How do you approach an ER negative, PR positive, Her2 negative breast cancer in terms of hormone manipulation based on PR status alone?

This is a very interesting question.

My practice is not to utilize anti-estrogen in true ER Negative /PR positive cases ( after repeating the assay on another tissue sample to rule out a false negative ER assay).

EBCTCG meta analysis of randomized trials published in Lancet in 2011 showed only slight early benefit from tamoxifen in ER Negative, PR positive cohort and the benefit was not significant and slight non significant benefit was possibly due  to inclusion in this category of few patients with false-negative ER assays.

Davies C, Godwin J, Gray R, Clarke M, Cutter D, Darby S, McGale P, Pan HC, Taylor C, Wang YC, Dowsett M, Ingle J, Peto R, Early Breast Cancer Trialists' Collaborative Group (EBCTCG): Relevance of breast cancer hormone receptors and other factors to the efficacy of adjuvant tamoxifen: patient-level meta-analysis of randomised trials. Lancet. 2011, 378: 771-784

With improvement in ER/PR testing, fewer and fewer cases are being reported as ER negative,  PR positive (4% in the early 1990s but only 1% in recent years in the SEER cancer registry data).

Based upon the ASCO/CAP guidelines from 2010, I would treat patients in the operable setting with anti-estrogen therapy if ER and/or PR > 1%. As described in guidelines (Hammond et al JCO 2010), the studies can vary in terms of the predictive role of PR, independent of ER values. In some retrospective studies, PR has been identified as predictive with as few as 1% of stained nuclei cells (Mohsin et al Mod Pathol 2004, Yamashita et al Breast Cancer 2006). In addition, per the guidelines, repeat testing may be considered on another sample if ER-/PR+ is identified to rule out a false-negative ER or false-positive PR assay.