Intracellular signaling by protein tyrosine phosphorylation is generally understood to govern many aspects of cellular behavior. The biological consequences of this signaling pathway are important because the levels of protein tyrosine phosphorylation are frequently elevated in cancer cells. In the classic paradigm, tyrosine kinases promote tumor cell growth, survival, and invasiveness, whereas tyrosine phosphatases negatively regulate these same behaviors. Here, we identify one particular tyrosine phosphatase, low molecular weight tyrosine phosphatase (LMW-PTP), which is frequently overexpressed in transformed cells. We also show that overexpression of LMW-PTP is sufficient to confer transformation upon non-transformed epithelial cells. Notably, we show that the EphA2 receptor tyrosine kinase is a prominent substrate for LMW-PTP and that the oncogenic activities of LMW-PTP result from altered EphA2 expression and function. These results suggest a role for LMW-PTP in transformation progression and link its oncogenic potential to EphA2.

Docosahexaenoic acid (DHA), a 22:6 n-3 polyunsaturated fatty acid, is the longest and most highly unsaturated fatty acid found in most membranes and has been shown to inhibit cancer cell growth in part by modifying cell signaling. In the current study, alterations to epidermal growth factor receptor (EGFR) signaling upon DHA supplementation are examined in A549 lung adenocarcinoma, WiDr colon carcinoma and MDA-MB-231 breast carcinoma cell lines. Interestingly, EGFR phosphorylation, most notably at the tyrosine 1068 residue, is dramatically upregulated, and EGFR association with the Sos1 guanine nucleotide exchange factor is concomitantly increased upon DHA supplementation. However, guanosine triphosphate-bound Ras and phosphorylated extracellular signal-regulated kinase (Erk)1/2 are paradoxically downregulated in the same treatments. Previous reports have noted changes in membrane microdomains upon DHA supplementation, and our findings confirmed that EGFR, but not Ras, is excluded from caveolin-rich lipid raft fractions in DHA-treated cells, resulting in a decreased association of Ras with Sos1 and the subsequent downregulation of Erk signaling. Xenografts of the A549 cell line implanted in athymic mice fed a control high-fat diet or a diet high in DHA confirmed our in vitro data. These results demonstrate for the first time a functional consequence of decreased EGFR protein in lipid raft microdomains as a result of DHA treatment in three different cancer models. In addition, we report the ability of DHA to enhance the efficacy of EGFR inhibitors on anchorage-independent cell growth (soft agar), providing evidence for the potential development of enhanced combination therapies.

Docosahexaenoic acid (DHA; C22:6n-3) depresses mammary carcinoma proliferation and growth in cell culture and in animal models. The current study explored the role of interrupting bioenergetic pathways in BT-474 and MDA-MB-231 breast cancer cell lines representing respiratory and glycolytic phenotypes, respectively and comparing the impacts of DHA with a non-transformed cell line, MCF-10A. Metabolic investigation revealed that DHA supplementation significantly diminished the bioenergetic profile of the malignant cell lines in a dose-dependent manner. DHA enrichment also resulted in decreases in hypoxia-inducible factor (HIF-1α) total protein level and transcriptional activity in the malignant cell lines but not in the non-transformed cell line. Downstream targets of HIF-1α, including glucose transporter 1 (GLUT 1) and lactate dehydrogenase (LDH), were decreased by DHA treatment in the BT-474 cell line, as well as decreases in LDH protein level in the MDA-MB-231 cell line. Glucose uptake, total glucose oxidation, glycolytic metabolism, and lactate production were significantly decreased in response to DHA supplementation; thereby enhancing metabolic injury and decreasing oxidative metabolism. The DHA-induced metabolic changes led to a marked decrease of intracellular ATP levels by 50% in both cancer cell lines, which mediated phosphorylation of metabolic stress marker, AMPK, at Thr172. These findings show that DHA contributes to impaired cancer cell growth and survival by altering cancer cell metabolism, increasing metabolic stress and altering HIF-1α-associated metabolism, while not affecting non-transformed MCF-10A cells. This study provides rationale for enhancement of current cancer prevention models and current therapies by combining them with dietary sources, like DHA.

SUMMARY The omega-6 polyunsaturated fatty acid linoleic acid (LA; C18:2 n-6) is prevalent in Western diets and has been shown to enhance tumorigenesis of several cancer models. However, the modes by which LA affects carcinogenesis have not been fully elucidated. In this study, a mechanism for LA-induced upregulation of cancer cell growth is defined. Cellular proliferation was enhanced with LA treatment in BT-474 human breast ductal carcinoma and A549 human lung adenocarcinoma cell lines. Enrichment of LA increased COX activity and led to increases in PGE2 production, followed by increases in MMP and TGF-α levels, which are all key elements involved in the enhancement of cancer cell growth. Further investigation revealed that LA supplementation in both BT-474 breast and A549 lung cancer cell lines greatly increased the association between the scaffolding protein Gab1 and EGFR, while at the same time dramatically decreasing Gab1 protein levels. These changes are concomitant with increases in activated Akt (pAkt), a downstream signaling component in the PI3K signaling pathway. Moreover, inhibitors of EGFR, PI3K and Gab1-specific siRNAs were capable of reversing LA-induced upregulation of pAkt, as well as observed increases in cell proliferation for these models. These data establish Gab1 as major target in LA-induced enhancement of tumorigenesis.

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