Induction of transformed cells to terminal differentiation and the modulation of gene expression. (1987)

Summary HMBA induces MELCs to terminal erythroid differentiation. The mechanism of HMBA action is not known. During induction there are early changes in cell function and gene expression during the latent period prior to commitment to terminal differentiation, changes which appear to accompany commitment, and changes which characterize the expression of differentiation in the committed cell. The inducer causes an early (within 2 h) decrease in diacylglycerol concentration and a decrease in Ca2+ and phospholipid-dependent protein kinase C activity. Early suppression (within 1–2 h) of c-myb and c-myc gene transcription and an increase in c-fos mRNA (within 4 h) may reflect alterations in protein kinase C-mediated signals required for expression of these protooncogenes. HMBA-induced commitment to terminal differentiation is detected by 12 h and over 95% are committed cells by 48 to 60 h. Commitment is associated with persistent suppression of c-myb gene transcription and elevated levels of c-fos mRNA whereas c-myc mRNA returns to control levels. Transcription of the erythroid-specific genes, α1- and βmaj-globin, is increased 10- to 30-fold while synthesis of rRNA is suppressed and there is activation or suppression of a number of additional genes which remain to be characterized. It is not yet clear how the protein products of protooncogenes elicit or modify cellular responses. Changes in expression of c-myb, c-myc, c-fos, and p53 genes which occur during HMBA-induced differentiation, as well as in several other systems, do suggest that products of these genes control multiple functions and, by implication, affect expression of multiple genes. Some clues exist, e.g., the observation that cells transfected with c-myc have an increased ability to promote the expression of certain endogenous genes. The evidence that increased expression of c-myc and c-myb is associated with the proliferative state and that induced suppression of these genes is associated with commitment to terminal cell division, while increased expression of c-fos is associated with differentiation, suggests that the protein products of these genes can affect genes with products that, in turn, are critical for cell proliferation or progression toward the terminal differentiated phenotype. The possible cause and effect relationships between changes in protooncogene expression and terminal differentiation are yet to be established but HMBA-mediated differentiation of MELC remains an attractive model for pursuing these questions. One possible application of the established pattern of HMBA-induced modulation of gene expression during MELC differentiation may be for following the effects of cytodifferentiation agents during treatment of clinical cancers. For example, clinical studies have been initiated to evaluate the effects of HMBA as a cytodifferentiation agent in human neoplasms (134). For most human tumors assay for cytological evidence of induced differentiation is difficult at best. Following the effects of a differentiation-inducing agent by determining c-myc, c-myb, or other mRNA levels may provide useful indicators of potential clinical effectiveness. Powerful tools are now available to dissect the structure and function of genes. Nevertheless, studies designed to unravel the mysteries of regulation of gene expression and the more complex questions related to the coordinated expression of multigene programs that characterize terminal cell differentiation are only in their infancy. Some understanding of multigenic regulation can be anticipated from structural studies comparing gene sequences, secondary structure, and associated selectively bound proteins for genes which are regulated in a coordinated fashion. It is already clear that even in a system as intensively studied as that of the α- and β-globin genes, the mechanisms regulating expression of these genes, which are coordinately expressed during normal erythropoiesis and HMBA-induced MELC differentiation, is very complex. Evidence has been reviewed for both cis-acting and trans-acting factors in the regulation of transcription of these genes and the evidence remains far from complete. To extend these insights to the less well-studied genes the expressions of which are also modulated during the course of induced terminal differentiation remains a formidable challenge.