Phosphodiesterase-resistant PKA I agonists inhibit thyroid cancer cell growth
S Lucchi1, D Calebiro2, T de Filippis1, P Porazzi1, P Beck-Peccoz3 & L Persani3
In thyroid cells TSH/cAMP stimulation is associated with proliferation and differentiation. The principal effector of cAMP is the protein kinase A (PKA) and events downstream PKA activation, signaling specificity and expression of PKA isoforms are largely unexplored. PKA is composed of two regulatory and two catalytic subunits. Two major isoforms of PKA (I and II) are determined by their specific regulatory subunits (RIα, RIβ, RIIα and RIIβ). A recent gene expression profile of differentiated FRTL5 and de-differentiated FRT rat thyrocytes revealed the lack of RIIβ expression in FRT cells, which was also associated with the loss of TSH-dependent effects. Here, we extended our observation to human thyroid cancer cell lines (ARO, anaplastic; NPA, papillary; WRO, follicular). As in FRT, RIIβ expression was absent in ARO cells. We then performed proliferative assays after stimulation with phosphodiesterase-resistant cAMP analog pair selective for PKA I or II. Only PKA II activation mimicked TSH proliferation in TSH-dependent FRTL5 cells. Conversely, PKA I action resulted in growth inhibition of FRT and human thyroid cancer cell lines, with a more pronounced effect in ARO and NPA (down to 40% of control growth) than in WRO cells. The antiproliferative effect of PKA I agonists was associated with the reduction of ERK phosphorylation. Such PKAI agonist effect on ERK phosphorylation was observed in cells endowed with constitutive MAPK activation through BRAF mutations, as in ARO and NPA. The effect of PKAI agonists was marginal in WRO cells consistently with the different mechanisms that sustain uncontrolled proliferation of these cells. These data indicate that RIIβ can be lost during de-differentiation and carcinogenesis process. In de-differentiated TSH-independent cells, selective PKA I activation has an antiproliferative effect which may open new therapeutic perspectives for thyroid cancer.