Regulation of Thy-1 and Pgp-1 glycoproteins in hybrids between T-cell lymphomas and Abelson-leukemia-virus-induced lymphomas

Fatty acids regulate Thy-1 antigen mRNA stability in T lymphocyte precursors

In this study, we report the effect of fatty acids on the Thy-1 antigen mRNA decay. Low serum and synthetic medium culture conditions were used to demonstrate that fatty acids, which are important metabolites involved as second messengers in signal transduction, also influence the steady-state mRNA level. Detailed analysis demonstrated that polyunsaturated lipids attached to bovine serum albumin, such as linoleic, linolenic, and arachidonic acids, modulate gene expression specifically in the S1A T lymphoma cell line by inducing a 3-5-fold increase in the steady-state Thy-1 mRNA level, concomitant with a twofold increase in cell surface expression. A similar modulation was observed in the immature CD4-CD8- T cell precursors but not in mature thymocytes. Nuclear run-on and transfection experiments indicated that the observed Thy-1 mRNA level is post-transcriptionally regulated and that the presence of the coding region is sufficient for this adaptive response. A mechanism without a requirement for protein kinase C activation, but involving Ca2+ entry, could account for this difference in Thy-1 mRNA stability.

The expression of several T cell-specific and novel genes is repressed by trans-acting factors in immature T lymphoma clones

Cell surface proteins encoded by members of the immunoglobulin supergene family are sequentially expressed during T cell ontogeny. The molecular mechanisms responsible for the regulation of these surface molecules are not well understood. To investigate this issue, we used a series of well characterized T lymphoma cell clones with phenotypes characteristic of distinct stages of early thymocyte maturation. Somatic cell hybrids formed from these cell lines were employed to detect the presence of negative regulatory molecules. The expression of CD4 and CD8 was strongly repressed in hybrids formed between a CD4+ CD8+ lymphoma clone and "immature" CD4- CD8- lymphoma clones. Individual subunits of the T cell receptor (TCR)/CD3 complex displayed independent regulation in unique patterns in hybrid cells. Hybrids formed by fusing CD3+ and CD3- cells completely repressed CD3-delta mRNA expression while CD3-gamma, -epsilon, and -zeta transcripts were moderately inhibited or codominantly regulated. Similar to CD3-delta, interleukin 2R-alpha(IL-2R-alpha), and TCR-beta mRNA accumulation was trans-negatively regulated. Transcription rate measurements demonstrated that the inhibition of CD4, CD8, CD3-gamma, CD3-epsilon, TCR-beta, and IL-2R-alpha mRNA accumulation in hybrid cells was exerted, at least in part, at the transcriptional level. To test whether repressional regulation is a general feature of T cells, we examined the regulation of six novel genes which were selected solely on the basis of their differential expression between two of the cell lines used in this study. Five of the six novel gene transcripts were repressed in the somatic cell hybrids. Thus, inhibitor factors appear to play a general role in controlling T cell gene expression. The model system presented here may be useful for the identification and characterization of repressor molecules responsible for the regulation of genes expressed during T cell ontogeny.

Transcripts encoding mouse CD44 (Pgp-1, Ly-24) antigen: strain variation and induction by mitogen

The cell surface molecule CD44, thought to be a cell surface receptor for hyaluronic acid, is expressed by thymocytes and peripheral T cells. Following T cell activation with mitogens or antigens, surface expression of CD44 increases. This increase in CD44 cell surface expression has been used as a means of identifying memory T cells in vivo. In this report, using Northern analysis, we have quantitated mouse CD44 transcripts in thymocytes and a cloned T hybridoma cell line. In contrast to a recent report by Nottenburg et al. (Proc. Natl. Acad. Sci. USA 1989. 86: 8521), we show that in all instances CD44 transcripts are polyadenylated. In all cells studied, three major mRNA species of approximately 4.5, 3.5 and 1.6 kb are detected. After stimulation with phorbol 12-myristate 13-acetate (PMA), there is a de novo protein synthesis-dependent increase in the level of CD44 transcripts. The increase in CD44 transcripts precedes the increase in cell surface expression. In contrast to CD44, PMA stimulation results in a decrease in the level of Thy-1 transcripts. Thymocytes from different mouse strains vary in their level of CD44 cell surface expression and Northern analysis indicates that this mouse strain variation correlates with a corresponding difference in the level of CD44 transcripts.

