Mitogen-activated Protein Kinase Phosphatase-3 Is a Tumor Promoter Target in Initiated Cells That Express Oncogenic Ras

Abstract

We have capitalized on the unique properties of the skin tumor promoter palytoxin, which does not activate protein kinase C, to investigate alternative mechanisms by which major signaling molecules can be modulated during carcinogenesis. We report here that palytoxin activates extracellular signal-regulated kinase (ERK) through a novel mechanism that involves inactivation of an ERK phosphatase in keratinocytes derived from initiated mouse skin (308 cells). Use of U0126 revealed that palytoxin requires the ERK kinase MEK to stimulate ERK activity, although palytoxin did not activate MEK. We found that 308 keratinocytes highly express mitogen-activated protein kinase phosphatase-3 (MKP-3), which selectively inactivates ERK. Palytoxin induced the loss of MKP-3 in a manner that corresponded to increased ERK phosphorylation. Complementary studies showed that sustained expression of exogenous MKP-3 inhibited palytoxin-stimulated ERK activation. As is characteristic of initiated keratinocytes, 308 cells express activated H-Ras. To investigate whether expression of oncogenic Ras is key to palytoxin-stimulated ERK activation, we determined how palytoxin affected ERK and MKP-3 in MCF10A human breast epithelial cells and in H-ras MCF10A cells, which stably express activated H-Ras. Palytoxin did not affect ERK activity in MCF10A cells, which had no detectable MKP-3. Like 308 cells, H-ras MCF10A cells highly express MKP-3. Strikingly, palytoxin stimulated ERK activity and induced a corresponding loss of MKP-3 in H-ras MCF10A cells. These studies indicate that in initiated cells palytoxin unleashes ERK activity by down-regulating MKP-3, an ERK inhibitor, and further suggest that MKP-3 may be a vulnerable target in cells that express oncogenic Ras.

Activating K-Ras mutations outwith ‘hotspot’ codons in sporadic colorectal tumours – implications for personalised cancer medicine

“…DUSPs are key regulators of the balance between kinase pathway activation and inactivation, and have previously been reported to be upregulated in an adaptive response, creating a negative feedback loop following MAPK pathway activation (Keyse, 2008). Consistent with our own data, DUSP4 expression has previously been shown to be increased in pancreatic tumours with K-Ras mutations (Yip-Schneider et al, 2001) and DUSP6 expression increased in a variety of tumour types with mutations in Ras or Raf pathway genes (Croonquist et al, 2003;Warmka et al, 2004;Bloethner et al, 2005). Of particular interest, however, was our observation that certain signalling cascades, including those mediated by Igf1 and Vegf, were differentially activated by cluster 1 and cluster 2 K-Ras mutants, suggesting that not only the presence but the specific molecular characteristics of individual K-Ras mutations may be important determinants of both tumour progression and treatment response.…”

ERK Signal Suppression and Sensitivity to CH5183284/Debio 1347, a Selective FGFR Inhibitor

“…Overexpression of DUSP6 was observed in response to activated RAS or BRAF (30)(31)(32)(33)(34), representing an increase in negative feedback of the MAPK pathway. Therefore, DUSP6 expression could reflect activity of the MAPK pathway.…”

Dual-specificity phosphatase DUSP6 has tumor-promoting properties in human glioblastomas

“…As stated by Bermudez et al (2011), upregulation of DUSP6 expression may be a way for cancer cells to maintain a level of ERK activity compatible with survival and cycle progression. In this respect, it may explain why DUSP6 expression is upregulated in many cancers harboring Ras mutations (Warmka et al, 2004 andBloethner et al, 2005).…”