Regulation of E2F1 activity via PKA-mediated phosphorylations

E2F1 becomes activated during the G1 phase of the cell cycle, and posttranslational modifications modulate its activity. Activation of G-protein coupled receptors (GPCR) by many ligands induces the activation of adenylate cyclases and the production of cAMP, which activates the PKA enzyme. Activated PKA elicits its biological effect by phosphorylating the target proteins containing serine or threonine amino acids in the RxxS/T motif. Since PKA activation negatively regulates cell proliferation, we thought that activated PKA would negatively affect the activity of E2F1. In line with this, when we analyzed the amino acid sequence of E2F1, we found 3 hypothetical consensus PKA phosphorylation sites located at 127-130, 232-235, and 361-364 positions and RYET, RLLS, and RMGS sequences. After showing the binding and phosphorylation of E2F1 by PKA, we converted the codons of Threonine-130, Serine-235, and Serine-364 to Alanine and Glutamic acid codons on the eukaryotic E2F1 expression vector we had previously created. We confirmed the phosphorylation of T130, S235, and S364 by developing monoclonal antibodies against phospho-specific forms of these sites and showed that their phosphorylation is cell cycle-dependent. According to our results, PKA-mediated phosphorylation of E2F1 by PKA inhibits proliferation and glucose uptake and induces caspase-3 activation and senescence.

AKT-mediated phosphorylation of TWIST1 is essential for breast cancer cell metastasis

Previously, it was shown that human TWIST1 (basic helix-loop-helix (b-HLH) is phosphorylated by Akt kinase at S42, T121, and S123. To show in vivo effect of these phosphorylations, we created mouse TWIST1 expression vector and converted the codons of S42, T125, and S127 to unphosphorylatable alanine and phosphorylation mimicking Glutamic acid. We hypothesized that alanine mutants would inhibit the metastatic ability of 4T1 cells while glutamic acid mutants would convert nonmetastatic 67NR cells into metastatic phenotype. To confirm this hypothesis, we created metastatic 4T1 and nonmetastatic 67NR cells expressing alanine mutants and glutamic acid mutants mouse TWIST1, respectively. Then, we injected 1 × 10⁶ 67NR and 1 × 10⁵ 4T1 cells overexpressing mutants of TWIST1 into the breast tissue of BALB/c mice. At the end of the 4th week, we sacrificed the animals, determined the numbers of tumors at lungs and liver. Although 67NR cells overexpressing wild-type TWIST1 did not show any metastasis, cells overexpressing S42E and T125E mutants showed 15–30 macroscopic metastasis to liver and lungs. Parallel to this, 4T1 cells expressing S42A and T125A mutants of TWIST1 showed no macroscopic metastasis. Our results indicate that phosphorylation of S42 and T125 by AKT is essential for TWIST1-mediated tumor growth and metastasis.