Constitutive expression of hyaluronan binding protein 1 (HABP1/p32/gC1qR) in normal fibroblast cells perturbs its growth characteristics and induces apoptosis.

Hyaluronan binding protein 1 (HABP1) is a ubiquitously expressed multifunctional phospho-protein that interacts with a wide range of ligands and is implicated in cell signalling. Recently, we have reported that HABP1 is an endogenous substrate for MAP kinase and upon mitogenic stimulation it is translocated to the nucleus in a MAP kinase-dependent manner (Biochem. Biophys. Res. Commun. 291(4) (2002) 829-837). This prompted us to investigate the role of HABP1 in cell growth or otherwise in low MAP kinase background. We demonstrate that HABP1, when overexpressed in normal rat skin fibroblasts, remained in the cytosol, primarily concentrated around the nuclear periphery. However, HABP1 overexpressing cells showed extensive vacuolation and reduced growth rate, which was corrected by frequent medium replenishment. Further investigation revealed that HABP1 overexpressing cells undergo apoptosis, as detected by TUNEL assay, induction of Bax expression, and FACS analysis, and they failed to enter into the S-phase. Periodic medium supplementation prevented these cells from undergoing apoptotic death. We also demonstrate that upon induction of apoptosis in HeLa cells by cisplatin, HABP1 level is upregulated, indicating a correlation between HABP1 and cell death in a normal cellular environment.