Expression and regulation of G₁ cell-cycle inhibitors (p16^INK4A , p15^INK4B, p18 ^INK4C, p19^INK4D ) in human acute myeloid leukemia and normal myeloid cells

Abstract

In hematological malignancies, structural alterations of genes for G₁-specific cyclin-dependent kinases inhibitors (CKIs) have been extensively investigated. G₁-CKIs might play an important role not only as tumor suppressor genes but also in cellular differentiation. We examined constitutive and differentiation-induced expression and regulation of the four members of the G₁-CKI family p16^INK4A, p15^INK4B, p18 ^INK4C and p19^INK4D in acute myeloid leukemia as well as their expression in normal granulocytes and monocytes. p18^INK4C and p19^INK4D mRNA were expressed constitutively at high levels in seven myeloid cell lines and 16 AML patient samples, whereas expression of p15^INK4B mRNA was very low and only detectable by nested RT-PCR analysis. During phorbol ester-induced monocytic differentiation of leukemic HL-60 cells expression of particular G₁-CKIs was disparately regulated. This process was associated with growth arrest of the majority of the cells (⩾80%) in G₁/G₀, and in parallel p15^INK4B were upregulated whereas p18^INK4C and p19^INK4D expression was downregulated. In contrast, granulocytic differentiation induced by DMSO was accompanied by an increase of p18^INK4C and p19^INK4D expression only. PMA treatment of blast cells from two AML patients confirmed these cell line results. Disparate regulation of p15^INK4B and p18^INK4C mRNA was dependent on intermediary protein synthesis and occurred at the post-transcriptional level as shown by nuclear run-on analysis and mRNA half-life studies. In normal granulocytes and monocytes low constitutive p15^INK4B and p18^INK4C mRNA expression was detectable by RT-PCR only, but p19^INK4D transcripts were noted by Northern blotting in both cell types. Disparate expression of G₁-specific cell cycle inhibitors indicates complex and divergent roles of particular CKIs during normal and leukemic myeloid hematopoiesis.