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Akt signaling is sustained by a CD44 splice isoform-mediated positive feedback loop
Tumor cells nearly invariably evolve sustained PI3K/Akt signaling as an effective means to circumvent apoptosis and maintain survival. However, for those tumor cells that do not acquire PI3K/Akt mutations to achieve this end the underlying mechanisms have remained obscure. Here we describe the discovery of a splice isoform-dependent positive feedback loop that is essential to sustain PI3K/Akt signaling in breast cancer. Splice isoform CD44s promoted expression of the hyaluronan synthase HAS2 by activating the Akt signaling cascade. The HAS2 product hyaluronan (HA) further stimulated CD44s-mediated Akt signaling, creating a feed-forward signaling circuit that promoted tumor cell survival. Mechanistically, we identified FOXO1 as a bona fide transcriptional repressor of HAS2. Akt-mediated phosphorylation of FOXO1 relieved its suppression of HAS2 transcription, with FOXO1 phosphorylation status maintained by operation of the positive feedback loop. In clinical specimens of breast cancer, we established that the expression of CD44s and HAS2 were positively correlated. Our results establish a positive feedback mechanism that sustains PI3K/Akt signaling in tumor cells, further illuminating the nearly universal role of this pathway in cancer cell survival.
VHL inactivation in precancerous kidney cells induces an inflammatory response via ER stress-activated IRE1{alpha} signaling
Mutations and epigenetic inactivation of the tumor suppressor gene von Hippel-Lindau (VHL) are major causes of clear-cell renal cell carcinoma (ccRCC) that may originate from chronic inflammation. However, the role of VHL loss-of-function in the development of ccRCC via inflammation remains poorly understood. VHL mutant cells exhibit metabolic abnormalities that can cause chronic endoplasmic reticulum (ER) stress and unfolded protein response (UPR). We hypothesize that unresolved ER stress induces the inflammatory responses observed in ccRCC. ER stress markers including BiP and XBP1s were significantly increased in cultured and primary VHL loss-of-function kidney cells. In epithelial cells, the kinase activity of IRE1α was required for the induction of NFκB and JNK and for the recruitment of macrophages. IRE1α kinase activity was also important for the development of fibrotic phenotype in conditional Vhlh knockout mice. Our results offer insights into the therapeutic potential against ccRCC development by relieving metabolic stress. Such cancer prevention strategy may be critical for high-risk cohorts such as the familial VHL disease patients.
