Cyclosporin A Increases Mitochondrial Buffering of Calcium: An Additional Mechanism in Delaying Mitochondrial Permeability Transition Pore Opening.

Regulation of mitochondrial free Ca2+ is critically important for cellular homeostasis. An increase in mitochondrial matrix free Ca2+ concentration ([Ca2+]m) predisposes mitochondria to opening of the permeability transition pore (mPTP). Opening of the pore can be delayed by cyclosporin A (CsA), possibly by inhibiting cyclophilin D (Cyp D), a key regulator of mPTP. Here, we report on a novel mechanism by which CsA delays mPTP opening by enhanced sequestration of matrix free Ca2+. Cardiac-isolated mitochondria were challenged with repetitive CaCl2 boluses under Na+-free buffer conditions with and without CsA. CsA significantly delayed mPTP opening primarily by promoting matrix Ca2+ sequestration, leading to sustained basal [Ca2+]m levels for an extended period. The preservation of basal [Ca2+]m during the CaCl2 pulse challenge was associated with normalized NADH, matrix pH (pHm), and mitochondrial membrane potential (ΔΨm). Notably, we found that in PO43- (Pi)-free buffer condition, the CsA-mediated buffering of [Ca2+]m was abrogated, and mitochondrial bioenergetics variables were concurrently compromised. In the presence of CsA, addition of Pi just before pore opening in the Pi-depleted condition reinstated the Ca2+ buffering system and rescued mitochondria from mPTP opening. This study shows that CsA promotes Pi-dependent mitochondrial Ca2+ sequestration to delay mPTP opening and, concomitantly, maintains mitochondrial function.

Parathyroid-hormone-related protein signaling mechanisms in lung carcinoma growth inhibition.

Parathyroid hormone-related protein (PTHrP) inhibits proliferation of several lung cancer cell lines, but the signaling mechanism has not been established. This study tested the hypotheses that growth inhibition is mediated through the PTHrP receptor, PTH1R, and that the process is modified by ERK activation. PTHrP-positive and negative clones of H1944 lung adenocarcinoma cells underwent stable PTH1R knockdown with lentiviral shRNA or transient transfection with ERK1 and ERK2 siRNA. Alternatively, cells were treated with 8-CPT cAMP, 8-CPT 2'-O-methyl cAMP, and N-6-phenyl cAMP analogs. H1944 cells expressing ectopic PTHrP showed 20-40% decrease in proliferation compared to the PTHrP-negative cells in the presence of normal levels of PTH1R (P < 0.01). PTH1R knockdown eliminated this difference and increased cell proliferation regardless of PTHrP status. The three cAMP analogs each inhibited proliferation over 5 days by 30-40%. ERK2 knockdown inhibited proliferation of PTHrP-positive cells alone and in combination with ERK1 knockdown. The growth inhibition mediated by cAMP analogs was unaffected by ERK1 knockdown. In conclusion, ectopic expression of PTHrP 1-87 inhibits H1944 cell proliferation. PTH1R knockdown blocks this effect and stimulates proliferation, indicating that the ligand exerts anti-mitogenic effects. cAMP, the second messenger for PTH1R also inhibits proliferation and activates ERK. PTHrP growth inhibition may be opposed by concomitant ERK activation.