Phorbol ester regulation of terminal deoxynucleotidyl transferase, proliferation, and TcR alpha in a pre-T cell line

Protein kinase C is not necessary for transforming growth factor beta-induced growth-arrest in leukemia cell lines

Growth and differentiation of blood cell precursors are regulated by cytokines and hormones by mechanisms which are incompletely understood. Protein kinase C (PKC) isozymes are widely regarded as being important in signal transduction pathways. We have shown that one isozyme, PKC beta, is uniquely important in mediating phorbol ester-induced growth-arrest in the HL-60 myeloid cell line. 1,25-dihydroxyvitamin D3 induces differentiation and growth-arrest in many cells. It upregulates the expression of PKC beta, potentiating the action of phorbol ester. We tested the hypotheses that cytokines, which arrest the growth of hematopoietic cells, do so by activating PKC beta, and that differentiation and growth-arrest induced by 1,25-dihydroxyvitamin D3 is caused by upregulation of PKC beta isozyme gene expression. The influence on growth of combinations of five cytokines (TNF alpha, TGF beta 1, gamma-IFN, IL-1, and G-CSF) and 1,25-dihydroxyvitamin D3 on ten human leukemia cell lines (THP-1, HL-60 S, HL-60 PET, U937, K562, Jurkat, MOLT-4, RPM1 8402, KG-1, and KG-1a) was determined. Four cell lines (THP-1, HL-60 S and PET, and U937) exhibited total growth-arrest when incubated with 1,25-dihydroxyvitamin D3 followed by TGF beta 1. The expression by each cell line of mRNA encoding PKC alpha, beta, and delta, both before and after 24 or 48 h of incubation with 1,25-dihydroxyvitamin D3, was determined. Cell lines sensitive to TGF beta 1 each expressed PKC delta endogenously, or expression was up-regulated with 1,25-dihydroxyvitamin D3. U937 cells underexpressed PKC alpha, and HL-60 PET cells underexpressed PKC beta. These data suggested that PKC delta could be responsible for mediating growth-arrest by TGF beta 1. To test this hypothesis directly, we incubated the cells with two bisindolylmaleimide PKC inhibitors during the addition of 1,25-dihydroxyvitamin D3 and TGF beta 1. Surprisingly, the PKC inhibitors did not block the growth-arrest induced by 1,25-dihydroxyvitamin D3 and TGF beta 1. This experiment strongly suggests that neither growth-arrest induced by TGF beta 1 nor the potentiation of this growth-arrest by 1,25-dihydroxyvitamin D3 is mediated by a PKC isozyme which is inhibitable by the bisindolymaleimides.

Expression of diverse and functional TCR gamma and Ig heavy chain transcripts in fetal liver cells cultured with interleukin-7

We have previously shown that specific T cell receptor (TCR) gamma V regions genes (V gamma 4 and V gamma 6) are rearranged and expressed by murine fetal liver (FL) cells cultured with IL-7. The present studies determined that the sequences of the TCR V region gene transcripts expressed in response to IL-7 included diverse and functional sequences expressed by thymocyte and peripheral V gamma 4+ and V gamma 6+ T cells, indicating that the IL-7-induced expression of these genes is functionally relevant and mimics normal in vivo developmental events of gamma delta T cells. We found that more than 50% of these TCR transcripts had N region diversity. The presence of N region diversity indicates that these TCR rearrangements took place in vitro, presumably in response to IL-7, because fresh (uncultured) FL cells do not produce detectable terminal deoxynucleotidyl transferase (TdT) mRNA or protein. We also found that 100% of immunoglobulin (Ig) VH7183-JH4 transcripts from FL cells cultured with IL-7 had N region diversity at the V-DJ region, while only 40% of Ig VH7183-JH4 transcripts from FL cells cultured in the absence of IL-7 had N region diversity at this region. FL cell cultures supplemented for 7 days with IL-7 had increased TdT mRNA and protein levels. However, since 1-day culture of FL cells with or without IL-7 resulted in induction of expression of TdT, IL-7 probably does not directly stimulate TdT expression, but increases the development and expansion of TdT+ lymphoid cells. These findings implicate IL-7 as a regulator of the molecular signals involved in controlling TCR gamma rearrangement and diversity, and provide an in vitro system for studying the regulation of TdT and N region diversity in B and T lymphoid progenitors by environmental signals.

Expression of the V(D)J recombinase gene RAG-1 is tightly regulated and involves both transcriptional and post-transcriptional controls

The V(D)J recombinase activating genes, RAG-1 and RAG-2, are coexpressed only in immature lymphocytes, and are sufficient and necessary for V(D)J recombination to occur in non-lymphoid cells. In order to examine control mechanisms operative in the regulation of RAG-1 and RAG-2, we have studied the pattern of expression of these genes in human pre-T cells, pre-B cells, and thymocytes treated with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA); an agent which mimics some of the lymphocyte maturation changes seen in vivo. The expression of RAG-1 and RAG-2 was tightly controlled in a rapid, yet very complex, manner with both positive and negative control elements operating. Treatment of immature lymphocytes with TPA caused the specific and rapid elimination of steady-state RAG-1 and RAG-2 RNA. Nuclear run-on assays showed that TPA completely repressed the transcription of RAG-1 within 30 min. In addition to repressing the transcription of RAG-1, TPA treatment caused the rapid and specific degradation of RAG-1 transcripts by decreasing the apparent half-life of RAG-1 mRNA more than two-fold. As judged by cycloheximide treatment of cells, the effects of TPA were not dependent on new protein synthesis. A labile transcriptional repressor, separate from the TPA-associated repression of transcription, was also active in cells transcribing RAG-1 and RAG-2 RNA. After depletion of this labile repressor by cycloheximide treatment, steady-state RAG-1 and RAG-2 RNA levels, and their transcription rates, were elevated four- to six-fold; but were still susceptible to elimination by TPA treatment. Treatment of pre-T CEM cells with interleukin-2, or theophylline (an agent that increases intracellular cAMP) resulted in a two-fold increase in RAG-1 RNA suggesting that lymphokines, either independently or through second messengers, may modulate RAG-1 and RAG-2 expression. The complex, rapid and precise regulation of RAG-1 and RAG-2 expression is consistent with the view that it is necessary for the cell to tightly regulate V(D)J recombinase levels; lower expression may result in inefficient recombination of Ig/TCR genes, whereas increased expression may lead to recombination errors that are deleterious to the cell.