This paper probably has the most explicit statement to the affirmative on tamoxifen and PgR in mammary tumors, saying "Thus, immuno-staining for progesterone receptor (PgR) is reduced in almost all cases by aromatase inhibitors, becoming undetectable in many. This contrasts with effects of tamoxifen in which the most common change on PgR is to increase expression." In table 3, we see that 24% the tamoxifen group had down-regulation of the PgR, 26% had no change, and 50% showed up-regulation. The full text of the paper explains, "In a small series of PgR-positive cancers treated with tamoxifen, a different pattern of change was seen. Thus, only 17% of cases showed a decrease and the most common change was a paradoxical increase in staining." They explain in the conclusion that this was a "paradoxical" or unexpected result and "may therefore represent early evidence of resistance to [tamoxifen] treatment." In other words, it may be because the tissue being tested was a mammary tumor, which can mutate and become resistant to tamoxifen.
In another paper on tamoxifen administration and PgR, again in breast cancer in women, 40% (21/52) had an increase in PgR, 37%(19/52) has a decrease in PgR, and the remaining (12/52) were negative for the receptor.
In contrast to those papers,
this study found what they described as "a modest decline" in PgR levels in all three histologies they tested with tamoxifen treatment, though it failed to achieve statistical significance (p values of .19, .82, and .15). The abstract explains that "There were trends for reductions in ER and PgR..."
I know there are other places where I've seen down-regulation of PgR in breast tissue. I'll do a little more research and see if I can dig up some more references when I have some free time.