Legumain regulates advanced prostate cancer via HSD17B4

Prostate cancer is the most common male cancer diagnosis. However, current treatment is limited and patients often develop resistant, metastatic disease. Novel targeted therapies which prevent recurrence and metastasis are essential to improve patient prognosis. A possible novel therapeutic target, the enzyme Legumain (LGMN), is overexpressed in advanced prostate cancers. Conversely, cystatin E/M (CST6), the endogenous inhibitor of LGMN, is repressed, suggesting a role for uncontrolled LGMN activity in prostate carcinogenesis. Despite this implied role, little is known about LGMN functionality and how it may regulate to tumorigenesis.

To clarify how LGMN signalling impacts prostate cancer formation and metastasis, novel small molecule inhibitors were developed and tested across a panel of normal cell lines and prostate cancer lines. Our data demonstrate that prostate cancer cells are ‘addicted' to LGMN signalling. Endogenous LGMN expression and enzymatic activity is heightened in prostate cancer cell lines, loss of which, via small molecule inhibitors or RNAi, is selectively toxic to cancer cells, with limited effect in normal cell lines. Further, LGMN appears to be pivotal to tumorigenic growth, with notable loss of colony formation in anchorage-independent growth assays when LGMN is depleted.

The biology surrounding LGMN is poorly characterised and to date few substrates have been confirmed. In order to identify novel LGMN interactors/substrates with potential roles in tumorigenic signalling, we performed BioID, a method which employs promiscuous biotin ligase labelling of proximal proteins. BioID identified a number of proteins which associate with LGMN including a known interactor, HSP90. A number of novel interactors were also identified including HSD17B4, a bifunctional steroidogenic enzyme previously been reported to associate with poor clinical outcomes in prostate cancer and to regulate resistance to anti-androgen therapies. Given the role of HSD17B4 in steroidogenesis we postulate that this LGMN-HSD17B4 interaction may represent one route through which LGMN promotes tumorigenesis and drug resistance (particularly to androgen deprivation therapies) in advanced disease. Reassuringly, knockdown of HSD17B4 reduced cell proliferation in a cancer-specific manner, similar to LGMN knockdown/inhibition. Ongoing work will delineate how LGMN and HSD17B4 interact to promote carcinogenesis and resistance to anti-androgen therapies.

There are limited options available to patients with advanced castrate-resistant prostate cancer. These data seek to clarify the role LGMN plays in prostate tumour growth and metastasis with the ultimate aim of improving patient outcomes and potentially providing novel therapeutic options, particularly for the clinical management of castrate-resistant prostate cancers.