Down-regulation and differential restoration of cAMP responses upon transient phorbol ester treatment of primary osteoblastic cells

Opposing regulatory effects of protein kinase C on the cAMP cascade in human HL-60 promyelocytic leukemia cells

The functional role of protein kinase C in the cAMP signaling cascade was investigated in human promyelocytic leukemia (HL-60) cells. Protein kinase C activation after short exposure to 100 nM phorbol 12-myristate 13-acetate (PMA) increased the intracellular cAMP level up to 3- to 5-fold after 30 min. Such enhancement was almost completely blocked by the selective protein kinase C inhibitor bisindolylmaleimide (GF 109203X). In addition, PMA, but not 4-alpha-PMA, synergistically elevated cAMP levels when adenylyl cyclase was activated directly by forskolin or indirectly by G protein activation after cholera toxin treatment or guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) treatment in digitonin-permeabilized cells. The results indicate that protein kinase C directly increases adenylyl cyclase activity and synergistically enhances it, when it is simultaneously activated otherwise. On the other hand, a 10-min treatment with PMA cut the cAMP accumulation induced by histamine, prostaglandin E2, or isoproterenol by 50-70%. However, the binding affinity and total binding of [3H]histamine to membrane receptors was not effected by PMA, suggesting that the site of protein kinase C's action is not at the receptor level. Western blot analysis of protein kinase C isozymes revealed that PMA (100 nM) caused translocation of cytosolic protein kinase C such as alpha, beta and epsilon to the particulate/membrane fraction. Treatment with a lower concentration of PMA (10 nM) translocated the protein kinase C-epsilon within 2 min, while it had little effect on the translocation of protein kinase C-alpha and -beta up to 20 min. However, simultaneous treatment with 10 nM PMA plus histamine for 5 min significantly inhibited the histamine-mediated cAMP generation, indicating that the protein kinase C-epsilon could be involved in the inhibition of receptor-mediated cAMP generation. Taken together, we conclude that PMA, through the activation of protein kinase C, has two opposite effects on the cAMP signaling cascade in HL-60 cells: a direct activation of adenylyl cyclase and an inhibition of receptor-mediated signal transduction.

Expression of the parathyroid hormone receptor and correlation with other osteoblastic parameters in fetal rat osteoblasts

Primary fetal rat calvarial cell cultures were examined for the expression of different osteoblastic parameters at the single cell level and in the whole population. The presence of the parathyroid hormone (PTH) receptor was studied by employing receptor autoradiography. After 3 days of culture, 10% of the cells expressed the PTH receptor. Immunolocalization of osteocalcin in 3-day-old cell cultures was found to be strongly correlated with the presence of the PTH receptor. Alkaline phosphatase (APase) localization in 3-day-old cultures correlated with only 69% of the PTH receptor expressing cells. Our results show that in 3-day-old rat calvarial cell cultures, only about 10% of the cells show markers of osteoblastic differentiation. The presence of the PTH receptor is strongly correlated with the presence of osteocalcin, but less with the presence of APase, indicating that it is the mature osteoblast that expresses the PTH receptor. After 7 days of culture, most receptor labeling, APase, and osteocalcin expression was found in multilayered areas of cells (nodules).