Overexpression of HMGA2 is present in many epithelial malignancies, such as pancreatic cancer, breast cancer, ovarian cancer, oral squamous cell carcinoma and lung cancer[8]. Accumulating evidences have suggested HMGA2 functions as a tumor promoter, which can promote tumor metastasis and enhance the formation of Epithelial-mesenchymal transition (EMT)[12, 27]. For example, HMGA2 can induce enhanced transcription of Twist1 and Snail by directly binding to the critical element of endogenous target promoters, resulting in EMT[28, 29]; the silencing of HMGA2 expression inhibited glioblastoma invasion via repression of the transcription of MMP2 gene[30]. Similarly, highly expressed HMGA2 was also observed in hepatocellular carcinoma cells, and the elevated expression of HMGA2 could promote hepatocellular carcinoma cell migration and invasion[10, 12]. In support of these studies, we also observed an abnormal expression of HMGA2 in the livers of DEN-treated rats; overexpression of HMGA2 increased the numbers of tumor metastatic nodules in a mouse model of lung metastasis.
Recently, several lines of reports focused on the regulation of HMGA expression have emerged. For example, long noncoding RNA VPS9D1 antisense RNA 1 (VPS9D1-AS1) promotes HMGA2 expression by sponging miR-532-3p in non-small cell lung cancer (NSCLC) cells[31]. Oxidized low density lipoprotein receptor 1 (OLR1) increased HMGA2 transcription by upregulating c-Myc expression in pancreatic cancer cells[32]. However, the mechanism underlying which regulates the transcriptional expression of HMGA2 in hepatocellular carcinoma cells remains largely unclear. Here, our findings suggested EGF stimulation can promotes HMGA2 expression at transcriptional level via activation of PI3K/Akt signaling pathways in hepatocellular carcinoma cells. In support of our results, Voon DC et al found exposure murine gastric epithelial (GIF-14) cells to EGF led to an increased HMGA2 transcription[33]. Certainly, we cannot rule out other mechanisms are involved in the regulation of HMGA2 expression in HCC. In fact, several long non-coding RNAs (lncRNAs), such as HULC, LSINCT5, LINC00473, SNHG16 and EGOT, have been found to promote HMGA2 expression in HCC[11, 34–37]. In addition, a series of microRNAs (miRs or miRNAs) including miR-663a, miR-107, microRNA-337 and microRNA-9 could inhibit hepatocellular carcinoma progression through suppression of HMGA2 expression[38–40]. However, whether these lncRNAs or microRNAs could interact with EGF and have a synergistic effect in regulation of HMGA2 transcription needs to be further explored.
Abnormal expression of p300 has been identified in esophageal squamous carcinoma, NSCLC cells and hepatocellular carcinoma[41–43]. Numerous studies have demonstrated p300 was associated with enhanced EMT and correlates with poor prognosis in these cancers[41, 43]. Overexpression of p300 promotes cell proliferation, migration, and invasion in NSCLC cells[42]; knockdown of p300 inhibited EMT, invasion and other malignant events of HCC cells[43]. However, we must note that p300 may act as a suppressor of tumor metastasis in some cancers. For example, Wang Y et al have reported that p300 can acetylate JHDM1A to inhibit the growth and metastasis of osteosarcoma[44]. Act as a transcriptional coactivator, p300 performs these functions through altering chromatin structure to stimulate tumor-related genes transcription[45, 46]. In detail, p300 controls these genes expression by transferring an acetyl group to lysine residues on histones and non-histone proteins[45]. Consistent with these reports, we demonstrated that p300 could acetylate histone H3 at K9 and promote HMGA2 transcription in hepatocellular carcinoma. To the best of our knowledge, we disclosed for the first time that p300 can stimulate HMGA2 expression at transcriptional level. As an acetyltransferase, the catalytic activity of p300 can be regulated by post-translational modification including methylation, acetylation as well as phosphorylation at some critical sites[47]. For instance, a wide array of protein kinases, such as protein kinase A (PKA), PKB, PKC, and mitogen-activated protein kinases (MAPK) can catalyze p300 phosphorylation at specific sites, resulting in activation or repression of target gene expression[47, 48]. Here, we also demonstrated that Akt could phosphorylate p300 at S1834 site to enhance the activity of p300, which promotes HMGA2 transcription in hepatocellular carcinoma cells. In support of our findings, Srivastava S et al also demonstrated Akt could directly phosphorylate p300 at S1834 in response to EGF stimulation in lung cancer cells.
Histone H3 acetylation has been shown to participate in the regulation of transcriptional activity by altering the structure of the chromatin[49]. In generally, hyperacetylation is largely associated with the activation of gene transcription, while hypoacetylation means repression of gene expression. Altered levels of acetylation of H3 at lysine (K) 4, 9, 14, 27 have been correlated with tumor progression and metastasis in a variety of cancers[50]. Although our results demonstrated H3 K9 acetylation plays an important role in promoting HMGA2 transcription, we could not exclude role of other lysine residues in regulation of HMGA2 gene expression, because several studies have noted that K4, K14 and K27 can be acetylated upon EGF stimulation[21, 51]. Histone H3 acetylation can be achieved by two groups of enzymes with opposing functions: histone acetyltransferases (HATs) and histone deacetylases (HDACs)[49]. Here we identified that p300, one member of HAT family, can add acetyl moieties to K9 residues upon EGFR activation in HCC. However, we cannot rule out the possibility that other HATs or HDACs are involved in catalyzing H3 K9 acetylation.