inhibits cell growth better than either
hormone alone.
siRNA knockdown of
IGFBP-3 expression partially reverses the growth inhibition by
calcitriol and by
androgens. Furthermore, we find that the growth inhibitory dose of
R1881 leads to increases in the
cyclin dependent kinase inhibitors (CDKIs), p21 and p27 as well as to G1 arrest. These changes can be blocked or partially reversed by
IGFBP-3 siRNA, indicating that the induction of CDKIs is downstream of
IGFBP-3. Our data suggest, for the first time, that
IGFBP-3 is involved in the antiproliferative action of high doses of
androgens partly through p21 and p27 pathways and that
IGFBP-3 may contribute significantly to
androgen-induced changes in LNCaP cell growth.