A steroid-binding protein mediates estrogen-dependent inhibition of growthof MCF-7 breast cancer cells

We have described two different proteins that mediate MCF-7 cell growth inh ibition (1). One is estrogen-dependent and is a heterodimer consisting of a heavy (54 kDa) and light (29 kDa) chain. This protein is distinct from cor tisol-binding globulin (CBG), sex hormone-binding protein (SBP or SHBG) and albumin (HSA). It may be fetal steroid binding protein (FSBP). The other i s not estrogen dependent, but may be related to the estrogen-dependent prot ein. Since these are large proteins they must be acting at the level of the plasma membrane not traditional intracellular estrogen receptors, and inhi bit MCF-7 cell growth.