The Drosophila homolog of human tumor suppressor TSC-22 promotes cellular growth, proliferation, and survival

Abstract

TSC22D1, which encodes transforming growth factor ␤-stimulated clone 22 (TSC-22), is thought to be a tumor suppressor because its expression is lost in many glioblastoma, salivary gland, and prostate cancers. TSC-22 is the founding member of the TSC-22/DIP/Bun family of leucine zipper transcription factors; its functions have not been investigated in a multicellular environment. Genetic studies in the model organism Drosophila melanogaster often provide fundamental insights into mechanisms disrupted in carcinogenesis, because of the strong evolutionary conservation of molecular mechanisms between flies and humans. Whereas humans and mice have four TSC-22 domain genes with numerous isoforms, Drosophila has only one TSC-22 domain gene, bunched (bun), which encodes both large and small protein isoforms. Surprisingly, Drosophila Bun proteins promote cellular growth and proliferation in ovarian follicle cells. Loss of both large isoforms has the strongest phenotypes, including increased apoptosis. Cultured S2 cells depleted for large Bun isoforms show increased apoptosis and less frequent cell division, with decreased cell size. Altogether, these data indicate that Drosophila TSC-22/DIP/Bun proteins are necessary for cellular growth, proliferation, and survival both in culture and in an epithelial context. Previous work demonstrated that bun prevents recruitment of epithelial cells to a migratory fate and, thus, maintains epithelial organization. We speculate that reduced TSC22D1 expression generally reduces cellular fitness and only contributes to carcinogenesis in specific tissue environments. The strong conservation of genes and molecular mechanisms between flies and humans makes this an ideal organism to evaluate candidate tumor-suppressor functions in vivo.

TSC-22D1 isoforms have opposing roles in mammary epithelial cell survival

“…However, whereas mutations that deleted the small bun isoforms (orthologues of isoform 2 in mammals) exerted no effect on cell viability or proliferation, 35 a knockdown of the TSC-22D1 isoform 2 in mammary epithelial cells increased colony formation and decreased TGFb3-induced cell death, indicating that the isoform 2 has an important role in controlling cell survival in the mammary epithelial system under these conditions. Similarly, whereas bunA depletion in Drosophila S2 cells increased cell death, 36 the TSC-22D1 isoform 1 did not seem to be necessary for cell survival in vitro, although this isoform reduced the cell death induced by TGFb3. This novel observation of the opposing functions of the TSC-22D1 isoforms in mammalian cells may help explain the recent findings in TSC-22D1 knockout mice.…”

“…Therefore, our data are in agreement with those recently observed for the Drosophila orthologue of TSC-22 domain proteins (bun). 35,36 The long isoform of bun (bunA) was found to promote cellular growth and proliferation of Drosophila ovarian follicle cells and polar cells, whereas the small isoforms exerted no or only minimal effects. 36 In addition, although bunA enhanced the growthpromoting activity of ribosomal protein S6 kinase when co-expressed in the dorsal compartment of the wing imaginal disc, the TSC-22D1 isoform 2 orthologues, bunB and bunC, antagonised this activity.…”

“…35,36 The long isoform of bun (bunA) was found to promote cellular growth and proliferation of Drosophila ovarian follicle cells and polar cells, whereas the small isoforms exerted no or only minimal effects. 36 In addition, although bunA enhanced the growthpromoting activity of ribosomal protein S6 kinase when co-expressed in the dorsal compartment of the wing imaginal disc, the TSC-22D1 isoform 2 orthologues, bunB and bunC, antagonised this activity. 35 The similarity between these results and our findings indicate a highly conserved function of the TSC-22D1 gene in both organisms.…”

Hippo signaling pathway and organ size control

“…Several transcriptional targets of the Hpo pathway have been identified, including cycE, diap1 and bantam, as well as two upstream Hpo pathway components: merlin (mer) and expanded (ex). 9,10,[13][14][15][18][19][20] Hpo signaling influences gene expression by regulating Yorkie (Yki), the Drosophila homolog of the mammalian transcriptional coactivator YAP, which binds to and is phosphorylated by Wts. 21 Overexpression of Yki phenocopies loss of Hpo signaling activity, suggesting that Hpo signaling restricts cell growth and promotes cell death by inhibiting Yki-mediated gene expression.…”

Regulation of C-type natriuretic peptide expression

“…Determination of a specific mechanism of action of TSC22D1 in regulating CNP transcription is further complicated by the existence of TSC22D1 splice variants that result in the formation of proteins of very different MW (18 kD vs. >100 kD) [28]. Recent studies [31,99] suggest that these protein variants could perform unrelated, or even antagonistic functions in the mammalian cell; e.g., although the short form of TSC22D1 has been described many times as a tumor suppressor, the long form in contrast promotes proliferation, growth and cell survival [99]. Interestingly, in our recent studies of the effects of TSC22D1 silencing on CNP expression in vascular smooth muscle cells, [52], we could only identify a long form of the protein (∼120 kD) in Western blots, yet the downregulation of this protein with TSC22D1 siRNA resulted in reduced production of CNP transcript in these cells.…”