Genetic regulation of hepatocellular carcinoma by PKM gene
As PKM2 is pivotal in the metastasizing mechanism of carcinogenic cells (termed the Warburg effect), studies have shown that HCC is linked with the PKM gene by numerous mechanisms (Fig. 2). A common feature of carcinogenic cells is the malfunction of biotransformation, for example, their ability to cause alterations in metabolic processes like glycolysis. In 2012, Wang et al. studied the genetic cross-walk of HCC alongside the oncogenic pathway protein kinase B (AKT) activation and lowering of the sprout 2 (spry2) gene. They reported that AKT-induced HCC development increased extensively through the overexpression of Spry2Y55F. This overexpression gave rise to increased cell proliferation, increasing glycolysis, and resulting in significantly increased frequency of PKM2 and MAPK pathway activity.17
Overexpression of PKM2 in HCC patients and its role in the enhancement of cell proliferation and ultimate progression of HCC occurs through various critical pathways including PRMT6-ERK-PKM2 regulatory axis, circMAT2B/miR-338-3p/PKM2 axis, and PARP14-JNK1-PKM2 regulatory axis.18,19 There are certain oncogenic factors like heat shock protein 90 (HSP 90) that bring about Thr-328 phosphorylation, or c-Myc, which drives the expression of MNX1-AS1 to consequently induce the upregulation of PKM2.20,21
Cellular metabolism is regulated by MicroRNA-122(MiR-122), and is abundantly expressed in normally functioning cells, however, metabolic alterations that occur in HCC result in the reduction of MiR-122. Studies reveal that genes demonstrating the interaction of MiR-122 at 3′UTR areas are PK, ALDOA, GNPDA1, PFKFB2, and AKR1B10, among which PK gene has maximum anticorrelation with MiR-122. In HCC cells, expression of PKM2 is comparably higher in the metastasizing state. Overexpression of MiR-122, therefore, is indicative of reduced lactate production, inhibited cell proliferation and increased chemosensitivity.22 Additionally, MiR-122-5p has been investigated for its assistance in the function of lncRNA-SOX2OT, and modulating the metabolism and metastasis of HCC cells. lncRNA-SOX2OT is a PKM2 regulator and fosters PKM2-facilitated glucose metabolism. As such, PKM2 inhibition hinders lncRNA-SOX2OT in promoting the metastasis of HCC.23
When PKM2 expressions in HCC patients were studied by immunohistochemistry analysis, overexpression of PKM2 was observed throughout, and was seemingly associated with the worse clinicopathologic characteristics in the patients. PKM2 overexpression was also correlated with endoplasmic reticulum (ER) stress. ER stress downregulates the expression of MiR-188-5p in the HCC cells, its target gene hnRNPA2B1, when downregulated, decreases PKM2 expression and enhances apoptosis.24
Wang et al. suggest lncRNA DACT3-AS1 is associated with the progression of HCC through the upregulation of PKM2, and this upregulation is brought about by the histone deacetylase 2/ forkhead box protein a3 (HDAC2/FOXA3) pathway.25 It promotes interaction with HDAC and FOXA3 and elicits the removal of an acetyl group from FOXA3, lowering FOXA3 protein levels.25 Another study by Zhang et al. suggests HDAC8 carries out the deacetylation of K62 residue of PKM2, which subsequently alters its regulation. This process is activated by hypoxia-inducible factor -1α, which transcriptionally activates DACT3 antisense RNA 1 (DACT3-AS1) expression under hypoxic conditions.18,25
Activation of the transcription of hypoxia-inducible factor-1α (HIF-1α) by STAT3 phosphorylation, at tyrosine705, is a potential mechanism for the proliferation of HCC cells. Further studies demonstrated that up-regulation of HIF-1α and Bcl-xL (B-cell lymphoma-extra-large) by PKM2 enhances cell proliferation in HCC.26
Guanosine triphosphate binding protein 4, one of the pivotal regulators of MAPK and cell cycle progression, encourages HCC progression by PKM2-regulated glucose metabolism.27
A study by Chen et al. identified E3 ligase zinc finger protein 91 (ZFP91) to have a suppressing effect on the reprogramming of metabolic processes in HCC and cell propagation and metastasis through in vitro and in vivo studies.28 According to this report, Lys48-linked ubiquitination of the oncoprotein hnRNP A1 at lysine 8 by ZFP91, and inhibiting hnRNP A1-dependent PKM splicing by proteasomal degradation results in greater production of PKM1 isoform than PKM2, leading to a suppressing effect on hepatocellular carcinogenesis.28
Tripartite motif-containing protein 35 (TRIM35) was identified as a novel tumor suppressor, particularly in HCC. It interacts with PKM2 and inhibits the phosphorylation of tyrosine residue 105 (Y105), which impedes glycolysis and suppresses cell proliferation.29,30
Genetic regulation of hepatitis by PKM gene
HCC is a highly aggressive malignancy, its onset is related to hepatitis virus infection, alcoholism, and other metabolic disorders.31 A recent study by Wu et al. screening the interactions of large viral surface antigens (LHBS) with the host factors set by affinity purification screen illustrated the role of aerobic glycolysis played in supporting the hepatitis B virus.32 PKM2 oligomerization in hepatocytes was affected by viral LHBS, increasing the lactate production in the cells.32
Chronic hepatitis B is a major risk factor for the development and progression of HCC. Hence the pharmacological targets that are involved in the HCC progression were investigated, including Hep-1 (Hepatocyte Parafn 1), CK18 (cytokeratin-18), CK19 (Cytokeratin 19), SUOX (sulfite oxidase), pERK (phosphorylation extracellular regulated protein kinases), TRIM35 (tripartite motif containing 35), and PKM2.33
Hepatitis C infections occur when the hepatitis C virus (HCV) core protein reduces the activity of (PPAR)-α/γ in hepatocytes.34 This core protein surrounds the lipid vesicle membrane of the liver and causes lipid deposition in the cells via the activation of sterol regulatory-element-binding protein-1c.35 In the glycolytic pathway, HCV down-regulates glucose transporter 2, resulting in less intracellular glucose transport. HCV also up-regulates the genes for phosphoenolpyruvate carboxykinase and glucose 6-phosphatase, which are pivotal enzymes for hepatic gluconeogenesis.36 Studies carried out by Wu et al. reported on the specific interaction of PKM2 isoforms through the hepatitis C virus NS5B. The group studied both virus-encoded proteins and cellular factors involved in the pathogenesis and replication of HCV, ascertaining the interactions of NS5B with PKM2. They also showed that shRNA downregulated the expression of PKM2 which subsequently inhibited the replication of HCV in 9B replicon HCV cell lines.37
By binding to and down regulating HIF-1α activity, PKM2 is also a potential mediator and therapeutic target for regulating sterile inflammation of the liver, alcoholic, and non-alcoholic steatohepatitis (NASH).38,39