Abstract

The SIL gene was discovered at the site of a cancer-associated interstitial deletion in which its promoter assumed the regulation of a second gene, SCL. The human SIL gene encodes a 1287-amino acid cytosolic protein that has been found to be highly conserved in the mouse. SIL is expressed in proliferating cells and is down-regulated when cellular proliferation ceases because of serum starvation, contact inhibition, or induction of terminal differentiation. SIL is induced within 1 h of stimulation by 20% serum in growth-arrested 3T3 cells. This induction is independent of protein synthesis because "superinduction" is observed in the presence of the protein synthesis inhibitor cyclohexamide. Thus, SIL is an immediate-early gene. Upon release from serum starvation of 3T3 fibroblasts, SIL mRNA and protein levels display a biphasic pattern during the first cell cycle. In contrast, in exponentially growing EL4 lymphoblasts, SIL mRNA is stable throughout the cell cycle, whereas SIL protein accumulates into G2 phase and then falls precipitously at the completion of the cell cycle. This pattern of cell cycle expression suggests that SIL may play an important role in cellular growth and proliferation.