Posttranscriptional lipopolysaccharide regulation of the lysozyme gene at processing of the primary transcript in myelomonocytic HD11 cells.

Lysozyme is increasingly expressed in macrophages in inflammatory response to bacterial LPS. In this study, we investigated the mechanisms that control expression of the lysozyme gene in myelomonocytic HD11 cells activated by LPS. Nuclear run-on transcription assays showed that LPS caused a 15-fold increase in the transcription rate of the lysozyme gene. However, Northern analyses with lysozyme cDNA and intron sequences revealed that the LPS-induced increase in nuclear lysozyme transcripts greatly exceeded the increase in transcription rate. Furthermore, nuclear lysozyme transcripts in untreated cells with a t(1/2) of <10 min were more unstable than those accumulated in LPS-activated cells. We suggested, therefore, that the increased lysozyme expression following LPS treatment was largely due to a nuclear stabilization of the primary transcript. Interestingly, the increase in stability of the lysozyme primary transcript was accompanied by changes in nuclear processing including an increase in poly(A) tail length, which gradually shortened after entering the cytoplasm. The long lysozyme poly(A) tail, however, did not result in any increase in polysomal recruitment for translation or in stability of the cytoplasmic lysozyme mRNA.