pioneering in vitro studies by the omalley group revealed that the transactivational potency of agonist bound progesterone receptor ( pr ) can be significantly enhanced by increasing the cellular level of members of the steroid receptor coactivator ( src / p160 ) family of coregulators , reviewed in [ mckenna and o'malley , 2002 ] . the src / p160 family consists of three members :   src-1 ( erap140/erap160/ncoa-1 ) ; src-2 ( tif-2/grip-1/ncoa-2 ) ; and src-3 ( p / cip / rac3/aib1/tram-1/actr / ncoa-3 ) ; reviewed in [ lonard and o'malley , 2005 ] .   to enhance nuclear receptor ( nr)-mediated transactivation , each src family member has been shown to directly contact - through discreet lxxll motifs within their nr interaction domain ( figure 1 ) - the highly conserved activation 2 domain located in the c - terminal region of nrs . furthermore , two activation domains ( ad1 and ad2 ) positioned in the c - terminal region of each src are responsible for recruiting secondary coregulators ( or co - coregulators ) . for example , ad1 is known to interact with histone acetyltransferases ( hats ) p300 and the related cyclic amp - response element binding protein ( creb)-binding protein ( cbp ) , whereas ad2 is known to recruit arginine methyltransferases such as coactivator - associated arginine methyltransferase 1 ( carm1 ) , reviewed in [ lonard and o'malley , 2006 ] .   the histone - modifying activities of these secondary coregulators ( in addition to the weak intrinsic hat activity of src members ) facilitate local chromatin remodeling that allows the general transcriptional machinery open access to promoter regions of nr target genes . apart from histones , these co - coregulators have been shown to posttranslationally modify other target proteins ( i.e. , other coregulators and transcription factors ) within the transcriptional complex . the n - terminally positioned basic helix loop helix - per / arnt / sim ( bhlh - pas ) domain is the most conserved structural motif among src members ( figure 1 ) and is also responsible for co - opting additional coregulators and transcription factors .   for src-2 [ kim et al . , 2003 ] , flightless - i ( fli - i ) [ lee and stallcup , 2006 ] , grip1-associated coactivator 63 ( gac63 ) [ chen et al . , 2005 ] , as well as the transcription factors myocyte - enhancer factor 2c ( mef-2c ) [ chen et al . , 2000b ] and tef4 [ belandia and parker , 2000 ] .   in the case of other regulatory proteins , the bhlh - pas motif has been shown to be involved in both dna and ligand binding [ gu et al . , 2000 ; huang et al . , 1993 ] , indicating that this structural domain feature may be involved in src regulatory functions beyond those currently known . superimposed on the myriad of protein - protein interactions that enable src-2 to relay ( and control ) signaling inputs dispatched from ligand - bound nr to the general transcriptional complex , a multiplicity of interacting signaling inputs ( i.e. , phosphorylation events triggered by extracellular growth and cell survival factors [ duong et al . , 2006 ; frigo et al . , 2006 ] ) are also being transduced by src-2 within the multicomponent transcriptional machinery . although src-2 has been primarily considered a coactivator , a subset of investigations provide strong support for a corepressor role for src-2 within certain cellular contexts [ gupta et al . , 2007 ; he and simons , 2007 ; rogatsky et al . , 2002 ; wang et al . , while in vitro experiments disclosed the existence of the src family , subsequent experimental mouse genetics would uncover important overlapping and non - overlapping roles for the three srcs in progestin - initiated signaling events in vivo . further underscoring their multifunctional properties , mouse studies have also uncovered critical roles for each src in signaling processes that reside outside the physiologic area of progestin control . the src-1 knockout ( ko ) mouse was shown to display an attenuated decidual response in the uterus [ xu et al . , 1998 ] , suggesting that this coregulator ( in concert with others ) is required for complete manifestation of this morphological response , which requires initial progesterone stimulation . although the src-3ko exhibits a normal decidual response , a partial block in hormone - induced mammary ductal side - branching and alveologenesis is observed in this animal [ xu et al . , 2000 ] . collectively , these observations support the proposal that src-1 and -3 are required for a subgroup of pr - mediated transcriptional responses in the