Mouse embryo fibroblasts (MEFs) derived from CPEB
knockout (KO) mice do not senesce as do MEFs derived from wild type (WT) mice, but instead are immortal. Senescence is rescued when ectopic CPEB is expressed in the KO MEFs and potentiated when expressed in WT MEFs7
. Human foreskin fibroblasts depleted of CPEB also bypass senescence and divide ~270 days compared to WT cells, which senesce after about 90 days. As with the mouse cells, ectopic expression of CPEB rescues senescence in knockdown cells and potentiates senescence in WT cells. CPEB controls the polyadenylation-induced translation of p53
mRNA, and indeed CPEB-induced senescence requires p53. Depletion of CPEB also induces the “Warburg Effect” where mitochondrial respiration is reduced and cells produce ATP primarily through glycolysis6
To investigate the possibility that CPEB control of p53
polyadenylation requires Gld2, human primary foreskin fibroblasts were stably transduced with lentiviruses expressing two different shRNAs against the Gld2 coding sequence. Surprisingly, Gld2 depletion () induced an increase in both p53 protein levels () and p53
mRNA polyadenylation (; Supplemental Fig. 1
). Also unexpectedly, depletion of Gld2 resulted in increased oxygen consumption () and entry into a senescence-like cell cycle arrest as evidenced by ß-galactosidase staining at acid pH (). In comparison, CPEB depleted cells had decreased oxygen consumption, fewer cells staining with ß-galactosidase, increased lifespan, and most importantly, reduced poly(A) tail size on p53
mRNA and ~50% reduction in p53 protein levels6
Figure 1 Depletion of Gld2 enhances p53 expression. (a) RT-PCR of Gld2 and tubulin RNAs following infection of human foreskin fibroblasts with lentiviruses expressing shRNA against Gld2 or GFP (mock, same in all panels). (b) Knockdown of Gld2-HA in cells expressing (more ...)
These paradoxical results prompted us to examine CPEB levels in Gld2 deficient cells since CPEB is required for normal p53
. After comparing the amounts of CPEB nuclear pre-mRNA by intron-specific quantitative RT-PCR and mostly cytoplasmic mRNA by exon-specific quantitative RT-PCR, we found that the pre-mRNA levels, which generally reflect transcription, were nearly unchanged while cytoplasmic mRNA levels increased by about five-fold (). Thus, in the absence of Gld2, CPEB
mRNA unexpectedly was more stable.
Figure 2 Gld2 knockdown increases CPEB reporter mRNA and translation by a post-transcriptional mechanism. (a) Fold change of nuclear (intron-containing) or predominantly cytoplasmic (exon-containing) CPEB RNA following Gld2 depletion (n=3, bars are s.e.m.) (b) (more ...)
Surmising that Gld2 might control p53 protein levels via CPEB, we next used a Renilla luciferase (Rluc) and firefly luciferase (Fluc) reporter system to investigate post-transcriptional regulation of CPEB by Gld2. As shown in , the entire CPEB 3’ UTR was translated about 40% less efficiently compared to a reporter lacking the 3’ most 455 nucleotides (Mock). However, in Gld2-deficient cells, the two reporters were translated equally. Additional deletions (Δ) of the CPEB 3’ UTR suggested that there might be multiple regions that elicited increases in reporter translation following Gld2 knockdown (i.e., ΔE translation was about two fold greater than ΔB, ΔC, or ΔD translation) ().
Analysis of the regions of the CPEB 3’ UTR that mediated translational repression by Gld2 revealed the presence of two potential miR-122 binding sites (Supplementary Fig. 2
). Although miR-122 is thought to be liver-specific and account for ~70% of the total population of microRNAs in that tissue13
, deletion of these specific sites, either individually or combined, alleviated translational repression in Gld2 depleted cells () and were nearly identical to that observed with the large deletions (). These results suggest that miR-122 might repress CPEB mRNA translation in human skin fibroblasts and indicate that this miRNA is more widely distributed than originally thought. Indeed recent evidence shows that miR-122 is present in human skin14
and even HEK293 cells15
Figure 3 miR-122 activates p53 mRNA translation by repressing CPEB. (a) Gld2 depleted fibroblasts were transduced with firefly and Renilla luciferase with CPEB full length or deletion mutant 3’UTRs lacking putative miR-122 sites (Supplementary Fig. 2). (more ...)
To assess directly whether miR-122 might repress CPEB
mRNA expression, we first cloned and sequenced it from human foreskin fibroblasts and found that it contained a nontemplated 3’ monoadenylate residue (; see discussion). Next, cells were electroporated with a locked nucleic acid (LNA) antagomir for miR-122, or as a control, a scrambled LNA. The miR-122 antagomir enhanced reporter expression by about 3.25 fold relative to control (), but had no stimulatory effect on a reporter whose 3’ UTR contained no miR-122 sites (Supplementary Fig. 3
). Based on evidence from Katoh et al16
, who demonstrated that in murine liver, Gld2 is essential for miR-122 stability, we suspected that Gld2 might influence CPEB expression and possibly p53
mRNA translation by controlling miR-122 steady-state levels. Indeed, demonstrates that depletion of Gld2 from skin fibroblasts reduced the level of miR-122 by nearly 40-fold. Importantly, when miR-122 LNA antagomir-transduced cells were incubated with the proteasome inhibitor MG132 and pulsed-labeled with 35
S-methionine for 15 min followed by p53 immunoprecipitation, there was a two-fold increase in the synthesis rate of p53 (). Taken together, these data demonstrate that human primary skin fibroblasts contain miR-122 and that Gld2 controls its steady state levels or activity.
