title: Proliferative suppression by CDK4/6 inhibition: complex function of the retinoblastoma pathway in liver tissue and hepatoma cells.
abstract: OBJECTIVE
  Hepatocellular carcinoma is the third leading cause of cancer mortality worldwide; current chemotherapeutic interventions for this disease are largely ineffective. The retinoblastoma tumor suppressor (RB) is functionally inactivated at relatively high frequency in hepatocellular carcinoma and hepatoma cell lines. Here, we analyzed the ability of CDK4/6 inhibition to inhibit hepatocyte proliferation and the effect of RB status on this process.
  
  
  METHODS
  Hepatoma cell lines and xenograft models harboring RB knockdown and mice harboring liver-specific Rb deletion were used to define the role of RB function in response to CDK4/6 inhibition.
  
  
  RESULTS
  Our study shows that CDK4/6-dependent cell cycle progression in hepatoma cells was readily arrested by inhibition of CDK4/6 by PD-0332991 or p16ink4a irrespective of RB status. Interestingly, upon CDK4/6 inhibition, p107 protein stability was dramatically increased as a function of RB loss. This engagement of compensatory mechanisms was critical for cell cycle inhibition in the absence of RB, because both the E1A oncoprotein and overexpression of E2F proteins were capable of overcoming the effect of CDK4/6 inhibition. These findings were recapitulated in xenograft models. Furthermore, to determine how these findings relate to hepatocyte proliferation in vivo, mice were exposed to carbon tetrachloride to induce liver regeneration followed by treatment with PD-0332991. This treatment significantly inhibited hepatocyte proliferation. Strikingly, this facet of PD-0332991 function was retained even in RB-deficient livers.
  
  
  CONCLUSIONS
  These data show that CDK4/6 inhibition is a potent mediator of cytostasis and that RB loss can be readily compensated for in the context of both hepatoma cell lines and liver tissue.
authors.affiliation: Kimmel Cancer Center, Thomas Jefferson University, Philadelphia, Pennsylvania, USA.
authors.forename: Dayana B
authors.initials: DB
authors.lastname: Rivadeneira
authors.forename: Christopher N
authors.initials: CN
authors.lastname: Mayhew
authors.forename: Chellappagounder
authors.initials: C
authors.lastname: Thangavel
authors.forename: Elena
authors.initials: E
authors.lastname: Sotillo
authors.forename: Christopher A
authors.initials: CA
authors.lastname: Reed
authors.forename: Xavier
authors.initials: X
authors.lastname: Graña
authors.forename: Erik S
authors.initials: ES
authors.lastname: Knudsen
mesh_terms: D017500:Adenovirus E1A Proteins / Q000378:metabolism; D000818:Animals; D002251:Carbon Tetrachloride; D006528:Carcinoma, Hepatocellular / Q000235:genetics / Q000378:metabolism* / Q000473:pathology; D002453:Cell Cycle / Q000187:drug effects; D049109:Cell Proliferation / Q000187:drug effects*; D056486:Chemical and Drug Induced Liver Injury / Q000235:genetics / Q000378:metabolism* / Q000473:pathology; D051358:Cyclin-Dependent Kinase 4 / Q000037:antagonists & inhibitors / Q000378:metabolism; D051361:Cyclin-Dependent Kinase 6 / Q000037:antagonists & inhibitors / Q000378:metabolism; D019941:Cyclin-Dependent Kinase Inhibitor p16 / Q000378:metabolism; D018844:Cyclin-Dependent Kinases / Q000037:antagonists & inhibitors* / Q000378:metabolism; D004195:Disease Models, Animal; D050684:E2F Transcription Factors / Q000378:metabolism; D055785:Gene Knockdown Techniques; D056945:Hep G2 Cells; D022781:Hepatocytes / Q000187:drug effects* / Q000378:metabolism / Q000473:pathology; D006801:Humans; D008113:Liver Neoplasms / Q000235:genetics / Q000378:metabolism* / Q000473:pathology; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D008819:Mice, Nude; D009368:Neoplasm Transplantation; D010766:Phosphorylation; D010879:Piperazines / Q000494:pharmacology; D047428:Protein Kinase Inhibitors / Q000494:pharmacology*; D011687:Purines / Q000494:pharmacology; D011725:Pyridines / Q000494:pharmacology; D034622:RNA Interference; D016160:Retinoblastoma Protein / Q000172:deficiency / Q000235:genetics / Q000378:metabolism*; D050720:Retinoblastoma-Like Protein p107 / Q000378:metabolism; D000077546:Roscovitine; D013997:Time Factors; D014162:Transfection
chemical_list: D017500:Adenovirus E1A Proteins; D019941:Cyclin-Dependent Kinase Inhibitor p16; D050684:E2F Transcription Factors; D010879:Piperazines; D047428:Protein Kinase Inhibitors; D011687:Purines; D011725:Pyridines; C493844:RBL1 protein, human; D016160:Retinoblastoma Protein; D050720:Retinoblastoma-Like Protein p107; D000077546:Roscovitine; D002251:Carbon Tetrachloride; C495900:CDK4 protein, human; C495911:CDK6 protein, human; C495902:Cdk4 protein, mouse; C495912:Cdk6 protein, mouse; D051358:Cyclin-Dependent Kinase 4; D051361:Cyclin-Dependent Kinase 6; D018844:Cyclin-Dependent Kinases; C500026:palbociclib