Phorbol ester prevents the thyroid‐stimulating‐hormone‐induced but not the forskolin‐induced decrease of cAMP‐dependent protein kinase activity in thyroid cell cultures
Authors: OMRI Boubaker , BRETON Marie‐France , HAYE Bernard , JACQUEMIN Claude , PAVLOVIC‐HOURNAC Miroslava 
Published: March 2005

The potent tumor promoter 12‐O‐tetradecanoyl‐phorbol 13‐acetate (TPA) affects several thyroid cell functions and interacts with thyroid‐stimulating hormone (TSH) either by inhibiting or potentiating its action on different cellular parameters. Since phorbol ester acts mainly through the activation of protein kinase C, which is its receptor, we studied this activation and its interaction with TSH and forskolin in suspension cultures of porcine thyroid cells.

In thyroid cell cultures, TPA has a dual effect on protein kinase C activity: immediately (2–5 min) after exposure of cells to TPA, it began to be translocated from the cytosol to the particulate fraction. The transfer of the cytosolic enzyme was total and could occur with or without a loss of activity. The translocated enzyme still needed Ca2+ and phospholipids for its activation. The basal activity increased transiently (2–4 h) in both the cytosol and particulate fractions during translocation.

The peak activity in the particulate fraction was reached 10–30 min after exposure of cells to TPA, and was followed by down‐regulation of protein kinase C and almost complete disappearance of its activity. The residual activity was about 13% of control after a 2‐day exposure to TPA. It was unequally distributed between cytosol (4%) and particulate fraction (9%).

Prolonged exposure of cells to TPA did not affect either the activity or the subcellular distribution of the CAMP‐dependent protein kinase activity. TPA interacted with TSH and prevented the decrease of this activity induced by prolonged exposure of cells to the hormone not only when it was introduced simultaneously with TSH, but also when it was added 24 h after TSH. However, the forskolin‐induced decrease in CAMP‐dependent protein kinase activity was not prevented by the presence of TPA.

TPA also affected the increases in cAMP accumulation mediated by TSH and forskolin. The TSH‐induced increase was significantly stimulated by TPA after short contacts (5–15 min), while longer preincubations of cells with TPA provoked a very strong inhibition of the TSH action. However, the forskolin‐induced stimulation of the cAMP accumulation was maintained and even further increased in the presence of TPA. Consequently, the actions of TSH and TPA are apparently interdependent, while those of forskolin and TPA seem to be parallel and independent.

Neither TSH nor forskolin prevented the TPA‐induced down regulation of protein kinase C. The biologically inactive phorbol ester analogue 4α‐phorbol 12,13‐didecanoate had no effect on protein kinase C activity, and did not interact with either TSH or forskolin.

Endogenous substrates of the two protein kinases in cultured thyroid cells were revealed by gel electrophoresis after phosphorylation of cytosolic proteins by these enzymes. Three protein bands (28, 30 and 33 kDa) were specifically phosphorylated by CAMP‐dependent protein kinase; the molecular masses of the proteins phosphorylated by protein kinase C were 35–38 kDa.

There was a close parallel between the two protein kinase activities and the phosphorylation of their specific substrates. These substrates were not labeled in any of the assays in which the corresponding protein kinase activity was lacking. The presence of their substrates in phosphorylated forms can therefore serve as markers of the respective protein kinase activities in thyroid cells.