Proteolytic degradation of Smad4 in extracts of AML blasts.
Loss of transforming growth factor (TGF) beta signaling has been implicated in malignant transformation of various tissues. To investigate a potential role of Smad4 in acute myeloid leukemia (AML), the expression of Smad4 was determined in blast cells from AML patients. Western analysis of nuclear extracts of nine AML samples indicated the absence of Smad4 protein in two cases. Smad4 RT-PCR analysis of these cases indicated normal Smad4 mRNA expression, and sequencing of one of these cases revealed no mutations as compared to wild type Smad4. Next, it was investigated whether Smad4 protein from these AML cases was subject to proteolytic degradation by incubating cell extracts of these Smad4-negative AML cells with extracts from COS-7 cells in which a tagged Smad4 was overexpressed. Inhibitor studies indicated that the extracts of AML blasts lacking Smad4 possessed a serine-dependent proteolytic activity, capable of degrading Smad4. Transfection studies using an SBE containing reporter construct as well as RT-PCR analysis of endogenous TGFbeta1 responsive genes indicated that the AML blasts were still able to respond to TGFbeta1, despite the observed degradation of Smad4. It was, therefore, concluded that the degradation of Smad4 was possibly AML subtype-dependent, in vitro phenomenon, occurring during the preparation of nuclear and cellular extracts despite the addition of a protease inhibitor cocktail. The results indicate that care should be taken when interpreting data obtained from protein expression studies using AML blast cells.
Wierenga
Albertus T J
AT
Department of Hematology, University Hospital Groningen, Hanzeplein 1, 9713 GZ Groningen, The Netherlands.
Eggen
Bart J L
BJ
Kruijer
Wiebe
W
Vellenga
Edo
E
D000208:Acute Disease; D001773:Blood Cells / Q000473:pathology; D015153:Blotting, Western; D002457:Cell Extracts; D004268:DNA-Binding Proteins / Q000172:deficiency / Q000378:metabolism*; D006801:Humans; D007951:Leukemia, Myeloid / Q000209:etiology / Q000378:metabolism* / Q000473:pathology; D010447:Peptide Hydrolases / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D051901:Smad4 Protein; D015534:Trans-Activators / Q000172:deficiency / Q000378:metabolism*; D015533:Transcriptional Activation / Q000187:drug effects; D016212:Transforming Growth Factor beta / Q000235:genetics / Q000494:pharmacology
D002457:Cell Extracts; D004268:DNA-Binding Proteins; C497796:SMAD4 protein, human; D051901:Smad4 Protein; D015534:Trans-Activators; D016212:Transforming Growth Factor beta; D010447:Peptide Hydrolases