#22

The methylated histone H3-K4 and K9 mark gene activation and repression, respectively [1]–[3]. LSD1, the first identified histone demethylase [6], has been shown to either repress a cohort of target genes through demethylation of H3-K4(M2) or activate other targets via removing the repressive mark H3-K9(M2), dependent on its interaction with different partners [6]–[8], [12], [17]. In the present study, we provide evidence that LSD1 is required for transcriptional repression of the hTERT gene in both normal and cancerous human cell lines.

Tranylcypromine treatment or depletion of LSD1 expression induces hTERT mRNA expression in MRC5 fibroblasts

The amino oxidase inhibitor tranylcypromine has been identified to potently suppress the enzymatic activity of LSD1 [17]. Therefore, to elucidate a potential role for LSD1 in the transcriptional regulation of the hTERT gene, we first incubated normal human MRC5 fibroblasts exhibiting a stable hTERT repression in the presence of tranylcypromine. As shown in Fig. 1A, hTERT transcripts were not observed in control MRC5 cells while tranylcypromine treatment induced a weak but detectable expression of hTERT mRNA. To rule out possible LSD1-independent, non-specific effects of tranylcypromine, we knocked-down LSD1 expression by using the specific LSD1 siRNA. siRNA treatment of MRC5 cells resulted in silencing of LSD1 expression, and consistent with the finding described above, hTERT mRNA was induced in these LSD1-depleted cells (Fig. 1B). Taken together, both pharmacological inhibition of LSD1 activity and depletion of its expression are capable of de-repressing the hTERT transcription in normal human fibroblasts.

The effect is even stronger when you combine tranylcypromine with an HDAC inhibitor.