Cell number-dependent regulation of Hsp70B' expression: evidence of an extracellular regulator.
Hsp70B' is a unique member of the human Hsp70 family of chaperones about which information is scarce. Unlike the major inducible Hsp72 protein, Hsp70B' is strictly inducible having little or no basal expression levels in most cells. We observed that Hsp70B' appears transiently in response to heat stress whereas Hsp72 levels persist for many days. Also, Hsp70B' is optimally induced when cell numbers are low, whereas Hsp72 levels are greatest at higher cell number. Hsp70B' promoter activation was measured by flow cytometry using an Hsp70B' promoter-driven GFP construct. In heat stressed cells, promoter activation is cell number independent over a broad range. However, when cell number increases beyond a certain population size, cells are less stress inducible for Hsp70B' and induction becomes highly cell number-dependent. Cell number differences in Hsp70 activation cannot be explained by changes in Hsf-1 DNA-binding activity or hyperphosphorylation. Cells with few or no cell matrix attachments (laminin-coated and low attachment plates, respectively) appear to be more sensitive to cell number-dependent inhibition. Medium conditioned by the low cell number (LCN) populations supports increased Hsp70B' promoter activation in high cell number (HCN) cultures. Likewise, medium conditioned in HCN culture conditions causes decreased activation of Hsp70B' promoter in LCN cultures. As HCN-conditioned medium has all the components necessary for cell growth, two possibilities for the activation of Hsp70B' gene expression exist: an inhibitory component that accumulates in culture medium at HCN, or an activator that accumulates at LCN.
Noonan
Emily J
EJ
Department of Molecular and Cell Biology, University of Connecticut, Storrs, Connecticut 06269, USA.
Place
Robert F
RF
Rasoulpour
Reza J
RJ
Giardina
Charles
C
Hightower
Lawrence E
LE
D002450:Cell Communication; D002452:Cell Count; D002467:Cell Nucleus / Q000378:metabolism; D049109:Cell Proliferation / Q000187:drug effects; D003110:Colonic Neoplasms / Q000378:metabolism* / Q000473:pathology; D017077:Culture Media, Conditioned / Q000494:pharmacology; D003600:Cytosol / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000378:metabolism; D017930:Genes, Reporter; D049452:Green Fluorescent Proteins; D055551:HSP27 Heat-Shock Proteins; D018840:HSP70 Heat-Shock Proteins / Q000235:genetics / Q000378:metabolism*; D050884:HSP72 Heat-Shock Proteins / Q000378:metabolism; D019073:HT29 Cells; D000076249:Heat Shock Transcription Factors; D006360:Heat-Shock Proteins / Q000378:metabolism; D006358:Hot Temperature; D006801:Humans; D007797:Laminin / Q000378:metabolism; D018832:Molecular Chaperones; D009363:Neoplasm Proteins / Q000378:metabolism; D010766:Phosphorylation; D011401:Promoter Regions, Genetic / Q000187:drug effects / Q000235:genetics; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D011485:Protein Binding; D013997:Time Factors; D014157:Transcription Factors / Q000378:metabolism; D014158:Transcription, Genetic
D017077:Culture Media, Conditioned; D004268:DNA-Binding Proteins; D055551:HSP27 Heat-Shock Proteins; D018840:HSP70 Heat-Shock Proteins; D050884:HSP72 Heat-Shock Proteins; C491129:HSPA7 protein, human; C057725:HSPB1 protein, human; D000076249:Heat Shock Transcription Factors; D006360:Heat-Shock Proteins; D007797:Laminin; D018832:Molecular Chaperones; D009363:Neoplasm Proteins; D014157:Transcription Factors; D049452:Green Fluorescent Proteins; D046988:Proteasome Endopeptidase Complex