uterus and mammary gland , respectively . investigations on the pr activity indicator ( prai ) model provide further support for this supposition [ han et al . , 2006 ; han et al . , mouse studies have also underscored important roles for src-1 and src-3 in areas of normal physiology and disease which are outside the realm of progesterone control , these include : cell growth [ wang et al . , 2000 ; xu et al . , 2000 ] , metabolism [ louet et al . , 2006 ; wang et al . , 2006 ] , thyroid hormone - based physiology [ ying et al . , 2005 ] , bone homeostasis [ modder et al . , 2004 ] , prostate biology [ zhou et al . , 2005 ] and b - cell lymphoma developmental progression [ coste et al . , in contrast to kos for src-1 and -3 , the global ko for src-2 ( termed : transcriptional intermediary factor 2 ko ( or tif2 ) ) displays striking reproductive abnormalities in both sexes [ gehin et al . , 2002 ] .   in the female , abrogation of src-2 function triggers placental hypoplasia , which results in a severe hypofertility defect . subsequent studies found that tif2 pups ( both sexes ) are significantly underrepresented in litters from tif2 crosses ( tif2 females resulting from such crosses are infertile ( personal observations ) ) . similar to kos for src-1 and -3 , global ablation of src-2 function results in physiological defects not directly linked to reproductive biology , such as a decrease in early postnatal survival [ mark et al . , 2004 ] , a breakdown in energy homeostasis [ jeong et al . , 2006 ] , as well as elaboration of insidious adrenocortical insufficiency [ patchev et al . , 2007 ] ) the placental defect exhibited by the global ko for src-2 , in conjunction with the recent observation that a subset of murine cell lineages express both pr and src-2 [ mukherjee et al . , 2006b ] , suggested that src-2 ( like src-1 and -3 ) may occupy a critical coregulator role in a subgroup of physiological processes that require pr function .   to test this hypothesis , 2005 ] with a src-2 mouse in which exon 11 of the src-2 gene was floxed to enable cre - mediated excision [ gehin et al . , 2002 ] ; exon 11 encodes the receptor - interacting domain ( rid ) . therefore , the prsrc-2 bigenic is designed to abrogate src-2 function specifically in cell lineages that are pr positive [ mukherjee et al . , 2006b ] . the utility of this genetic strategy is that src-2 s role in pr - regulated transcriptional programs can be directly investigated at the whole - animal level without interference from other , unrelated phenotypes resulting from src-2 s absence ( a key advantage over the global ko for src-2 ) . to date , female and male prsrc-2 mice show normal postnatal development ; however , the prsrc-2 female is sterile [ mukherjee et al . , 2006b ] . unlike the tif2 mouse , male prsrc-2 mice exhibit normal fertility and neither sex displays phenotypes ( outside progestin control ) previously described for the tif2 model [ gehin et al . the absence of implantation sites along the uterine horn of the prsrc-2 mouse ( 5.5 days post - coitum ) is the primary underlying cause for the infertility defect displayed by the prsrc-2 female ( figure 2a ) . this result suggests a pivotal role played by src-2 in the early cellular changes in the uterus that are required for embryo implantation . although the src-2 uterus ( a positive control ) exhibits a full decidual response to an artificial deciduogenic stimulus ( figure 2b and c ) , the prsrc-2 uterus displays only a partial decidual response ( figure 2b and c ) . these findings support the proposal that a subgroup of pr - mediated transcriptional events are dependent on src-2 to launch a complete decidual reaction .   the incomplete decidual response phenotype shared by the src-1ko [ xu et al . , 1998 ] and prsrc-2 mouse suggests that both src coregulators may be required together in pr - mediated signaling cascades that result in a fully decidualized uterus . to test this hypothesis , the src-1ko mutation was introduced into the prsrc-2 germline to generate a prsrc-2 src-1ko trigenic model . figure 2b and c shows that the trigenic uterus fails to mount a decidual response , thereby furnishing essential in vivo support for a cooperative involvement for src-1 and src-2 in the progesterone - dependent decidual reaction . note that the prsrc-2 uterine phenotype is not explained by changes in the normal levels of uterine src-1 and/or src-3 protein ( figure 2d ) . in contrast to the uterus , ovarian and pituitary function is not compromised in the prsrc-2 mouse , suggesting that pr enlists other coregulators in these systems ( normal ovarian and pituitary functionality is severely diminished in the prko [ lydon et al . , 1995 ] ) . furthermore , src-2 is not required for progesterone - inhibition of pr expression or suppression of estrogen - induced luminal epithelial proliferation in the uterus .   