The CD3 delta gene encodes multiple transcripts regulated by transcriptional and post-transcriptional mechanisms

CD3 is a multi-subunit complex of proteins noncovalently associated with the T cell receptor (TcR) for antigen. Considerable evidence indicates a role for CD3 molecules in the transduction of activation signals in T cells. The murine CD3 delta gene encodes a 0.7-kb transcript present in mature T cells. Here we report the characterization of several additional CD3 delta transcripts; two nuclear transcripts, 4-4.5 kb in size, and two predominamtly cytoplasmic transcripts of 1.5 kb and 2.5 kb. Both T lymphoma cell lines and normal thymocytes express the 1.5-kb and 2.5-kb CD3 delta transcripts. These cytoplasmic transcripts have long 3'-untranslated sequences which extend beyond the polyadenylation site of the predominant 0.7-kb transcript. The protein synthesis inhibitor cycloheximide (CHX) increases the expression of all three cytoplasmic CD3 delta transcripts, indicating that their level of expression may be regulated by a labile inhibitor protein(s). The CHX elicited increase in CD3 delta mRNA appears to result from post-transcriptional events since the rate of CD3 delta gene transcription remains constant. In contrast to CHX, the calcium ionophore A23187 increases the rate of CD3 delta gene transcription and, like CHX, also increases the level of cellular CD3 delta mRNA. The immunosuppressive agent cyclosporin A inhibits A23187-mediated stimulation of transcription, but has no effect on the CHX-mediated induction of CD3 delta mRNA. We conclude that both transcriptional and post-transcriptional mechanisms can regulate the amount of all three cytoplasmic CD3 delta transcripts.

Evidence from neuronal heterokaryons for a trans-acting factor suppressing Thy-1 expression during neuronal development

The expression of Thy-1 on developing sensory neurons cultured in vitro is strictly regulated and follows a temporal course identical to that observed in situ. We have investigated the role of trans-acting factors in this innate regulation by constructing heterokaryons between mature Thy-1.1+ expressing neurons and immature Thy-1.2- neurons. It has been observed by immunofluorescence that within 16 hr of fusion, Thy-1.1 expression is suppressed in such heterokaryons. However, this suppression is reversible, and after 3-4 days in vitro, at the time at which Thy-1.2 is normally expression, there is re-expression of the Thy-1.1 molecule producing Thy-1.1+/Thy-1.2+ co-expressing heterokaryons. As nuclear fusion does not occur, it appears that a developmentally regulated diffusible suppressor molecule is responsible for these observations.

Evidence from neuron/lymphoma heterokaryons for a common trans-acting factor suppressing Thy-1 expression

We have investigated the tissue specificity of a trans-acting regulatory factor observed to suppress Thy-1 expression during normal neuronal differentiation. Heterokaryons were constructed between Thy-1.1+ expressing mouse T cell lymphoma cells and Thy-1.2- mouse sensory neurons and their surface phenotypes determined by immunofluorescence histochemistry 16 h after fusion. Thy-1.1 expression was observed to be specifically suppressed in such heterokaryons whilst the expression of the lymphoma cell surface marker, Ly-1, was not altered. As the nuclei did not fuse in the heterokaryons, it appears that the developmentally regulated diffusible suppressor factor active in sensory neurons is not tissue specific and can down-regulate Thy-1 expression in lymphoma cells. We also report the suppression of the H-2Kk surface antigen in heterokaryons by a similar but independent regulatory mechanism.

