The importance of SREBPs in the homeostatic control of cholesterol and fatty acid synthesis in somatic cells is well established (19
). However, the finding of a constitutively active, sterol-insensitive form of SREBP2 that is expressed in a developmentally regulated manner during spermatogenesis indicated a broader role for this factor not limited to lipid metabolism alone (50
). The present findings directly implicate SREBP2gc in the stage-dependent expression of the spermatogenic cell-specific gene proacrosin
, which is expressed in both spermatocytes and spermatids. SREBP2gc is only the second spermatogenic cell-enriched transcription factor (CREMτ is the first) shown to regulate a germ cell-specific promoter, and it is the first such factor shown to activate a gene expressed during male meiosis. Further, it is likely that SREBP2gc regulates multiple spermatogenic-cell-specific genes, not proacrosin
alone. Thus, this factor may be an integral part of a more global differentiation program, and defining additional target promoters for SREBP2gc in male germ cells is an important future goal. In particular, disruption of SREBP2gc function during spermatogenesis will establish the extent to which this factor is involved in directing spermatogenic differentiation as well as the nature of its gene targets. It also should provide the first insight into the cell-specific transcriptional mechanisms operating in meiotic spermatocytes.
Based on the present results, it appears that a ubiquitous somatic factor (SREBP2) was adapted by spermatogenic cells to function in an entirely new manner as a trans
regulator of germ cell-specific genes. In fact, precedent for this notion already exists in the form of CREMτ: analogous to SREBP2gc, it is a spermatogenic cell-specific variant of a generally expressed transcription factor family generated by alternative splicing. Both factors also possess unique properties that circumvent regulatory mechanisms operating in somatic cells and that are critical for their function as spermatogenic cell trans
regulators. For CREMτ, alternative splicing converts the CREM repressor into a germ cell-specific activator of CREs (12
). Further, phosphorylation mechanisms normally required for interactions with the CREB coactivator CBP do not apparently operate in spermatids. Instead, CREMτ interacts with the phosphorylation-independent coactivator ACT, which is expressed only in haploid spermatogenic cells along with CREMτ (11
). This alternative pathway apparently evolved to provide for both stage- and cell-specific activation of CRE-dependent promoters in germ cells. Similarly, alternative RNA processing in spermatogenic cells generates an SREBP2 isoform that bypasses sterol-dependent inhibitory mechanisms, permitting stage-dependent up-regulation of a constitutively active factor and its target promoters in late spermatocytes and early spermatids.
It is of interest that SRE- and CRE-binding proteins act together to regulate numerous promoters in somatic cells (40
). It therefore seems likely that SREBP2gc and CREMτ coordinately regulate common spermatogenic cell-specific promoters in spermatids. This may reflect coevolution of functionally related transcription factors, in which interacting partners take on cell-specific functions in parallel. In fact, these two proteins may be members of a larger group of factors, including Y/CAAT- and GC box binding factors, as well as YY1-like proteins, specifically arising from more generally expressed trans
-regulator families to control gene expression in the male germ line. Such adaptation may be an efficient means for generating germ cell-specific transcription factors since it utilizes generally expressed, and perhaps ancient (52
factors as well as response elements commonly found in RNA polymerase II promoters. Notably, many germ cell-specific promoters expressed in late spermatocytes and/or round spermatids contain CRE, YY1, and Y- and GC-box elements (23
), and unique, spermatogenic-cell- or testis-enriched nuclear factors that bind these sites have been previously identified (16
). Additional, novel coregulator isoforms also may function in late spermatogenesis.
Analysis of the proacrosin gene, which contains binding sites for all major SREBP coregulators and which is expressed in both of these stages, provides an excellent opportunity to explore the role of coregulators in both cell- and stage-dependent activation by SREBP2gc. Such analyses ultimately will expand our understanding of the transcriptional network regulating spermatogenesis and the unique placement of SREBP2gc within it. GC-4spc cells should prove useful in this regard due to their expression of SREBP2gc as well as the cell-specific regulation of proacrosin promoter activity that they exhibit.
Finally, what is the significance of SREBP2gc expression for cholesterol synthesis during spermatogenesis? Recent studies have shown that loss or inhibition of the function of dhcr24, a terminal reductase in the cholesterol biosynthetic pathway, disrupts spermatogenesis (41
). Several cholesterol biosynthesis genes also are specifically up-regulated during late spermatogenesis (46
), which likely involves trans
activation by SREBP2gc. However, a number of observations indicate that enhancement of cholesterol synthesis per se is not the role of this transcription factor in meiotic and haploid germ cells. For one thing, not all cholesterol biosynthetic genes are coordinately up-regulated during late spermatogenesis (46
). Accordingly, cholesterol synthesis actually declines in pachytene spermatocytes and round spermatids (36
), as does testicular cholesterol content during sexual maturation (46
). These facts further argue that SREBP2gc has major functions distinct from cholesterol synthesis and are consistent with the switch to a sterol-independent mechanism of SREBP2 production in these spermatogenic stages. While this may involve an increased synthesis of certain cholesterol intermediates, such as T-MAS (46
), it is likely that a major role of SREBP2gc is to regulate a totally new set of promoters uniquely expressed in spermatocytes and spermatids.