Mammalian Ste20-like Kinase (Mst2) Indirectly Supports Raf-1/ERK Pathway Activity via Maintenance of Protein Phosphatase-2A Catalytic Subunit Levels and Consequent Suppression of Inhibitory Raf-1 Phosphorylation

Abstract

Many tumor suppressor proteins act to blunt the effects of mitogenic signaling pathways. Loss of function mutations in the merlin tumor suppressor underlie neurofibromatosis type 2 (NF2), a familial autosomal dominant cancer syndrome. Studies of Drosophila suggest that Hippo (hpo) is required for inhibition of cell proliferation mediated by dMer, the orthologue of human merlin. Mammalian sterile 20-like kinase-2 (Mst2) is a mammalian Hpo orthologue, and numerous studies implicate Mst2 as a tumor suppressor. Mst2 is negatively regulated by the proto-oncoprotein Raf-1 in a manner independent of the kinase activity of Raf-1. We sought to determine whether, in mammalian cells, merlin could positively regulate Mst2. We also sought to determine whether Mst2, in addition to being negatively regulated by Raf-1, might itself reciprocally regulate Raf-1. In contrast to findings from Drosophila, we find no evidence that mammalian merlin positively regulates mammalian Mst2. Instead, surprisingly, RNA interference silencing of Mst2 leads to elevated inhibitory phosphorylation of Raf-1 at Ser-259 and impaired Raf-1 kinase activity. Consequent to this, ERK pathway activation and cell proliferation are attenuated. Phosphatase-2A (PP2A) dephosphorylates Raf-1 Ser-259 in response to mitogens. Interestingly RNA interference silencing of Mst2 triggers a striking proteasome-dependent decrease in the levels of the catalytic subunit of PP2A (PP2A-C). A similar effect is achieved upon silencing of large tumor suppressor (LATS)-1 and LATS2, direct substrates of Mst2. Our studies reveal a more complex role for Mst2 than previously thought. The Mst2 3 LATS1/2 pathway, by maintaining PP2A-C levels, may, in some situations, positively affect mitogenic signaling.

The Tumor Suppressor RASSF1A Prevents Dephosphorylation of the Mammalian STE20-like Kinases MST1 and MST2

“…This conclusion is in complete agreement with a new report published while this manuscript was in revision that shows that in Drosophila PP2A is the phosphatase that dephosphorylates the MST ortholog, the Hippo kinase (35). The situation is most likely complex and may involve feedback loops because recent results suggest that an intact MST2 signaling pathway is necessary for maintenance of PP2Ac levels in the cell (36).…”

“…Physiological inputs upstream of MST kinases other than apoptotic inducers may be important for MST signaling but remain to be identified. For example, in contrast to findings from Drosophila, no evidence was found that mammalian merlin positively regulates mammalian MST2 (36). Of note, there are several reports that have linked the function of MST kinases to centrosome duplication (41), mitotic control (7,42,43), and lymphocyte trafficking (44).…”

The Mammalian Ste20-like Kinase 2 (Mst2) Modulates Stress-induced Cardiac Hypertrophy

“…We found a trend of increased PP2A-C in Mst2-overexpressing cardiomyocytes (Fig. 7, I-J), which is consistent with previously published observations (25). This indicates that PP2A might be involved in Mst2-dependent regulation of Raf1/ERK1/2 in cardiomyocytes.…”

“…Previously, it has been reported that Mst2 positively regulated Raf1 through modulation of phosphatase 2A (PP2A) (25). Therefore, we examined the expression of the catalytic C subunit of PP2A in NRCM-overexpressing Mst2.…”

Organ-specific alteration in caspase expression and STK3 proteolysis during the aging process

“…However, whereas all caspases generate a 34 kDa STK3 fragment, CASP6 also generates a 39 kDa fragment. The apoptotic functions of the STK4 36 kDa fragment are well described in the literature (Deng, Pang, and Wang 2003;Kilili and Kyriakis 2010;Kim et al 2010;O'Neill et al 2004;Qin et al 2013) and some studies suggest that the 34 kDa STK3 fragment may have a similar role (Deng, Pang, and Wang 2003) . However, to our knowledge, the existence and the production of the STK3 39 kDa fragment by a caspase has not been previously described.…”