While consistent with the hypothesis that miR-122 mediates p53 mRNA translation via CPEB, these data do not eliminate the possibility that miR-122 could act via another molecule to regulate p53 synthesis (note that p53 mRNA has no miR-122 sites according to Targetscan.org or Microrna.org). Consequently, we infected cells with a lentivirus expressing shRNA for CPEB as well as the miR-122 antagomir followed by a 15 minute pulse of 35S-methionine and p53 immunoprecipitation. shows that although miR-122 antagomir alone elicited an increase in p53 synthesis, the antagomir plus shRNA for CPEB induced no increase. Taken together, these data demonstrate that Gld2 activity stabilizes miR-122, which in turn reduces CPEB expression; CPEB then acts directly on p53 mRNA to control poly(A) tail length and translation.
If not Gld2, what poly(A) polymerase modifies p53
mRNA polyadenylation and translation? We surmised that an alternative non-canonical poly(A) polymerase, i.e., one that lacks an RNA binding domain and thus would require another factor such as CPEB to be tethered to the RNA, would most likely be involved. Two cytoplasmic enzymes have this characteristic: Gld4 (PAPD5)17
and MitoPAP (PAPD1)18
. Both polymerases were depleted with shRNAs (Supplementary Fig. 4
) but only the loss of Gld4 reduced p53 protein levels (). To investigate whether Gld4 interacts with p53
mRNA in a CPEB-dependent manner, FLAG-Gld4 was expressed in cells (Supplementary Fig. 5
) containing shRNA for tetracycline repressor (TETR, a control) or CPEB. Gld4 was then immunoprecipitated and the extracted RNA was examined for p53 and GAPDH (a control) RNAs by RT-PCR (). p53
mRNA was detected only when CPEB was present, suggesting that Gld4 is anchored to p53
mRNA by CPEB, and indeed, CPEB co-immunoprecipitated Gld4 but not Mcl1, a nonspecific control (). Finally, depletion of Gld4 reduced p53
mRNA polyadenylation (), which probably then induced p53 mRNA destabilization (; depletion of Gld4 reduced mostly cytoplasmic p53
mRNA as examined by RT-PCR using exon-specific primers but had no effect on p53
transcription as examined by RT-PCR using intron-specific primers).
Figure 4 Gld4 controls p53 mRNA expression. (a) p53 and actin western blots from fibroblasts expressing GFP (mock), shGld4, or shMitoPAP (shMPAP). (b) Fibroblasts containing shTETR or CPEB were transfected with Gld4-FLAG followed by FLAG antibody or IgG immunoprecipitation (more ...)
The results presented here and in Katoh et al. (ref. 16
) suggest a model for homeostatic control of p53 synthesis in human skin fibroblasts (). Gld2 stabilizes miR-122 by catalyzing the addition of a single adenylate residue to its 3’ end16
. miR-122 then base-pairs to two regions of the CPEB 3’ UTR, causing instability and/or translational inhibition of the mRNA. CPEB, whose levels are modulated by these events, binds to the p53
3’UTR and recruits Gld4, which in turn maintains p53
mRNA polyadenylation and translation. We envision this hierarchical regulation of p53 to resemble a rheostat, where translation is turned up or down rather than a switch, where translation is turned on or off19
, although p53
mRNA translation can also be controlled by a switch mechanism in response to DNA damage20,21
. A 50% change in p53 synthesis can toggle a cell between growth and senescence6
, demonstrating that drastic biological consequences result from a relatively modest change in protein level.
Although ectopically-expressed Gld2 immunoprecipitated from hepatocarcinoma cells adds a single adenosine to miR-122 in vitro16
, Gld2 tethered to a small non-coding RNA by MS2 adds >300 adenylate residues in injected oocytes22
, and about that same amount to mRNA when bound to CPEB10
. How the enzyme can modulate its catalytic activity depending on the substrate is unknown, but we postulate that components of the RNA-induced silencing complex (RISC) might be responsible. In addition to our demonstration that miR-122 is 3’ mono-adenylated in skin fibroblasts, ~20% of all RNA deep sequencing reads from cloned neuroblastoma miRNAs contain a non-templated adenylate residue23
, suggesting that miR-122 may be one of several miRNAs that are mono-adenylated by Gld2.
In conclusion, our results demonstrate that Gld2 control of miR-122 stability in human skin fibroblasts tunes CPEB expression, which in turn regulates p53 mRNA polyadenylation and translation by Gld4. The coordinated activities of these factors then gate entry into senescence. These studies also bring up two additional aspects of CPEB-related activities: how does Gld4, but not Gld2, associate with CPEB on p53 mRNA, and what molecular machinery is responsible for miR-122 destruction upon Gld2 depletion? Deciphering the mechanisms involved would likely require analysis of the combinatorial associations of factors on different RNA substrates.