TITLE:
Signal Transduction Pathways Mediated by Secreted and Non-Secreted Forms of Intact Insulin-Like Growth Factor Binding Protein-3 (IGFBP-3) and Its 1-97 N-Terminal Fragment in PC-3 Human Prostate Cancer Cells
AUTHORS:
Hanief Mohammad Shahjee, Benjamin Kefas, Nisan Bhattacharyya, Mohamed K. Radwan
KEYWORDS:
N-Terminal Fragment; Apoptosis; Caspases; Human Prostate Cancer Cells
JOURNAL NAME:
Journal of Cancer Therapy,
Vol.4 No.8,
September
18,
2013
ABSTRACT: Our previous
results indicated that both the secreted and the intracellular form of full
length and 1-97 N-terminal fragment of IGFBP-3 induce apoptosis in PC-3 human
prostate cancer cells in an IGF-dependent and independent manner. This study
was undertaken to delineate possible down-stream signaling pathways that are
involved in this process. Intact IGFBP-3 and its N-terminal 1-97 fragments with
or without a signal propeptide were fused to YFP and expressed in PC-3 human
prostate cancer cells. In some cases, the putative IGF-binding site was presented
in full length IGFBP-3 and its N-terminal fragment was also mutated. Extent of
apoptosis was quantified using FACS. Up-regulation of total Stat-1 and
activation of phospho-Stat-1 were shown by western blot. TGF-β signal was measured by luciferase
reporter assay. Results from inhibitor studies indicated that both the Caspase
8 and caspase 9 pathways are involved in IGFBP-3 (non-secreted form) which
induced apoptosis in PC-3 cells. Exogenous addition of IGFBP-3 to PC-3 cells increased
Stat-1 protein expression/tyrosine phosphorylation.
Interestingly, results also showed that knockdown of Stat-1 by siRNA potentiated
the IGFBP-3 induced apoptosis in PC-3 cells. In addition, both full-length
IGFBP-3 and its 1-97 Nterminal fragments inhibited TGF-β signaling in these cells. This is the first report that compares
the signal transduction pathways involved in apoptotic pathways mediated by
IGFBP-3 in PC-3 human prostate cancer cells. Non-secreted form of full length
IGFBP-3 and its N-terminal fragments induced apoptosis in PC-3 cells via
activation of caspase 8 and caspase 9. Although, only non-secreted form of
IGFBP-3 is involved in inducing apoptosis in PC-3 cells via caspase 8 and
caspase 9 activation pathways but both secreted and non-secreted forms of
IGFBP-3 are involved in modulating Stat-1 and TGF-β pathways to induce apoptotic actions in PC-3 cells. Non-secreted
intact IGFBP-3 and its N-terminal fragments induced apoptosis in PC-3 cells via
activation of caspase 8 and caspase 9 pathways. Modulation in STAT-1 and TGF-β pathways may also be important for
IGFBP-3 induced apoptosis in PC-3 cells in general. These studies clearly
demonstrate that secreted and non-secreted FL and 1-97 N-terminal fragments
induce apoptosis in PC-3 cells by regulating different mechanistic pathways.