TGF-beta1 regulates the expression of multiple max-interacting transcription factors in Balb/MK cells: implications for understanding the mechanism of action of TGF-beta1

Appropriate transforming growth factor-beta1 (TGF-beta1) signaling is required to preserve homeostasis of diverse tissues during development. At the cellular level, one function of TGF-beta1 that is critical for preserving homeostasis is the ability to arrest cell growth. TGF-beta1 arrests growth by blocking the function of the c-myc proto-oncogene. c-myc function is determined by the level of c-myc expression relative to other Max-interacting transcription factors, and TGF-beta1 has been shown to inhibit c-myc expression by inhibiting c-myc transcription. However, whether TGF-beta1 might also increase the expression of a Max-interacting factor that blocks myc function by competing with myc for Max binding is not known. Therefore, we determined the effect of TGF-beta1 on the expression of Max-interacting transcription factors in Balb/MK cells. We found unexpectedly that Balb/MK cells express both N-myc and c-myc. The pattern of N-myc expression during the cell cycle differs from that of c-myc, indicating that mRNA accumulation is controlled by mechanisms specific to each gene. TGF-beta1 rapidly inhibits N-myc mRNA expression; thus N-myc is a novel target of TGF-beta1 in Balb/MK cells. More importantly, we found that TGF-beta1 induces the expression of the putative tumor suppressor genes Mad4 and Mxi1 in both the Balb/MK and Mv1Lu cell lines. Mad4 and Mxi1 are novel targets of TGF-beta1, known to inhibit cell growth by antagonizing the interaction of Myc with Max. Thus, our results suggest that the induction of Mad4 and Mxi1 may function in tandem with the inhibition of N-myc and c-myc to mediate the growth inhibitory function of TGF-beta1.

Inhibition of topoisomerase IIalpha expression by transforming growth factor-beta1 is abrogated by the papillomavirus E7 protein

Transforming growth factor-beta (TGF-beta) protects normal cells from etoposide-induced cell death, yet the mechanism has remained speculative. Studies have shown that etoposide modifies the activity of the topoisomerase IIalpha (topo IIalpha) enzyme, thereby causing DNA damage and inducing cell death. Expression of topo IIalpha is necessary for etoposide-induced cell death, and peak expression of topo IIalpha normally occurs during the G2 phase of the cell cycle. We predicted that by arresting growth in the G1 phase, TGF-beta1 would prevent the induction of topo IIalpha expression that normally occurs subsequent to the G1-S transition, thereby protecting cells from etoposide-induced cell death. Accordingly, we hypothesized that the inhibition of topo IIalpha expression by TGF-beta1 would be dependent on the ability of TGF-beta1 to arrest cell cycle progression in G1. Using mink lung epithelial cells (MvlLu), we found that TGF-beta1 decreases topo IIalpha mRNA expression, and the decrease occurs as cells begin to accumulate in the G1 phase of the cell cycle. Topo IIalpha protein expression decreases subsequent to the fall in mRNA expression. In contrast, topo IIalpha expression is not affected by TGF-beta1 in cells that fail to undergo G1 arrest because of inactivation of the retinoblastoma tumor suppressor protein (pRb) by the papillomavirus type 16 E7 protein. Our studies suggest that inhibition of topo IIalpha by TGF-beta1 is the principal mechanism that protects mink lung epithelial cells (Mv1Lu) from etoposide-induced toxicity. Furthermore, the inhibition of topo IIalpha protein expression by TGF-beta1 is dependent on pRb-mediated cell cycle arrest, suggesting that TGF-beta1 will not reduce the sensitivity of pRb-deficient cancers to etoposide.

TGF-beta1 inhibits BRCA1 expression through a pathway that requires pRb

TGF-beta1 inhibits BRCA1 expression, which contradicts the model that TGF-beta1 prevents carcinogenesis by activating tumor suppressor genes. To resolve this apparent contradiction, we examined BRCA1 expression in Mv1Lu cells, a well-established model system for studying the TGF-beta1 tumor suppressor pathway. We found that inactivation of pRb by the papillomavirus type 16 E7 protein increased BRCA1 expression and abolished the ability of TGF-beta1 to inhibit BRCA1 expression. We conclude that TGF-beta1 inhibits BRCA1 expression through a pathway that requires pRb. We propose a model to explain the inhibition of BRCA1 as a target in the TGF-beta1 tumor suppressor signaling pathway. Our results suggest that the tumor suppressor functions of BRCA1 are initiated by the inactivation of pRb, and therefore that the activation of pRb by TGF-beta1 might alleviate the requirement for BRCA1 function.

Inhibition of cell growth by TGF beta 1 is associated with inhibition of B-myb and cyclin A in both BALB/MK and Mv1Lu cells

The concept of positive and negative regulation of normal cellular growth by diffusible factors is well illustrated by the effects of epidermal growth factor and transforming growth factor beta 1 (TGF beta 1) on mouse keratinocytes (MK) and mink lung epithelial cells (Mv1Lu). MK and Mv1Lu are nontransformed cell lines that reversibly arrest at a point in late G1 in response to TGF beta 1. Previously, we have shown that expression of the protooncogene c-myc is induced upon epidermal growth factor stimulation of quiescent MK and Mv1Lu cells and that transcriptional suppression of c-myc by TGF beta 1 treatment is important in the TGF beta 1 growth inhibition pathway. Using epidermal growth factor-stimulated synchronized MK and Mv1Lu cells, we have investigated the mRNA expression of a large number of growth factor-inducible genes that are critical regulators of growth in G1 and at the G1/S transition. These genes, often found to be dysregulated in cancer, include transcription factors as well as cyclins and their associated kinases, that promote growth, and tumor suppressor genes, that inhibit growth. As reported here, TGF beta 1 significantly inhibited mRNA expression of B-myb and cyclin A in both cell lines, suggesting that these may be important common downstream targets in the growth inhibition pathway. In contrast, the expression patterns of cyclins D1 and D2 and the transcription factors E2F1 and E2F2 were unaffected in MK cells treated with TGF beta 1 but were significantly inhibited in TGF beta 1-treated Mv1Lu cells. We cite the evidence suggesting that the inhibition of B-myb and cyclin A may contribute to the late G1 arrest caused by TGF beta 1 and that these events may be linked through the actions of the product of the retinoblastoma susceptibility gene (Rb) or an Rb family member.