collectively , these observations suggest that the selective enlistment of src-2 by pr in female reproductive tissues may provide one explanation as to why different reproductive tissues display different responses to the same progesterone signal . having disclosed a central role for uterine src-2 in murine peri - implantation biology , future questions to be addressed include : ( 1 ) is uterine src-2 expression in the epithelial , stromal , or both cellular compartments required for the development of the receptive uterus ? ( 2 ) is src-2 expressed in the embryonic - derived trophectoderm ? if so , is trophectoderm - derived src-2 required for embryo implantation ? ( 3 ) does uterine src-2 possess coregulator functions necessary for later stages of pregnancy ? for example , in the regulation of the onset of parturition ; and ( 4 ) does src-2 have a role in the etiopathogenesis of such endometrial disorders as uterine hyperplasia and/or endometriosis ? the detection of src-2 protein in mammary epithelial cells that are pr positive [ mukherjee et al . , 2006b ] suggested that mammary src-2 may occupy a crucial role in pr - mediated proliferative programs which result in ductal side - branching and alveolar morphogenesis in the mammary gland of the adult . this assumption was supported by the observation that the prsrc-2 mammary gland fails to exhibit the typical morphological changes that occur with combined estrogen and progestin exposure ( figure 3a - d ) .   like the prko [ lydon et al . , 1999 ] , the underlying cause of the prsrc-2 mammary phenotype is a failure of the mammary epithelium to proliferate in response to hormone ( figure 3e ) . these results support an essential role for src-2 in progesterone - induced signaling programs which are required for mammary morphogenesis in the adult .   of note , the prsrc-2 mammary defect was not compensated for by src-3 ( figure 3e ) .   although src-3 has been shown to be involved in steroid - induced mammary morphogenesis [ xu et al . , 2000 ] , as well as tumorigenesis [ kuang et al . , 2005 ; kuang et al . , 2004 ; torres - arzayus et al . , 2004 ] , our data to date suggest that src-2 and -3 are operationally distinct in the murine mammary epithelial cell . irrespective of the functional interrelationships between src-2 and other members of the src family in this tissue , our studies reveal src-2 to be an important coactivator for progestin - initiated signaling in the mammary epithelium . an important question for the future will be to determine whether src-2 ( like src-3/aib-1 [ anzick et al . , 1997 ] ) can act as a mammary oncogene . although state - of - the - art genetics demonstrated a critical role for src-2 in a subset of progesterone responses in the uterus and mammary gland of the mouse , whether these findings translate to the human is now an important research focus . as previously shown [ hofman et al . , 2002 ] , the transactivational potency of the human pr ortholog is significantly enhanced with increasing levels of human src-2 ( figure 4a ) . these observations provide strong support for a coregulator involvement for src-2 in progestin - dependent physiological processes in the human .   as further support for this supposition , immunohistochemistry demonstrates that src-2 protein is expressed in a subset of steroid - responsive target tissues in the human ( figure 4b - f ) .   in the case of human prostate , ( figure 4b ) , src-2 expression is regionally restricted to the epithelial compartment , a known cellular target - site for androgen receptor - mediated signaling and neoplastic transformation [ berrevoets et al . , 2004 ; culig et al . , 2002 ; ye et al . , , immunohistochemical studies clearly demonstrate that src-2 and pr are expressed in identical cell types within the stromal and epithelial compartments ( figure 4c - d ) ; similar findings have been described for the mouse [ mukherjee et al . , , 2006b ] , immunohistochemical investigations have shown that a subgroup of epithelial cells within the normal human breast express src-2 ( figure 4e and f ) . interestingly , the punctate spatial expression pattern for src-2 in the human breast parallels a similar spatial expression