AMPK Promotes Aberrant PGC1β Expression To Support Human Colon Tumor Cell Survival

Abstract

A major goal of cancer research is the identification of tumor-specific vulnerabilities that can be exploited for the development of therapies that are selectively toxic to the tumor. We show here that the transcriptional coactivators peroxisome proliferatoractivated receptor gamma coactivator 1␤ (PGC1␤) and estrogen-related receptor ␣ (ERR␣) are aberrantly expressed in human colon cell lines and tumors. With kinase suppressor of Ras 1 (KSR1) depletion as a reference standard, we used functional signature ontology (FUSION) analysis to identify the ␥1 subunit of AMP-activated protein kinase (AMPK) as an essential contributor to PGC1␤ expression and colon tumor cell survival. Subsequent analysis revealed that a subunit composition of AMPK (␣2␤2␥1) is preferred for colorectal cancer cell survival, at least in part, by stabilizing the tumor-specific expression of PGC1␤. In contrast, PGC1␤ and ERR␣ are not detectable in nontransformed human colon epithelial cells, and depletion of the AMPK␥1 subunit has no effect on their viability. These data indicate that Ras oncogenesis relies on the aberrant activation of a PGC1␤-dependent transcriptional pathway via a specific AMPK isoform.A third of all human cancers, including a substantial percentage of colorectal, lung, and pancreatic cancers, are driven by activating mutations in Ras genes. Activating K-Ras mutations are present in 35 to 40% of colon tumors and are thought to be both drivers of tumorigenesis and determinants of therapeutic regimens (1). Therapeutic disruption of Ras function has been clinically ineffective to date, but investigation of Ras pleiotropy continues to yield a diversity of downstream effectors with obligate roles in the maintenance and adaptation of Ras-driven tumors to changing environments. The Raf-MEK-extracellular signal-regulated kinase (ERK) signaling pathway is essential for the oncogenic properties of mutated K-Ras (2). However, numerous potent and specific MEK inhibitors have been developed yet have failed to demonstrate single-agent efficacy in cancer treatment (3). As a molecular scaffold of the Raf-MEK-ERK kinase cascade (4, 5), kinase suppressor of Ras 1 (KSR1) is necessary and sufficient for Ras V12 -induced tumorigenesis (4), mouse embryo fibroblast (MEF) transformation (5, 6), and pancreatic cancer growth (7) but dispensable for normal development (4). KSR1 is overexpressed in endometrial carcinoma and is required for both proliferation and anchorage-independent growth of endometrial cancer cells (8). Except for minor defects in hair follicles, KSR1 knockout mice are fertile and develop normally (4).This observation predicts that small molecules targeting KSR1 and functionally related effectors should preferentially target Rasdriven tumors while leaving normal tissue largely unaffected. More generally, this observation demonstrates that tumor cells, while under selective pressure to adapt to inhospitable environments and proliferate without constraint, will adopt strategies that, while advantageous to that singular purpose, create...

KSR1 and EPHB4 Regulate Myc and PGC1β To Promote Survival of Human Colon Tumors

“…KSR1 regulates the oncogenic potential of activated Ras (14). Our laboratory has recently shown that KSR1 also promotes anchorageindependent growth and tumor maintenance in human colon tumor cell lines (16 (16,19,44,45), the similarity of siKSR1-and siEPHB4-dependent gene expression signatures suggests that EPHB4 is likely to support colon tumor cell survival similarly to KSR1. EPHB4 expression is elevated in a variety of human cancers, including cancers of the head and neck, prostate, bladder, ovaries, large intestine, lung, brain, pancreas, and esophagus (46)(47)(48)(49)(50)(51)(52)(53)(54).…”

“…The gene expression-based high-throughput screen has been described previously (16,19). The gene expression-based signature measured in the screen is based on six genes (ACSL5, BNIP3L, ALDOC, LOXL2, BNIP3, and NDRG1 genes) that are consistently affected by KSR1 depletion, as well as two housekeeping genes (PPIB and hypoxanthine phosphoribosyltransferase [HPRT] genes) that were included for normalization.…”

“…We have recently shown that tumor-specific expression of peroxisome proliferator-activated receptor gamma coactivator 1␤ (PGC1␤) is required for colon cancer survival (16). Previous work has shown that PGC1␤ is a direct downstream target of Myc (33,34).…”

“…KSR1 is a scaffold of the RAF/MEK/ERK kinase cascade and is required for maximal MAPK-dependent signaling (14,15). While KSR1 is required for the survival of CRC cells, it is dispensable in normal colon epithelial cells (16). KSR1…”

“…Given that KSR1 is dispensable for normal cells but indispensable for colorectal cancer cells, we sought to detect and exploit further vulnerabilities in human colon tumor cells. To do this, we developed a gene expression-based high-throughput screen and used functional signature ontology (FUSION) (16,19) to identify functional analogs of KSR1. From this screen, we identified EPH (erythropoietin-producing hepatocellular carcinoma) receptor B4 (EPHB4) as a KSR1-like, cancer-specific vulnerability that may be exploited by targeted therapies.…”