The LCLAT1/LYCAT acyltransferase is required for EGF-mediated phosphatidylinositol-3,4,5-trisphosphate generation and Akt signaling.

Receptor tyrosine kinases such as EGF receptor (EGFR) stimulate phosphoinositide 3 kinases to convert phosphatidylinositol-4,5-bisphosophate [PtdIns(4,5)P2] into phosphatidylinositol-3,4,5-trisphosphate [PtdIns(3,4,5)P3]. PtdIns(3,4,5)P3 then remodels actin and gene expression, and boosts cell survival and proliferation. PtdIns(3,4,5)P3 partly achieves these functions by triggering activation of the kinase Akt, which phosphorylates targets like Tsc2 and GSK3ß. Consequently, unchecked upregulation of PtdIns(3,4,5)P3-Akt signaling promotes tumor progression. Interestingly, 50-70% of PtdIns and PtdInsPs have stearate and arachidonate at sn-1 and sn-2 positions of glycerol, respectively, forming a species known as 38:4-PtdIns/PtdInsPs. LCLAT1 and MBOAT7 acyltransferases partly enrich PtdIns in this acyl format. We previously showed that disruption of LCLAT1 lowered PtdIns(4,5)P2 levels and perturbed endocytosis and endocytic trafficking. However, the role of LCLAT1 in receptor tyrosine kinase and PtdIns(3,4,5)P3 signaling was not explored. Here, we show that LCLAT1 silencing in MDA-MB-231 and ARPE-19 cells abated the levels of PtdIns(3,4,5)P3 in response to EGF signaling. Importantly, LCLAT1-silenced cells were also impaired for EGF-driven and insulin-driven Akt activation and downstream signaling. Thus, our work provides first evidence that the LCLAT1 acyltransferase is required for receptor tyrosine kinase signaling.

HER2 expression defines unique requirements for flotillin and c-Src in EGFR signaling.

The epidermal growth factor receptor (EGFR) controls many cellular functions. Upon binding its ligand, the receptor undergoes dimerization, phosphorylation and activation of signals including the phosphoinositide-3-kinase (PI3K)-Akt pathway. Although some studies have indicated that EGFR signaling may be controlled by signal enrichment within various membrane rafts, such as flotillin nanodomains, others have found a limited effect of disruption of these nanodomains on EGFR signaling, suggesting that specific factors may define context-specific control of EGFR signaling. Ligand-bound EGFR can homodimerize or instead undergo heterodimerization with the related receptor HER2 (also known as ERBB2) when the latter is expressed. We examined how EGFR signaling in the presence of HER2 distinctly requires flotillin nanodomains. Induction of HER2 expression altered EGFR signaling duration, which is consistent with EGFR-HER2 heterodimer formation. EGFR and c-Src (also known as SRC) localized within plasma membrane structures demarked by flotillin-1 more prominently in HER2-expressing cells. Consistently, HER2-expressing cells, but not cells lacking HER2, were dependent on flotillin-1 and c-Src for EGFR signaling leading to Akt activation and cell proliferation. Hence, HER2 expression establishes a requirement for flotillin membrane rafts and c-Src in EGFR signaling.

Control of cell metabolism by the epidermal growth factor receptor.

The epidermal growth factor receptor (EGFR) triggers the activation of many intracellular signals that control cell proliferation, growth, survival, migration, and differentiation. Given its wide expression, EGFR has many functions in development and tissue homeostasis. Some of the cellular outcomes of EGFR signaling involve alterations of specific aspects of cellular metabolism, and alterations of cell metabolism are emerging as driving influences in many physiological and pathophysiological contexts. Here we review the mechanisms by which EGFR regulates cell metabolism, including by modulation of gene expression and protein function leading to control of glucose uptake, glycolysis, biosynthetic pathways branching from glucose metabolism, amino acid metabolism, lipogenesis, and mitochondrial function. We further examine how this regulation of cell metabolism by EGFR may contribute to cell proliferation and differentiation and how EGFR-driven control of metabolism can impact certain diseases and therapy outcomes.

Targeted enhancement of flotillin-dependent endocytosis augments cellular uptake and impact of cytotoxic drugs.

Cellular uptake is limiting for the efficacy of many cytotoxic drugs used to treat cancer. Identifying endocytic mechanisms that can be modulated with targeted, clinically-relevant interventions is important to enhance the efficacy of various cancer drugs. We identify that flotillin-dependent endocytosis can be targeted and upregulated by ultrasound and microbubble (USMB) treatments to enhance uptake and efficacy of cancer drugs such as cisplatin. USMB involves targeted ultrasound following administration of encapsulated microbubbles, used clinically for enhanced ultrasound image contrast. USMB treatments robustly enhanced internalization of the molecular scaffold protein flotillin, as well as flotillin-dependent fluid-phase internalization, a phenomenon dependent on the protein palmitoyltransferase DHHC5 and the Src-family kinase Fyn. USMB treatment enhanced DNA damage and cell killing elicited by the cytotoxic agent cisplatin in a flotillin-dependent manner. Thus, flotillin-dependent endocytosis can be modulated by clinically-relevant USMB treatments to enhance drug uptake and efficacy, revealing an important new strategy for targeted drug delivery for cancer treatment.