pattern reported for pr in the rodent and human breast [ clarke et al . , however , whether ( like the mouse [ mukherjee et al . , 2006b ] ) src-2 and pr localize to identical cells in the human breast has yet to be demonstrated . further progress in this area is important , as separation of pr positive mammary epithelial cells from pr negative cells that undergo cell division in response to progesterone is now recognized as an evolutionarily conserved feature that underpins a proposed paracrine mechanism action for pr in the normal breast , reviewed in [ fernandez - valdivia et al . , 2005 ] . despite over 200 known coactivators reported [ lonard and o'malley , 2006 ] , it is significant that pr action is singularly dependent on the coregulator functions of src-2 for a subgroup of progesterone - induced physiological responses that are necessary for the maintenance of female fertility and postnatal mammary morphogenesis in the mouse . overtly distinct from src-1 and src-3 , coregulator properties of which subserve only a selection of progesterone - initiated transcriptional responses either in the uterus or mammary gland , src-2 exerts potent coregulator activities in both progesterone target tissues .   from a clinical standpoint , the indispensable role of src-2 in murine peri - implantation biology demands further study , since recurrent implantation failure is now recognized as a key underlying factor that precludes the establishment of a successful pregnancy [ norwitz et al . , 2001 ] ) . importantly , abnormal increases in endometrial src-2 levels have also been associated with infertility in women with polycystic ovarian syndrome ( pcos ) and with a subset of endometrial cancers [ gregory et al . , 2002 ; pathirage et al . , although preliminary , these latter observations suggest a possible connection between perturbation in src-2 protein levels and the etiopathogenesis of these uterine disorders . in the case of the murine mammary gland , previous studies demonstrated that pr action is necessary for parity - induced mammary proliferation , which represents a prerequisite developmental step prior to terminal differentiation of this tissue ; importantly , the progesterone signal can also influence breast cancer susceptibility , reviewed in [ fernandez - valdivia et al . , the finding that src-2 ablation results in a mammary phenotype similar to the prko mammary defect has prompted three key questions : ( 1 ) can upregulation of src-2 expression promote neoplastic transformation in the murine mammary gland ? ( 2 ) if so , does src-2 have an involvement in hormone - responsive breast cancers in the human ?   and ( 3 ) does the established oncogenic effects of src-3 require the presence of src-2 ? obviously , addressing these questions will extend our current understanding of progesterone s role in breast cancer promotion and/or progression , and thus may enable the formulation of more powerful diagnostic , prognostic and/or therapeutic approaches in the future clinical containment of this cancer .
although the importance of the progesterone receptor ( pr ) to female reproductive and mammary gland biology is firmly established , the coregulators selectively co - opted by pr in these systems have not been clearly delineated .   a selective gene - knockout approach applied to the mouse , which abrogates gene function only in cell types that express pr , recently disclosed steroid receptor coactivator 2 ( src-2 , also known as tif-2 or grip-1 ) to be an indispensable coregulator for uterine and mammary gland responses that require progesterone . 
 uterine cells positive for pr ( but devoid of src-2 ) were found to be incapable of facilitating embryo implantation , a necessary first step toward the establishment of the materno - fetal interface . 
 importantly , such an implantation defect is not exhibited by knockouts for src-1 or src-3 , underscoring the unique coregulator importance of src-2 in peri - implantation biology . 
 moreover , despite normal levels of pr , src-1 and src-3 , progesterone - dependent branching morphogenesis and alveologenesis fails to occur in the murine mammary gland in the absence of src-2 , thereby establishing a critical coregulator role for src-2 in signaling cascades that mediate progesterone - induced mammary epithelial proliferation . 
 finally , the recent detection of src-2 in the human endometrium and breast suggests that this coregulator may represent a new clinical target for the future management of female reproductive health and/or breast cancer .