Abbreviations
AGPG
actin gamma 1 pseudogene
APC/C
anaphase‐promoting complex
ESCC
esophageal squamous cell carcinoma
lncRNA
long non‐coding RNA
PFKFB3
6‐Phosphofructo‐2‐Kinase/Fructose‐2,6‐Biphosphatase 3
siRNA
small interfering RNA
Over the years, thousands of long non‐coding RNAs (lncRNAs) have been identified to
be exclusively expressed in specific cancer types and for their unique functions in
tumorigenesis. This has led to an increasing interest in elucidating the vital roles
[1] and underlying mechanism [2, 3] of such non‐coding genome in driving cancerous
phenotypes. A number of studies have pinpointed the key functions of lncRNAs in diverse
biological events including chromatin interactions, transcriptional regulation, RNA
processing, mRNA stabilization, signal transduction, and metabolic regulation; highlighting
their essential roles in both physiology and diseases such as cancer [4].
A classic hallmark of cancer is the reprogramming of glucose metabolism that occurs
to redirect glycolytic intermediates toward the biosynthetic production of macromolecules
needed for cancer progression [5, 6, 7]. The metabolic role of lncRNAs has been also
discovered [8, 9], while the mechanistic details on how lncRNAs regulate metabolic
processes remain to be investigated. Notably, a recent study conducted by Liu and
colleagues [10] revealed a novel lncRNA, named as actin gamma 1 pseudogene (AGPG),
as a key regulator of 6‐Phosphofructo‐2‐Kinase/Fructose‐2,6‐Biphosphatase 3 (PFKFB3),
in driving glycolysis and cell cycle progression, and a biomarker in esophageal squamous
cell carcinoma (ESCC; Figure 1).
FIGURE 1
Illustration of the oncogenic role of lncRNA AGPG in driving p53‐deficient ESCC development.
Loss of p53 promotes AGPG expression, which in turn forms a complex with PFKFB3 to
induce glycolysis and cell cycle progression. This AGPG‐dependent oncogenic alteration
contributes to the ESCC development and could be used as a therapeutic target for
ESCC treatment
1
AGPG IS A NOVEL STIMULATOR OF GLYCOLYSIS AND CANCER DEVELOPMENT
In that study [10], the authors aimed to identify oncogenic lncRNAs in ESCC with a
focus on their involvement in glucose metabolism. To achieve this, they conducted
a small interfering RNA (siRNA) screening by taking cell viability and lactate production
as readouts, where AGPG stood out as one of the top candidates. The authors further
validated their findings by measuring the glycolytic flux and found that glycolysis
was significantly diminished when AGPG was depleted. Additionally, the downregulation
of AGPG inhibited cancer cell proliferation and cell cycle progression. Discovery
of such a novel lncRNA adds to the developing body of literature, showing the importance
of lncRNAs in metabolic regulation. However, a lack of understanding in the field
is how specific can lncRNAs mechanistically modulate cancer metabolism. To address
this, the authors further performed mass spectrometry analysis and successfully discovered
PFKFB3 as a putative binding partner for AGPG.
2
AGPG ACTS THROUGH PFKFB3 INTERACTION
Through a variety of in vitro and in vivo experiments, the authors successfully elucidated
the functional significance of AGPG and PFKFB3 binding in reprogramming glucose metabolism.
So far, this is the first study to report the lncRNA binding partner of PFKFB3, making
it possible to study PFKFB3 from a new perspective. Mechanistically, AGPG sterically
blocks the association between anaphase‐promoting complex (APC/C) and PFKFB3, ultimately
halting the ubiquitination and degradation of PFKFB3. As previously reported, PFKFB3
is an important target for cancer therapeutics because of its role in driving glycolysis
and cell proliferation in cancer cells [11]. Therefore, such AGPG‐mediated PFKFB3
stabilization nicely explained how AGPG is involved in these two oncogenic processes.
Notably, a previous study reported a lncRNA‐PFKFB2 complex in promoting metastasis
through the alteration of glycolysis [12]. Together with current findings for the
AGPG‐PFKFB3 complex, it would be interesting to see if a general lncRNA‐based regulation
of the PFKFB family of enzymes could exist. Taken together, these data provide the
first step for understanding the intricate mechanism of the fundamental concepts missing
regarding the stabilization of PFKFB3 and its novel regulator, AGPG, in metabolic
remodeling.
3
AGPG SERVES AS A NON‐CODING LINKER BETWEEN P53 AND GLUCOSE METABOLISM
To explore the upstream regulation of AGPG, the authors further discovered p53 as
a putative transcription factor that negatively regulates AGPG expression. This intriguing
finding not only connected AGPG with various cellular stresses signaling via p53,
but also explained the oncogenic upregulation of AGPG in ESCC with p53 deficiency.
Given the critical role of p53 in controlling glucose metabolism [13], the discovery
of AGPG as a non‐coding transcript of p53 provides a novel insight into this process.
4
AGPG IS A BIOMARKER AND A THERAPEUTIC TARGET FOR ESCC
While metabolic reprogramming in cancer is vital for cancer initiation and progression,
efficacious therapeutics are limited. Despite the numerous available therapies, ESCC
is currently the foremost cause of cancer associated deaths, thus calling for a need
to identify novel biomarkers [14]. Pathologically, the authors showed that the overexpression
of AGPG was correlated with poor survival rates in ESCC patients. Notably, upon depletion
of AGPG using an optimized lncRNA inhibitor, a dramatic decrease in ESCC patient‐derived
xenograft tumor growth was observed. Therefore, identification of novel cancer‐associated
lncRNAs, such as, AGPG, makes it an attractive biomarker and a therapeutic target
in ESCC. With this important information, it would be also interesting to see AGPG
exploited pharmaceutically into inhibitors for personalized cancer therapy.