Phorbol ester-induced surface transferrin receptor modulation. No correlation with decreased cell proliferation.

Both phorbol ester or diacylglycerol (DAG) reduce cell surface transferrin receptor (TFR) number in CEM cells (a human T-cell acute lymphoblastic leukemia line) and HL-60 cells (a human promyelocytic leukemia cell line). This effect occurs with a t1/2 of approx. 30 min and is mimicked by addition of phospholipase C to cell cultures. Although cell surface TFR number is reduced to 25-30% of the control level 5 h after phorbol ester administration, apparent cell proliferation (as measured by [3H]thymidine incorporation) remains unaffected. Although independent of extracellular calcium (EGTA is slightly enhancing), the phenomenon is completely blocked by 30-min pretreatment with the calcium channel blocker diltiazem. Dilitazem pretreatment, while preventing receptor redistribution, does not completely block the phorbol ester-induced increase in TFR phosphorylation thought to be associated with receptor redistribution. Thus, calcium channel blockade effectively dissociates the effects of tetradecanoylphorbol acetate (TPA) on TFR internalization and phosphorylation. Our results also demonstrate that phorbol ester-induced effects on the TFR can be mimicked by the endogenous stimulator of protein kinase C, DAG, whether added directly to cultures or produced by the cells in response to exogenous phospholipase C. Furthermore, the phenomenon of TFR redistribution here described is not associated with a decreased proliferative capacity.