Characterization of the effects of Ca2+ depletion on the synthesis, phosphorylation and secretion of caseins in lactating mammary epithelial cells.
We have examined the effects of depleting lumenal Ca2+ on the synthesis, phosphorylation and secretion of caseins in lactating mouse mammary cells by using inhibitors of the endoplasmic reticulum Ca(2+)-ATPase or the ionophore ionomycin in the absence of external Ca2+. Treatment with these drugs resulted in a transient increase in the cytosolic Ca2+ concentration due to Ca2+ mobilization. Protein synthesis over a 1 h period was substantially inhibited by Ca2+ depletion, but in a pulse-chase protocol secretion of pre-synthesized proteins was unaffected by Ca2+ depletion. Analysis of polysome profiles showed that Ca2+ depletion resulted in a loss of polysomes, consistent with an inhibition of initiation of protein synthesis. Neither treatment with Ca(2+)-ATPase inhibitors to deplete endoplasmic reticulum Ca2+ nor treatment with ionomycin/EGTA had any effect on an early phase of phosphorylation of alpha- or beta/gamma-caseins, but Ca2+ depletion resulted in a decrease in a late phase of casein phosphorylation. These results indicate that lumenal Ca2+ is required to maintain protein synthesis in lactating mammary cells but is not required for protein secretion, and that Ca2+ accumulation in the Golgi cisternae is required for a late but not for an early phase of casein phosphorylation.
Duncan
J S
JS
Physiological Laboratory, University of Liverpool, UK.
Burgoyne
R D
RD
D000818:Animals; D002118:Calcium / Q000172:deficiency*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D002364:Caseins / Q000378:metabolism*; D002469:Cell Separation; D004305:Dose-Response Relationship, Drug; D004533:Egtazic Acid / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004847:Epithelial Cells; D004848:Epithelium / Q000187:drug effects / Q000378:metabolism; D005260:Female; D006056:Golgi Apparatus / Q000378:metabolism; D006873:Hydroquinones / Q000494:pharmacology; D007476:Ionophores / Q000494:pharmacology; D007774:Lactation / Q000378:metabolism*; D008321:Mammary Glands, Animal / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D051379:Mice; D010766:Phosphorylation / Q000187:drug effects; D011132:Polyribosomes / Q000187:drug effects; D014176:Protein Biosynthesis / Q000187:drug effects; D013729:Terpenes / Q000494:pharmacology; D019284:Thapsigargin
D002364:Caseins; D004791:Enzyme Inhibitors; D006873:Hydroquinones; D007476:Ionophores; D013729:Terpenes; D004533:Egtazic Acid; D019284:Thapsigargin; C018855:2-tert-butylhydroquinone; D000252:Calcium-Transporting ATPases; D002118:Calcium