Increased transcription of the Eμ-myc transgene and mRNA stabilisation produce only a modest elevation in Myc protein

Abstract

Mice bearing the Em-myc transgene, which links the immunoglobulin heavy chain enhancer (Em) with c-myc, are predisposed to developing B cell lymphomas. Several B lineage cell lines have been isolated from these animals, and some have been converted to macrophages following infection with v-raf. In this study we compared the regulation of myc expression in Em-myc B lymphoma lines, their macrophage counterparts and other non-myc transformed B cell lines. Nuclear run-on analyses demonstrated that transcription of the transgene was elevated in Em-myc B cell lines. Moreover, the presence of a 600 bp fX174 marker in the 3' end of the transgene produced a marked stabilisation of this RNA species. Consequently, steady state myc mRNA levels in the Emmyc B lymphoma cells were tenfold higher than the macrophage derivatives and non-myc transformed B lineage lines. Despite the considerable dierence in myc RNA levels, the Em-myc B cell lines contained only 30 ± 50% more Myc protein than the other cell lines. This discrepancy between RNA and protein content was not due to increased degradation of the protein as the half life was normal in the transgenic cell lines. These results indicate that both Em and fX174 sequences in¯uence transgenic myc expression and that protein levels do not correlate with RNA content in Em-myc cell lines.Keywords: Em-myc transgene; mRNA stabilisation; transcription; Myc protein Expression of the myc gene is subject to an elaborate array of transcriptional, post-transcriptional and translational regulatory mechanisms and perturbation of these controls has been implicated in the genesis of a wide range of tumours (Spencer and Groudine, 1991). Translocations involving immunoglobulin loci and cmyc have been identi®ed in Burkitt's lymphoma and murine plasmacytomas (Cory, 1986). To investigate the role of the immunoglobulin heavy chain enhancer (Em) in these tumours, Adams et al. (1985) generated transgenic mice coupling this enhancer to c-myc. In addition, a fragment of fX174 was introduced into the 3' untranslated region (3'UTR) of the transgene to generate a larger transcript which could easily be distinguished from the endogenous myc mRNA. Expression of Em-myc was restricted to the B lymphoid compartment and transgenic animals contained abnormally high levels of early B cells before succumbing to B lineage lymphomas Langdon et al., 1986;Langdon et al., 1988). Transgene RNA levels were ®ve to sixfold greater in Em-myc splenocytes and cell lines than c-myc in the spleens of non-transgenic littermates (Alexander et al., 1987). It was presumed that`the malignancy is a consequence of high constitutive myc expression within the lymphoid compartment, forced by enhancers which are activated by lymphoid factors . et al., 1985). Labelled transcripts were hybridised to membranes containing probes for myc (Stanton et al., 1983), Cm (Gough et al., 1980) and GAPDH (Piechaczyk et al., 1984). Annealed transcripts were detected by phosphorimager (a) and their levels expressed relative to GAPDH (b). Black histograms ...

Glucocorticoids, oxysterols, and cAMP with glucocorticoids each cause apoptosis of CEM cells and suppress c-myc

“…But much more rapidly, the oxysterol causes loss of c-Myc protein. Dissociation of regulation of the protein and its message has been reported in other systems [28][29][30][31]. CEM-C7 subclone M10R5, selected for resistance to 25-hydroxycholesterol, failed to show any change in c-myc message or protein in the face of lethal concentrations of oxysterols.…”

The Spectrum of Lymphoma with 8q24 Aberrations: A Clinical, Pathological and Cytogenetic Study of 87 Consecutive Cases

“…Although cmyc aberration is strongly implicated in these cases, detailed molecular characterizations of the breakpoint and partner gene are difficult. Over-expression of c-myc protein is very common in NHL and immunostaining does not correlate directly with c-myc translocation or prognosis [11]. Furthermore, non-classical Burkitt translocations are not covered by existing DNA Southern probes [12].…”

RNA decay defines the response to transcriptional perturbation in leukaemia

“…As short-lived transcripts are able to be more rapidly induced and cleared in comparison to stable mRNA, it has been suggested that they have evolved to enable acute alterations of total mRNA levels 61 . This is critical for biological processes such as transcription, cellular proliferation and inflammation, which without rapid modulation and proper control can lead to cellular transformation and inappropriate immunological responses 55,62,63 . Maintenance of the steady state abundance of short-lived transcripts requires high RNA production rates 61 .…”