Expression of p21(Waf1/Cip1) and cyclin D1 is increased in butyrate-resistant HeLa cells

Sodium butyrate induced cell cycle arrest in mammalian cells through an inc rease in p21(Waf1/Cip1), although another study showed that this arrest is related to pRB signaling. We isolated variants of HeLa cells adapted to gro wth in 5 mM butyrate. One of these variants, clone 5.1, constitutively expr essed elevated levels of p21(Waf1/Cip1) when incubated in regular growth me dium and in the presence of butyrate. Despite this elevated level of p21(Wa f1/Cip1), the cells continue to proliferate, albeit at a slower rate than p arental HeLa cells. Western blot analyses showed that other cell cycle regu latory proteins were not up-regulated to compensate for the elevated expres sion of p21(Waf1/Cip1). However, cyclin D1 was downregulated by butyrate in HeLa cells but not in clone 5.1. We conclude that continued expression of cyclin D1 allowed clone 5.1 to grow in the presence of butyrate and elevate d levels of p21(Waf1/Cip1).