Insights into the molecular regulatory mechanisms of tumor macropinocytosis

<sec><p indent="0mm">Macropinocytosis is a highly conserved, actin-dependent endocytic pathway in eukaryotic cells that facilitates the non-selective engulfment of extracellular soluble substances by macropinosomes. These macropinosomes can either fuse with lysosomes for degradation or recycle back to the plasma membrane, to replenish membrane receptors. As a vital cellular process, macropinocytosis is important in various biological functions, including immune responses, cellular metabolism, and the progression of several types of advanced cancers. </sec><sec> The function of macropinocytosis in the progression of malignant tumors, such as pancreatic ductal adenocarcinoma (PDAC), has been a focus of research. An intriguing aspect of PDAC is how tumor cells exploit macropinocytosis to scavenge nutrients from the surrounding tumor microenvironment. Specifically, collagen, a major component of the extracellular matrix, is an important source of nutrients. Tumor cells internalize collagen via macropinocytosis, followed by its degradation into amino acids and intermediates of the tricarboxylic acid (TCA) cycle. These metabolites facilitate tumor growth and metastasis, particularly to the liver. Inhibitors of macropinocytosis, alone or in combination with other antitumor therapies such as autophagy inhibitors, chemotherapy, or immune checkpoint inhibitors, significantly reduce tumor growth and metastasis. These findings suggest that targeting the macropinocytosis pathway is a promising therapeutic approach. Understanding the molecular regulatory mechanisms that underlie macropinocytosis in tumors is thus essential for the development of novel treatments for cancer. </sec><sec> There has been considerable progress in understanding the molecular regulatory mechanisms that control macropinocytosis, particularly in the context of tumor biology. This review first focuses on macropinocytosis to provide readers with insight into this physiological pathway, followed by a discussion of recent findings related to its molecular regulation. Tumor cells often experience nutrient stress due to their rapid growth and the abnormal vascularization of solid tumors. The activation of macropinocytosis in response to this stress promotes the acquisition of extracellular nutrients, such as amino acids and glucose, to meet the metabolic demands of the tumor. Several key signaling pathways regulate macropinocytosis in tumors under nutrient stress. One important pathway is the EGFR-PAK axis, which activates macropinocytosis in tumor cells, in response to glutamine deprivation. Unlike cancer cells, in cancer-associated fibroblasts (CAFs), macropinocytosis is initiated by calcium ion (Ca²⁺)-dependent pathways, such as Ca²⁺-CaMKK2-AMPK-RAC1 and Ca²⁺-ARHGEF2-RAC1 under glutamine starvation. Furthermore, the DDR1-NF-κB-p62/SQSTM1-NRF2 pathway mediates macropinocytosis under both glutamine-deprivation and glucose-depletion conditions. Notably, NRF2 is a critical regulator of the transcription of macropinocytosis-related genes, which promote the survival of cancer cells when autophagy, another nutrient-scavenging pathway, is inhibited. By contrast, when macropinocytosis is downregulated, the resulting low ATP level activates autophagy, suggesting a compensatory relationship between these two pathways. In addition to these pathways, several novel macropinocytosis-related proteins have been identified, including V-ATPase, SDC1, LYSET, TPC, TMEM206, and Slc15A. These proteins are involved in several aspects of macropinocytosis, such as cholesterol trafficking, RAC1 activation, lysosomal enzyme trafficking, macropinosome resolution, and macropinocytosis activation in <italic>Dictyostelium discoideum</italic>. </sec><sec> Despite the above-mentioned advancements, our understanding of macropinocytosis is incomplete. More research is needed to identify the proteins localized to macropinosomes, the mechanisms of macropinosome acidification, and the molecular machinery that connects macropinocytosis and autophagy. Additionally, the identification of highly specific and effective inhibitors of macropinocytosis would enable the development of novel therapeutics. Future research will benefit from the integration of high-resolution mass spectrometry, ultra-high-resolution confocal microscopy, and structural biology techniques. These advanced tools will enable a detailed examination of the membrane composition of macropinosomes and identification of the mechanisms by which macropinocytosis inhibitors exert their effects. Further exploration of these issues will not only provide a more comprehensive understanding of the formation and molecular regulatory mechanisms of macropinocytosis but also accelerate the translation of these pathways from basic biology to clinical applications. </sec>