Interleukin-6 Mediates G₀/G₁ Growth Arrest in Hepatocellular Carcinoma Through a STAT 3-Dependent Pathway

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 demonstrates 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.

Key Words: hepatocellular carcinoma, interleukin-6, mitogenesis, growth arrest, cyclin-dependent kinases