Diltiazem inhibits transferrin receptor expression and causes G1 arrest in normal and neoplastic T cells.

Abstract

Transferrin receptor expression is essential for the proliferation of both normal and malignant T cells. While transferrin receptor expression in normal T cells is tightly coupled to interleukin-2 receptor expression, transferrin receptor expression in malignant cells is usually constitutive and is released from this constraint. Temporally, the appearance of these membrane receptors is preceded by changes in the expression of the proto-oncogenes c-myc and c-myb. In addition, although an increase in the level of intracellular free calcium occurs early in the sequence of T-cell activation, the activation events dependent on this calcium flux have not been resolved. In the present study we report that diltiazem, an ion channel-blocking agent that inhibits calcium influx, arrested the growth in vitro of both normal and malignant human T cells in the Gl phase of the cell cycle. However, diltiazem did not inhibit the expression of c-myc or interleukin-2 receptor mRNA and protein in normal mitogen-activated T cells or the constitutive expression of c-myc and c-myb mRNA in malignant T cells (T acute lymphoblastic leukemia cells). In contrast, diltiazem prevented the induction of transferrin receptor (mRNA and protein) in normal T cells and caused a progressive loss of transferrin receptor (mRNA and protein) in malignant T cells. These data demonstrate that diltiazem can dissociate several growth-related processes normally occurring in Gl and thereby disrupt the biochemical cascade leading to cell proliferation.

Regulation of murine plasmacytoma transferrin receptor expression and G1 traversal by plasmacytoma cell growthfactor

“…Furthermore, diltiazem (250 p M ) was able to inhibit TfR expression in a PCT-GF-independent cell line (M460D) as well (Table 1). These results are in agreement with recent findings that diltiazem effectively inhibits constitutive TfR expression in IL-2-independent T cell lines (Neckers et al, 1986a).…”

“…Thus, diltiazem prevents PCT-GF from maintaining TfR expression and cell growth and has the same effect on PCT-GF-independent PCTs. Analogous results have been reported studying IL-2dependent and -independent T cell lines (Neckers et al, 1986a). It would appear that functional calcium channels are necessary for TfR expression to be maintained in growth factor-dependent as well as -independent cells.…”

“…Given the fact that the half-life of the TfR protein is about 10 h r (Mattia et al, 1984;Neckers et al, 1986a) and that the total length of G, in these cells is also about 10 hr, the latter explanation would not appear likely. However, we cannot state categorically that the turnover time of the TfR is constant throughout G1.…”

“…Since we recently reported that treatment with diltiazem prevented IL-2 from inducing TfR expression in T cells (Neckers et al, 1986a), we studied whether the calcium channel blocker would have a similar effect on PCT-GF-dependent PCTs. We examined surface TfR expression by flow cytometric analysis and found that addition of diltiazem (250 pM) reduced TfR expression in the presence of growth factor as effectively as removal of the growth factor itself (Table 1).…”

Transcriptional inactivation of c-myc and the transferrin receptor in dibutyryl cyclic AMP-treated HL-60 cells.

“…Expression of the proto-oncogene c-myc, like TfR, has been proposed to be essential for cell growth and particularly for traversal of the G1 phase of the cell cycle (3,14,19 30 min in paraformaldehyde, the cells underwent three 5-min washes in Tris-buffered saline containing 1% normal goat serum and were stained with either affinitypurified rabbit anti-human c-myc immunoglobulin (45) (5 ,ug per slide) or normal rabbit immunoglobulin. The slides were then processed as described by Hsu et al (18).…”