Cells undergoing apoptosis are characterized by decreased cell size due to changes in intracellular ion concentration and rapid, aquaporin (AQP)-dependent water movement out of the cell, events required for the activation of pro-apoptotic enzymes. The current study demonstrates AQP 8 and 9 expression is significantly decreased in hepatocellular carcinoma (HCC) versus normal liver. Isolation of hepatic tumor cells (H4IIE) and hepatocytes confirmed a lack of water movement across the H4IIE cell membrane via AQPs and identified an inherent resistance of H4IIE cells to apoptotic stimuli. In contrast, hepatocytes rapidly responded to osmotic challenge through AQP-dependent water movement and underwent cell death following apoptotic stimulation. IINDEXING TERMS (KEY WORDS)

Interleukin-6 (IL-6) is a pleiotropic cytokine that regulates diverse cell functions including proliferation and differentiation. Within the liver IL-6 signaling plays a central role during normal hepatic growth and regeneration yet can inhibit the proliferation of hepatocellular carcinoma (HCC) cells. The aim of the current study was to identify underlying mechanisms whereby IL-6 induces cell cycle arrest in HCC cells. These studies demonstrate that IL-6 inhibits cell cycle progression at the G0/G1 interface through inhibition of cyclin dependent kinase (cdk) 2 and cdk4 activity in the absence of changes in total cyclin (A, D1, D3 and E) or cdk (cdk2, 4 and cdc2 p34) expression. Inhibition of signal transduction pathways associated with IL-6 receptor activation demonstrate that IL-6-dependent inhibition of G0-G1 progression occurs via Janus tyrosine kinase-signal transducers and activators of transcription-3 (Jak-STAT3)-dependent induction of p21 waf1/cip1 and is independent of ERK-MAPK signaling. These data demonstrate that, while IL-6 plays a central role in hepatocyte priming and proliferation in vivo, the pronounced inhibition of proliferation observed in HCC cells occurs due to IL-6-STAT3-dependent regulation of cdk2/cdk4 activity and p21 waf1/cip1 expression.