Loss of Mel-18 induces tumor angiogenesis through enhancing the activity and expression of HIF-1α mediated by the PTEN/PI3K/Akt pathway

Abstract

Mel-18 has been implicated in several processes in tumor progression, in which the Akt pathway is involved as an important key molecular event. However, the function of Mel-18 in human cancers has not been fully established yet. Here, we examined the effect of Mel-18 on tumor angiogenesis in human breast cancer, and found that Mel-18 was a novel regulator of HIF-1a. Mel-18 negatively regulated the HIF-1a expression and its target gene VEGF transcription during both normoxia and hypoxia. We demonstrated that Mel-18 regulated the HIF-1a expression and activity via the PI3K/Akt pathway. Loss of Mel-18 downregulated Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) expression, consequently activating the PI3K/Akt/MDM2 pathway, and leading to an increase of HIF-1a protein level. Mel-18 modulated the HIF-1a transcriptional activity via regulating the cytoplasmic retention of FOXO3a, a downstream effector of Akt, and recruitment of HIF-1a/CBP complex to the VEGF promoter. Furthermore, our data shows that Mel-18 blocked tumor angiogenesis both in vitro and in vivo. Mel-18 overexpression inhibited in vitro tube formation in human umbilical endothelial cells (HUVECs). Xenografts in NOD/SCID mice derived from stably Mel-18 knocked down MCF7 human breast cancer cells showed increased tumor volume, microvessel density, and phospho-Akt and HIF-1a expression levels. In conclusion, our findings provide that Mel-18 is a novel regulator of tumor angiogenesis through regulating HIF-1a and its target VEGF expressions mediated by the PTEN/PI3K/Akt pathway, suggesting a new tumor-suppressive role of Mel-18 in human breast cancer.

KLF5 promotes hypoxia-induced survival and inhibits apoptosis in non-small cell lung cancer cells via HIF-1α

“…The expression and activity of HIF-1α are regulated in tumor development by multiple signaling pathways and proteins, including PKC/P13K/AKT/mTOR (35)(36)(37), Ras/MEK/ERK (38), MAPK (39,40), EGFR and IGF1R autocrine signaling (41), PTEN (37,42,43). Under hypoxic condition, stabilized HIF-1α protein dimerizes with HIF-1β in the nucleus and the complex binds to a core DNA hypoxia response element (HRE) (44), activating the expression of numerous tumor promoter genes, including GLUT1 (10), VEGF (45), NOS (46), IGF-2 (47) and ID2 (48).…”

MEL-18 loss mediates estrogen receptor–α downregulation and hormone independence

“…Human breast cancer cell lines were cultured in DMEM lacking phenol red (Welgen Inc.) containing 10% FBS. The 293T cell lines were cultured as previously described (13). CHX and MG132 were obtained from Calbiochem, and ginkgolic acid, E2, 4-hydroxytamoxifen, R5020, and LY294002 were obtained from Sigma-Aldrich.…”

“…After the tumors were dissected, IHC analysis was performed using consecutive sections of tumors from 3 independent xenografted mice as described previously (13,48). The results were scored by multiplying the percentage of positive cells by the staining intensity as described in the IHC staining section.…”

“…Furthermore, studies have demonstrated that MEL-18 is strongly involved in the regulation of several molecules that are associated with basal-like cancer or TNBC, including p53, PTEN, PI3K, Wnt, Notch, and miR-205 (12,13,15). For example, a high percentage of TNBC cases lack PTEN expression (39,40), and we have demonstrated that MEL-18 loss induces PTEN downregulation and PI3K/AKT activation (13). The loss of p53 function is characteristic of TNBC (41).…”

“…Although further clinical evidence is needed, accumulating studies have suggested that MEL-18 acts as a tumor suppressor in several human tumors, including breast cancer (7)(8)(9)(10)(11). Our previous studies have also revealed that MEL-18 loss facilitates stem cell activity, cell growth, angiogenesis, and epithelial-mesenchymal transition (EMT) in breast cancer (12)(13)(14)(15), implying an association of MEL-18 loss with an aggressive phenotype. MEL-18 is involved in multiple posttranslational modifications of target proteins and in epigenetic gene regulation.…”