A study of the protein kinase MK5's effect on melanoma cell proliferation

Abstract

The mitogen-activated protein kinases (MAPKs) play a crucial role in cellular processes such as proliferation, differentiation, apoptosis, metabolism and motility. Both conventional and atypical MAPKs can phosphorylate other proteins referred to as MAPK- activated protein kinases (MAPKAPK) such as MK5 or p38-regulated /activated protein kinase. Mutations in the MK5 gene have been detected in lung, melanoma and skin cancer tissue. Several studies pointed to MK5 as an important player in mediating senescence in skin and hematopoietic cells and in inhibiting tumor- migration in osteosarcoma U2OS cells. MK5 can also reduce the invasiveness in breast cancer. However, once the tumor is established, MK5 promotes angiogenesis. The latter finding indicates that MK5 may possess oncogenic potentials. In this study we investigated the possible anti-proliferative role of MK5 in melanoma cells with BRAF (A375 and WM266-4) and NRAS mutation (SK-MEL-2). Melanoma cells that transiently overexpressed wild-type, constitutive active or kinase dead MK5, or cells that were stably transfected with constructs for these MK5 variants were monitored for cell proliferation using MTT assay and BrdU incorporation. The expression of the senescence markers senescence-activated--galactosidase, p21Cip1/Waf1 and p16INK4A was also tested in these melanoma cell lines.The p21 transcripts were expressed and p21 promoter activity or senescence –activated β-gal activity was elevated in A375 and WM266-4 cells expressing constitutively active MK5, suggesting a possible role for MK5 in mediating senescence in these cells. Transiently activation of MK5 by forskolin treatment of WM266-4 and SK-MEL-2 cells also reduced the proliferation rate compared to non-treated cells. However, others experiments with SK-MEL-2 cells indicated that inactive MK5 reduced proliferation, whereas MK5 wild type overexpression had the opposite effect, suggesting a pro-oncogenic role of MK5 in these cells which depends on the enzymatic activity of the kinase. Further studies are required to elucidate the exact role of MK5 in melanoma. Since skin cancer is one the most aggressive cancers, determining the exact contribution of MK5 in melanoma may be helpful in using MK5 as therapeutic target.