PP2A activity is controlled by methylation and regulates oncoprotein expression in melanoma cells: A mechanism which participates in growth inhibition induced by chloroethylnitrosourea treatment

Abstract

Abstract. Protein phosphatase 2A (PP2A), an Akt pathway inhibitor, is considered to be activated by methylation of its catalytic subunit. Also PP2A downregulation was proposed to take part in carcinogenesis. Recently, PP2A activation was shown to be activated in response to DNA damage. To obtain further information on the role of PP2A in tumors and response to DNA damage, we investigated the relationship between PP2A methylation and activity, cell proliferation, Akt activation, c-Myc expression and PTEN activity in B16 melanoma cells untreated and after chloroethylnitrosourea (CENU) treatment. In untreated cells, okadaic acid, an antagonist of PP2A methylation, inhibited PP2A activity, stimulated cell proliferation, increased Akt activation and c-Myc expression. Xylulose-5-phosphate, an agonist of PP2A methylation, increased PP2A activity, decreased cell proliferation, Akt activation and c-Myc expression. However, both PP2A methylation modulators increased PTEN activity. During the response to CENU treatment, PP2A methylation and activity were strongly increased, Akt activation and c-Myc expression were decreased. However PTEN activity was increased. After tumor cell growth recovery, these modifications were moderately decreased. PP2A methylation was quantified and correlated positively with PP2A activity, and negatively with criteria for cell aggressiveness (cell proliferation, Akt activation, c-Myc expression). Based on these data, PP2A methylation status controls PP2A activity and oncoproteins expression and PP2A is strongly activated after CENU treatment thus partly explaining the growth inhibition in response to this agent. It follows that PP2A promethylating agents are potential candidates for anticancer drugs.

Serine/threonine protein phosphatases in DNA damage response

“…Upon DNA damage, PP2A is activated, partially regulated by methyla-tion on the catalytic domain [74]. PP2A regulates the DNA damage-induced G1/S checkpoint, and several checkpoint proteins, including Rb (retinoblastoma), p107, and p53, which are PP2A substrates [70,75].…”

The Akt signaling pathway

“…One study using mouse melanoma cells showed that PP2A expression is regulated by methylation of a catalytic subunit. 72 Inhibition of methylation by okaidic acid then reduced PP2A activity, resulting in increased Akt phosphorylation and proliferation. 72 Agents such as chloroethylnitrosourea (CENU) augment methylation of PP2A to reduce cell proliferation and survival.…”

“…72 Inhibition of methylation by okaidic acid then reduced PP2A activity, resulting in increased Akt phosphorylation and proliferation. 72 Agents such as chloroethylnitrosourea (CENU) augment methylation of PP2A to reduce cell proliferation and survival. 72 Therefore, PP2A promethylating agents are potential candidate melanoma inhibitors.…”

“…72 Agents such as chloroethylnitrosourea (CENU) augment methylation of PP2A to reduce cell proliferation and survival. 72 Therefore, PP2A promethylating agents are potential candidate melanoma inhibitors. Other mechanisms such as ubiquitination also have been shown to regulate Akt activity in several cancers, 70,75 but in melanomas this mechanism of regulation remains to be explored.…”

“…Phosphatases control the overall phosphorylation status of Akt in melanoma cells. 72 Key phosphatases in melanomas include (1) PTEN, (2) the protein phosphatase 2A (PP2A), and (3) PH domain leucine-rich repeat protein phosphatase (PHLPP). Whereas PTEN dephosphorylates PtdIns(3,4,5)P 3 to form PtdIns(4,5)P 2 , PP2A and PHLPP dephosphorylate T308 and S473, respectively, to deactivate Akt kinases.…”