Sequential activation and loss of the pre-B cell Thy-1 gene in T-cell X pre-B cell somatic hybrids

In somatic cell hybrids between the pseudodiploid Thy-1- Abelson-leukemia-virus-induced pre-B cell lymphoma RAW 253.1 and the Thy-1+ T-cell lymphoma, AKR1 (Thy-1+), all cells express the Thy-1 allele of the T-cell parent but most hybrid cells do not express the Thy-1 allele of the pre-B cell lymphoma parent. The Thy-1 allele of the pre-B cell parent, however, is spontaneously activated in a minor proportion of hybrid cells. By sorting for cells expressing the Thy-1 allele of the pre-B cell parent, derivative clones in which 100% of cells express both parental Thy-1 alleles can be isolated. Revertants with a phenotype identical with that of the original hybrid cell line can be isolated from these derivatives by sorting for nonexpression of the Thy-1 allele of the pre-B cell parent. These first-generation revertant cell lines have lost one copy of the Thy-1 gene derived from the pre-B cell lymphoma parent. By a further cycle of sorting, derivatives in which 100% of cells express both parental Thy-1 alleles can again be obtained. Second-generation revertants isolated by sorting these Thy-1+ hybrid cells for nonexpression of the Thy-1 allele of the pre-B cell parent no longer contain a normal copy of the pre-B cell Thy-1 allele and this surface antigen is no longer expressed by any cells in the population. These results are consistent with a mechanism that sequentially activates each copy of the Thy-1 gene derived from the pre-B cell lymphoma parent. Hybrids between the class D Thy-1- mutant, AKR1 (Thy-1- d), in which the 5' region of the Thy-1 structural gene has been deleted, and RAW 253.1 cannot be activated to express either Thy-1 allele. This result indicates that a sequence upstream of exon 2 of the active Thy-1 allele is critical for the initial activation event.

Differential control of mRNA levels for Thy-1 antigen and laminin in rat mammary epithelial and myoepithelial-like cells

Thy-1 antigen and laminin are two components often associated with the basement membrane of the rat mammary gland and are thought to be synthesized, at least in part, by the adjacent myoepithelial cells in vivo. The relative levels of Thy-1 mRNA and laminin mRNA are compared in a rat mammary cuboidal epithelial cell line and a derivative elongated myoepithelial-like cell line by hybridizing cloned cDNAs to cellular mRNA isolated from these cell types. Although the elongated myoepithelial-like cells synthesize four times as much laminin protein as the cuboidal epithelial cells, there is only a 1.7-fold increase in laminin mRNA between the two cell types. In contrast the 17-fold increase in Thy-1 antigen between the elongated cells and the cuboidal cells can be accounted for completely by a 14-18-fold increase in Thy-1 mRNA, suggesting that changes in the steady-state levels of Thy-1 mRNA in these cell lines are modulated at either a transcriptional or a post-transcriptional level. Run-off transcription by nuclei isolated from the cell lines does not distinguish between these two possibilities. The comparative results on Thy-1 antigen and laminin show that the enhanced production of two proteins often associated with the basement membrane of the rat mammary gland can be controlled at different levels in the elongated myoepithelial-like cells.

A Thy-1- mutant defining a gene acting in trans position to regulate cell-surface Thy-1 glycoprotein expression and Thy-1 messenger RNA content

The Thy-1 glycoprotein is a differentiation antigen which exhibits tissue-specific regulation. A mutant of a Thy-1.1+ T-cell lymphoma has been isolated which does not express Thy-1 glycoprotein on the cell surface and does not accumulate Thy-1 mRNA in the cytoplasm. Hybrids between the mutant and a Thy-1.2+ T-cell lymphoma express 20-30-fold lower levels of Thy-1 glycoprotein on their cell surface compared to wild-type T-cell lymphomas, and they have correspondingly low levels of cytoplasmic Thy-1 mRNA. A revertant of one hybrid was isolated which expressed wild-type levels of both Thy-1 alleles on its surface and contained correspondingly increased levels of Thy-1 mRNA. A Thy-1+ revertant of the Thy-1- mutant was isolated by cell sorting. A second generation Thy-1- mutant could be isolated from this revertant which also did not accumulate Thy-1 mRNA and which behaved in a way similar to the first generation mutant when hybridized to a Thy-1.2+ lymphoma. No changes in the structure or copy number of the Thy-1 structural gene could be detected in this series of mutants and revertants. These properties are consistent with a mutation in one (or more) gene(s) which acts in trans position to regulate Thy-1 glycoprotein expression.