Hedgehog acyltransferase: A key player in Sonic hedgehog signalling in cancer cells

Protein acyltransferases (PATs), a group of enzymes that catalyze acylation of a variety of proteins, have recently been shown to play an important part in protein sorting and signalling in the cell. In the Sonic hedgehog (Shh) signalling pathway the PAT Hedgehog acyltransferase (Hhat) is responsible for addition of a palmitic acid (C16) to the N-terminal cysteine residue of Shh. In mice, this modification has been shown to be necessary for proper packaging and release of Shh from the signalling cell as well as for transmitting a potent signal to the receiving cell, rendering Hhat crucial for a functioning Shh signal. Aberrant regulation of the Shh signalling pathway leads to a variety of malignant diseases in adults. Oncogenic activation of the Shh signalling pathway is known to arise from excessive production of the Shh signal or oncogenic mutations in the signal receiving cell. To date, drug therapies targeting the pathway in cancer have been directed at Shh-independent mutations in downstream regulators, Ptch, Smo, and Su(Fu), in the signal-receiving cell. To complement this, we hypothesize that Hhat is a potential drug target in cancers dependent on excessive Shh production for their growth. Here, we validate the potential of Hhat as a therapeutic target in cancer cell lines dependent on Shh for their growth. By knocking down Hhat in established cancer cell lines using siRNA we show that proper distribution and signalling by Shh can be inhibited. We measure the effect of the knock-down on target gene expression of the pathway (Ptch and Gli1), cancer cell growth and invasive properties of the cells. The effect is validated against inhibition of the Shh pathway by the Shh blocking antibody 5E1, and KAAD-cyclopamine, a Smo antagonist