Changes in intracellular Ca2+ distribution during the transition of fibroblasts from the proliferating to the stationary state

Alterations in intracellular calcium transients of fibroblasts from progressive systemic sclerotic patients: a digital imaging microscopic study

Intracellular Ca2+ concentrations ([Ca2+]i) in cultured skin fibroblasts from normal subjects and progressive systemic sclerosis (PSS) patients were determined by using Fura-2 and fluorescent videomicroscopy. With the exception of fibroblasts from one PSS patient showing a higher [Ca2+]i, no significant difference was observed in resting [Ca2+]i between the two groups of fibroblasts. Bradykinin (BK) (10 microM) induced a transient [Ca2+]i increase in normal fibroblasts, whereas the BK-induced [Ca2+]i increase was reduced or not detectable in PSS fibroblasts. Removal of extracellular Ca2+ did not eliminate the BK-induced [Ca2+]i increase in normal fibroblasts. These findings suggest that BK stimulates Ca2+ release from intracellular stores in human fibroblasts, and also that the BK-mediated Ca2+ release is impaired in PSS fibroblasts.

Effects of synthetic calcium phosphates on the 3H-thymidine incorporation and alkaline phosphatase activity of human fibroblasts in culture

Gingival fibroblasts were cultured with four different calcium phosphate minerals (hydroxyapatite, whitlockite, beta-tricalcium phosphate, and octocalcium phosphate). 3H-thymidine incorporation into DNA and alkaline phosphatase specific activity were determined after different incubation periods. As a consequence of the phagocytosis of calcium phosphates crystals, we pointed out, compared to control, a stimulation of the rate of 3H-thymidine incorporation and sharp decreases in alkaline phosphatase activity. The magnitude of the alkaline phosphatase activity inhibition was observed to be increased with the solubility of the materials. We propose that the effects of calcium phosphates on alkaline phosphatase and 3H-thymidine incorporation could be calcium-mediated events, resulting from intracellular dissolution of phagocytized materials. We suggest that in vitro determination of 3H-thymidine incorporation and alkaline phosphatase activity, which are highly sensitive tests, could be involved in evaluation procedures of calcium phosphates biomaterials.

Endocytosis is required for the mitogenic effect of basic calcium phosphate crystals in fibroblasts

Basic calcium phosphate (BCP) crystals stimulate mitosis of cultured fibroblasts and synoviocytes in vitro. Although intimate crystal-cell contact is required for mitogenesis, and lysosomotropic agents such as chloroquine and ammonium chloride block the mitogenic effect of crystals, the requirement of endocytosis has not been demonstrated. Synthetic BCP crystals, uniformly trace-labeled with 45Ca, were added to cultured, quiescent, confluent human foreskin fibroblasts in the presence or absence of ammonium chloride. After 4 or 24 hours, cultures were pulsed with 3H thymidine. The cells were then released with EDTA and trypsin, and fractionated on preformed Percoll density gradients. Fractions were analyzed for incorporation of 45Ca and [3H]thymidine and cell number. The cells containing 45Ca crystals also were heavily labeled with thymidine. Ammonium chloride decreased the amount of crystals endocytosed, and inhibited mitogenesis. These data suggest that mitogenesis induced by BCP crystals is preceded by endocytosis and dissolution in the acidic environment of phagolysosomes.

Cultured human monocytes and fibroblasts solubilize calcium phosphate crystals

Two rheumatic syndromes associated with deposition of calcium phosphate crystals in soft tissues of the shoulder have prompted us to study the cellular mechanisms of calcium phosphate crystal solubilization. Synthetic 45Ca labeled calcium phosphate crystal aggregates were solubilized by cultured human fibroblasts or monocytes. Such solubilization required crystal cell contact and was inhibited by chloroquine and ammonium. We hypothesize that the mechanism of crystal solubilization involves phagocytosis followed by dissolution in the acidic environment of secondary lysosomes. Study of the mechanism of calcium phosphate solubilization may be important in understanding resorption of extra-osseus as well as osseus calcification. Intracellular release of calcium from calcium phosphate crystals may also explain our previous observation that hydroxyapatite and other calcium-containing crystals are mitogenic stimuli for fibroblasts and synovial cells.

Cyclic nucleotides and calcium transport in cultured dermal fibroblasts from progressive systemic sclerosis and rheumatoid arthritis patients

Levels of cAMP were decreased and cGMP increased in progressive systemic sclerosis (PSS) fibroblasts compared with normal fibroblasts. PSS cells took up unusually large quantities of Ca++. The epinephrine-induced increases in cAMP were diminished sixfold and epinephrine-induced Ca++ uptake was abnormal in PSS cells. Considerable heterogeneity was observed in rheumatoid arthritis fibroblasts. Membrane-bound abnormalities in (PSS) cells may be of importance in the defective control of collagen production in progressive systemic sclerosis.