Klotho: a tumor suppressor with a potential glycosidase activity

Klotho is an anti-aging hormone and a tumor suppressor shown to play a key role in energy metabolism and cancer. Downregulation of Klotho results in enhanced proliferation and reduced apoptosis of cancer cells, while overexpression inhibits colony formation of breast cancer cells in vitro. While its importance as a master regulator in human health and in malignancy is clear, the precise mechanism of action of Klotho remains obscure. Several studies suggested that Klotho has a carbohydrate cleaving enzymatic activity, but it remains controversial. We hypothesize that Klotho can act as a sialidase, to remove the terminal sialic acid carbohydrates from glycans, thereby affecting downstream signalling in cancer cells. Here we show that Klotho removes sialic acid from a purified glycoprotein, as measured by HPLC with fluorescence detection of the released sialic acids. Moreover, exposure of Klotho against diverse glycoproteins printed microarrays revealed an altered recognition by carbohydrate-binding lectins, with a decreased recognition of sialic acids and reciprocal increased recognition of galactose, suggesting that Klotho cleaves sialic acid thereby exposing the underlying galactose. To evaluate the potential downstream effects on cells, we compared proteomics of breast cancer cell lines that were either control-treated or exposed to Klotho for 5 minutes or 2 hours. Klotho significantly affected 1236 different proteins of various biological pathways. Some of these proteins are potential substrates for Klotho, while others likely contribute to downstream signalling pathways. Further studies will validate some of these participating proteins. Better understanding of Klotho’s activity may contribute to the design of novel cancer treatment approaches.