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1 Lactation and Mammary Gland Biology Group, Department of Animal Science, University of Vermont, Burlington 05405
2 Department of Animal Sciences, University of Illinois, Urbana 61801
Corresponding author: Thomas B. McFadden; e-mail: tmcfadde{at}zoo.uvm.edu.
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ABSTRACT |
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 TOP ABSTRACT ACKNOWLEDGEMENTSREFERENCES |
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Key Words: photoperiod • suppressors of cytokine signaling • prolactin • mammary gland
Abbreviation key: CIS = cytokine-inducible SH2-containing protein, LD = long-day photoperiod, PRL = prolactin, PRL-R = prolactin receptor, SD = short-day photoperiod, SOCS = suppressors of cytokine signaling.
Recently, Miller et al. (2000) reported that cows exposed to short-day photoperiod (SD) during the dry period produced 3.5 kg/d more milk in the subsequent lactation than cows exposed to long-day photoperiod (LD). Because exposure to LD during the dry period results in increased concentrations of prolactin (PRL) in circulation (Miller et al., 2000; Auchtung et al., 2005), and PRL concentrations are negatively correlated with PRL-receptor (PRL-R) expression in various tissues (Barash et al., 1983; Di Carlo et al., 1995; Auchtung et al., 2003), altered PRL signaling could play a role in mediating photoperiodic effects. In addition, it has been demonstrated in mice that PRL signaling in the mammary gland depends on a threshold of PRL-R expression (Hennighausen et al., 1997; Ormandy et al., 2003). We recently reported that exposure to SD during the dry period enhances mammary cell proliferation in dairy cows (Wall et al., 2005), and that altered PRL signaling to the mammary gland may mediate these photoperiodic effects.
Sensitivity of target tissues to PRL is regulated by the inverse relationship between concentrations of PRL in circulation and PRL-R expression (Hennighausen et al., 1997) as well as the recently discovered suppressors of cytokine signaling (SOCS). Suppressors of cytokine signaling are induced by the action of cytokines including PRL that use the Janus kinase/signal transducers and activators of transcription pathway (Aman and Leonard, 1997), and act through negative feedback to modulate cytokine signaling. To date, 8 members of the SOCS family have been identified: SOCS-1 through 7, and cytokine-inducible SH2-containing protein (CIS) (Larsen and Ropke, 2002).
The function of SOCS in the mammary gland has only recently been investigated. Overexpression of CIS results in impaired mammary gland development in pregnant mice, whereas CIS–/– mice have no obvious phenotype (Liu et al., 1998). Gene deletion studies demonstrated that SOCS-1 plays a critical role in the mouse mammary gland. For example, PRL-R–/– mice fail to lactate due to impaired mammary development, and this defect is resolved by deletion of a single SOCS-1 allele (Lindeman et al., 2001). In addition, SOCS-1–/–mice undergo precocious lactation, indicating that SOCS-1 is required for the prevention of lactation before parturition (Lindeman et al., 2001).
There is also evidence that photoperiod affects SOCS expression. Exposure of Siberian hamsters to LD increased SOCS-3 mRNA expression in the hypothalamic arcuate nucleus relative to hamsters exposed to SD (Tups et al., 2004). The increase in SOCS-3 led to resistance to leptin, a cytokine hormone that, like PRL, utilizes the Janus kinase/signal transducers and activators of transcription signaling pathway (Tups et al., 2004). In addition, leptin is thought to have a functional relationship with PRL in the mammary gland and may stimulate mammary development by increasing PRL concentrations (Motta et al., 2004). Therefore, it is plausible that SOCS may be involved in mediating effects of photoperiod on mammary gland development. We hypothesized that exposure to SD during the dry period would reduce SOCS expression in the mammary gland of dairy cows. Our objectives were to compare expression of SOCS mRNA in mammary tissue of cows exposed to either LD or SD during the dry period and to characterize the expression of SOCS-1, SOCS-2, SOCS-3, and CIS mRNA in bovine mammary gland during pregnancy and early lactation.
Cows used in this study were a subset (n = 6 cows per treatment) of animals from a larger study (Auchtung et al., 2005). Details on animal management and treatments were described previously (Wall et al., 2005). The University of Vermont and University of Illinois Institutional Animal Care and Use Committees approved all animal use.
Primers for all target genes are presented in Table 1
. Primer design and PCR for SOCS-2 and CIS were performed as previously described (Wall et al., 2005). For SOCS-1 and SOCS-3, primers and probes were designed using Primer Express (version 1.5; Applied Bio-systems, Foster City, CA). The PCR reaction included 1µL of diluted cDNA and 12.5 µL of TaqMan 2x PCR Master Mix (Applied Biosystems). The PCR reaction started with a 2-min uracil glycosylase step at 50°C followed by a 10-min Taq activation step at 95°C. Samples were then subjected to 40 cycles of 15-s denaturation at 95°C and 1-min annealing and extension at 60°C. Expression of the reference genes ß-actin and ß2-microglobulin was measured and found to be stable over time and between treatments. Therefore, all gene expression values were normalized to that of ß-actin in the same sample. Statistical analyses were performed as previously described (Wall et al., 2005).
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Mammary expression of SOCS-1 and SOCS-3 mRNA increased late in the dry period and in lactation (P < 0.002; Figure 1a, 1b). Although mammary expression of SOCS-1 mRNA was not affected by photoperiod (P = 0.2; Figure 1a), expression of SOCS-3 was lower overall in SD cows relative to LD cows (P = 0.03; Figure 1b). We also observed a treatment by time interaction in SOCS-3 expression (P = 0.03). The photoperiod response that we observed is in agreement with Tups et al. (2004), who reported lower expression of SOCS-3 mRNA in the brain of hamsters exposed to SD compared with those on LD.
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). Mammary expression of SOCS-2 was lower overall in SD cows than LD cows (P = 0.05; Figure 1c), whereas expression of CIS mRNA was lower in SD cows than in LD cows only at d 11 of lactation (P = 0.02; Figure 1d). The temporal changes in SOCS expression observed in the present study imply that SOCS may play a role in regulating or responding to mammary gland development and function during pregnancy and lactation in dairy cows. It has been shown that SOCS-1 is required for the prevention of lactation before parturition in mice (Lindeman et al., 2001), and it may have a similar role in the bovine mammary gland. Expression of SOCS-3 mRNA in the mammary gland of rats is regulated in part by filling of the mammary gland with milk (Tam et al., 2001). Thus, it is possible that the marked increase we observed in SOCS-1 and SOCS-3 expression during late gestation may be due to engorgement of the gland with mammary secretion. Treatment differences in expression of SOCS during the dry period and during lactation may have contributed to differences in mammary proliferation (Wall et al., 2005) and milk yield (Auchtung et al., 2005) in these cows.
We conclude that SOCS genes are expressed in bovine mammary gland. The pattern of SOCS expression during pregnancy and lactation indicates that they play a role in regulating bovine mammary gland development and lactogenesis. Relative to LD, exposure to SD during the dry period resulted in decreased expression of SOCS-2, SOCS-3, and CIS mRNA in bovine mammary gland. These data suggest that lower SOCS expression in cows exposed to SD during the dry period may enhance PRL signaling to the mammary gland, thereby augmenting mammary development during pregnancy and milk production in the subsequent lactation.
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ACKNOWLEDGEMENTS |
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FOOTNOTES |
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Received for publication March 23, 2005. Accepted for publication May 13, 2005.
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