Phorbol diesters and transferrin modulate lymphoblastoid cell transferrin receptor expression by two different mechanisms

Abstract

Expression of transferrin receptors (TfR) by activated lymphocytes is necessary for lymphocyte DNA synthesis and proliferation. Regulation of TfR expression, therefore, is a mechanism by which the lymphocyte's proliferative potential may be directed and controlled. We studied mechanisms by which lymphoblastoid cells modulate TfR expression during treatment with phorbol diesters or iron transferrin (FeTf), agents which cause downregulation of cell surface TfR. Phorbol diester-induced TfR downregulation occurred rapidly, being detectable at 2 min and reaching maximal decreases of 50% by 15 min. It was inhibited by cold but not by agents that destabilize cytoskeletal elements. Furthermore, this downregulation was reversed rapidly by washing or by treatment with the membrane interactive agent, chlorpromazine. In contrast, FeTf-induced TfR downregulation occurred slowly. Decreased expression of TfR was detectable only after 15 min and maximal downregulation was achieved after 60 min. Although FeTf-induced downregulation also was inhibited by cold, it was inhibited in addition by a group of microtubule destabilizing agents (colchicine, vinblastine, podophyllotoxin) or cytochalasin B, a microfilament inhibitor. Furthermore, FeTf-induced downregulation was not reversed readily by washing or by treatment with chlorpromazine. The inactive colchicine analogues, beta- and gamma-lumicolchicine, did not inhibit FeTf-induced TfR downregulation. Similarly, when cells were pretreated with taxol to stabilize microtubules, colchicine no longer inhibited FeTf-induced downregulation. Therefore, FeTf causes TfR downregulation in lymphoblastoid cells by a cytoskeleton-dependent mechanism. Phorbol diesters cause TfR downregulation by a cytoskeleton-independent mechanism. In other experiments, treatment of cells with both a phorbol diester and FeTf, either simultaneously or sequentially, produced additive effects on TfR expression. These data indicate that TfR expression is regulated by two independent mechanisms in lymphoblastoid cells, and they provide the possibility that downregulation of TfR by different mechanisms may result in different effects in these cells.

Disparity between expression of transferrin receptor ligand binding and non‐ligand binding domains on human lymphocytes

“…In addition, in other studies we have shown that PMA treatment of cultured T-lymphoblastoid cell lines caused rapid down-regulation of TfR. This effect was detectable within 2 min, was mediated by a cytoskeleton-independent mechanism, and was reversed by membrane interactive agents (Alcantara et al, 1986). In those experiments PMA down-regulation clearly differed from Fen-induced downregulation, which required an intact cytoskeleton, occurred more slowly, and was not readily reversed by membrane interactive agents.…”

Effects of protein kinase modulators on transferrin receptor expression in human leukaemic HL‐60 cells

“…The effects of PMA and dbcAMP on the down-regulation of TfR gene expression in leukaemic cells were shown to be mediated at transcription level [9,22]. It is noteworthy that the promoter sequence of the TfR gene contains a TRE/CRE-like element as revealed by sequence homology and DNAase footprinting [23].…”