Yes-associated protein 1 (YAP1) is a transcriptional coactivator in the Hippo signaling pathway. Increased YAP1- activity promotes the growth of tumors, including that of colorectal cancer (CRC). Verteporfin, a drug that enhances phototherapy to treat neovascular macular degeneration, is an inhibitor of YAP1. Here, we found that verteporfin inhibited tumor growth independently of its effects on YAP1 or the related protein TAZ in genetic or chemical-induced mouse models of CRC, in patient-derived xenografts and in enteroid models of CRC. Instead, verteporfin exhibited in vivo selectivity for killing tumor cells in part by impairing the global clearance of high molecular weight oligomerized proteins, particularly p62 (a sequestrome involved in autophagy) and STAT3 (a transcription factor). Verteporfin inhibited cytokine-induced STAT3 activity and cell proliferation and reduced the viabilty of cultured CRC cells. Although verteporfin accumulated to a greater extent in normal cells than in tumor cells in vivo, experiments with cultured cells indicated that the normal cells efficiently cleared verteporfin-induced protein oligomers through autophagic and proteasomal pathways. Culturing CRC cells in hypoxic or nutrient-deprived conditions (modeling a typical CRC microenvironment) impaired the clearance of protein oligomers and resulted in cell death; whereas culturing cells in normoxic or glucose-replete conditions protected cell viability and proliferation in the presence of verteporfin. Furthermore, verteporfin suppressed the proliferation of other cancer cell lines even in the absence of YAP1, suggesting that verteporfin may be effective against multiple types of solid cancers.

Colorectal cancer (CRC) is the third-leading cause of cancer mortality in the United States and other industrialized countries. A hypoxic microenvironment is a hallmark for solid tumors. The hypoxia-induced signal transduction is transcriptionally mediated by hypoxia-inducible factor (HIF). Three major HIF isoforms, HIF-1α, HIF-2α, and HIF-3α, are present in the intestine. Our previous work demonstrates that HIF-2α is essential for CRC growth and progression. However, the mechanisms mediating cell proliferation after hypoxia or HIF-2α activation in CRC are unclear. Data mining of RNA-Seq experiments with mouse models of intestinal HIF-2α or Yes-associated protein 1 (YAP1) overexpression indicates a significant overlap of genes in these conditions. YAP1 is a transcriptional co-activator in the Hippo signaling pathway, and YAP1-induced transcriptional responses are essential in cancer cell proliferation. Here, we report that HIF-2α robustly increases YAP1 expression and activity in CRC-derived cell lines and in mouse models. The potentiation of YAP1 activity by HIF-2α was not via canonical signaling mechanisms such as Src (non-receptor tyrosine kinase), PI3K, ERK, or MAPK pathways. Moreover, we detected no direct interaction of HIF-2α with YAP1. Of note, YAP1 activation was critical for cancer cell growth under hypoxia. Our findings indicate that HIF-2α increases cancer cell growth by up-regulating YAP1 activity, suggesting that this pathway might be targeted in potential anti-cancer approaches for treating CRC patients.

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